Release 1.5.0.1

Add --enable-tests to test
Make the lexer and parser safe
2025-06-19 13:50:45 +02:00 · 2025-01-18 16:54:11 +01:00 · 2025-01-18 16:33:13 +01:00 · 2024-12-03 19:49:33 +01:00 · 2024-12-01 21:53:01 +01:00 · 2024-11-10 22:23:54 +01:00
45 changed files with 3662 additions and 2500 deletions
--- a/.gitea/deploy.awk
+++ b/.gitea/deploy.awk
@ -0,0 +1,3 @@
+END {
+    system("cabal upload --username belka --password "ENVIRON["HACKAGE_PASSWORD"]" "$0)
+}
--- a/.gitea/workflows/build.yml
+++ b/.gitea/workflows/build.yml
@ -0,0 +1,33 @@
+name: Build
+
+on:
+  push:
+    branches:
+      - '**'
+  pull_request:
+    branches: [master]
+
+jobs:
+  audit:
+    runs-on: buildenv
+    steps:
+      - uses: actions/checkout@v4
+      - run: hlint -- src tests
+
+  test:
+    runs-on: buildenv
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install dependencies
+        run: cabal update
+      - name: Prepare system
+        run: cabal build graphql-test --enable-tests
+      - run: cabal test --test-show-details=streaming --enable-tests
+
+  doc:
+    runs-on: buildenv
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install dependencies
+        run: cabal update
+      - run: cabal haddock --enable-documentation
--- a/.gitea/workflows/release.yml
+++ b/.gitea/workflows/release.yml
@ -0,0 +1,17 @@
+name: Release
+
+on:
+  push:
+    tags:
+      - '**'
+
+jobs:
+  release:
+    runs-on: buildenv
+    steps:
+      - uses: actions/checkout@v4
+      - name: Upload a candidate
+        env:
+          HACKAGE_PASSWORD: ${{ secrets.HACKAGE_PASSWORD }}
+        run: |
+          cabal sdist | awk -f .gitea/deploy.awk
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@ -6,6 +6,145 @@ The format is based on
 and this project adheres to
 [Haskell Package Versioning Policy](https://pvp.haskell.org/).

+## [1.5.0.1] - 2025-06-19
+### Fixed
+- Allow any 2.x QuickCheck version.
+- Make the lexer and parser safe.
+
+## [1.5.0.0] - 2024-12-03
+### Removed
+- Remove deprecated 'gql' quasi quoter.
+
+### Changed
+- Validate the subscription root not to be an introspection field
+  (`singleFieldSubscriptionsRule`).
+
+## [1.4.0.0] - 2024-10-26
+### Changed
+- `Schema.Directive` is extended to contain a boolean argument, representing
+  repeatable directives. The parser can parse repeatable directive definitions.
+  Validation allows repeatable directives.
+- `AST.Document.Directive` is a record.
+- `gql` quasi quoter is deprecated (moved to graphql-spice package).
+
+### Fixed
+- `gql` quasi quoter recognizeds all GraphQL line endings (CR, LF and CRLF).
+
+### Added
+- @specifiedBy directive.
+
+## [1.3.0.0] - 2024-05-01
+### Changed
+- Remove deprecated `runCollectErrs`, `Resolution`, `CollectErrsT` from the
+  `Error` module.
+
+## [1.2.0.3] - 2024-01-09
+### Fixed
+- Fix corrupted source distribution.
+
+## [1.2.0.2] - 2024-01-09
+### Fixed
+- `gql` removes not only leading `\n` but also `\r`.
+- Fix non nullable type string representation in executor error messages.
+- Fix input objects not being coerced to lists.
+- Fix used variables are not found in the properties of input objects.
+
+## [1.2.0.1] - 2023-04-25
+### Fixed
+- Support hspec 2.11.
+
+## [1.2.0.0] - 2023-02-28
+### Added
+- Schema printing.
+- `Semigroup` and `Monoid` instances for `AST.Document.Description`.
+- Support for vector 0.13.0.0 and transformers 0.6.1.0.
+
+### Fixed
+- Fix resolvers returning a list in the reverse order.
+
+### Removed
+- GHC 8 support.
+- Cabal -json flag.
+- `Test.Hspec.GraphQL`: moved to `graphql-spice` package.
+- CPP `ifdef WITH_JSON` blocks.
+
+## [1.1.0.0] - 2022-12-24
+### Changed
+- Removed deprecated `Language.GraphQL.Error` functions: `addErr`, `addErrMsg`,
+  `singleError`.
+- Deprecate `Resolution`, `CollectErrsT` and `runCollectErrs` in the `Error`
+  module. It was already noted in the documentation that these symbols are
+  deprecated, now a pragma is added.
+- `Language.GraphQL`: Added information about the *json* flag and switching to
+  *graphql-spice* for JSON support.
+
+### Added
+- Partial schema printing: operation type encoder.
+
+## [1.0.3.0] - 2022-03-27
+### Fixed
+- Index position in error path. (Index and Segment paths of a field have been
+  swapped).
+- Parsing empty list as an argument.
+
+### Added
+- quickCheck Parser test for arguments. Arbitrary instances for Language.GraphQL.AST.Document.
+- Enhanced query error messages. Add tests for these cases.
+- Allow version 2.0 of the text package.
+
+## [1.0.2.0] - 2021-12-26
+### Added
+- `Serialize` instance for `Type.Definition.Value`.
+- `VariableValue` instance for `Type.Definition.Value`.
+- `Json` build flag, enabled by default. JSON and Aeson support can be disabled
+  by disabling this flag.
+
+## [1.0.1.0] - 2021-09-27
+### Added
+- Custom `Show` instance for `Type.Definition.Value` (for error
+  messages).
+- Path information in errors (path to the field throwing the error).
+- Deprecation notes in the `Error` module for `Resolution`, `CollectErrsT` and
+  `runCollectErrs`. These symbols are part of the old executor and aren't used
+  anymore, it will be deprecated in the future and removed.
+- `TH` module with the `gql` quasi quoter.
+
+### Fixed
+- Error messages are more concrete, they also contain type information and
+  wrong values, where appropriate and possible.
+- If the field with an error is Non-Nullable, the error is propagated to the
+  first nullable field, as required by the specification.
+
+## [1.0.0.0] - 2021-07-04
+### Added
+- `Language.GraphQL.Execute.OrderedMap` is a map data structure, that preserves
+  insertion order.
+- `Language.GraphQL.Schema.schemaWithTypes` constructs a complete schema,
+  including an optional schema description and user-defined types not referenced
+  in the schema directly (for example interface implementations).
+- `Language.GraphQL.Schema.description` returns the optional schema description.
+- All errors that can be associated with a location in the query contain
+  location information.
+
+### Fixed
+- Parser now accepts empty lists and objects.
+- Parser now accepts all directive locations.
+- `valuesOfCorrectTypeRule` doesn't check lists recursively since the
+  validation traverser calls it on all list items.
+- `valuesOfCorrectTypeRule` doesn't check objects recursively since the
+  validation traverser calls it on all object properties.
+- Validation of non-nullable values inside lists.
+- `executeField` shouldn't assume that a selection has only one field with a
+  given name, but it should take the first field. The underlying cause is a
+  wrong pattern, which (because of the laziness) is executed only if the field
+  has arguments.
+
+### Changed
+- `AST.Document.Value.List` and `AST.Document.ConstValue.ConstList` contain
+  location information for each list item.
+- `Error`: `singleError`, `addErr` and `addErrMsg` are deprecated. They are
+  internal functions used by the executor for error handling.
+
 ## [0.11.1.0] - 2021-02-07
 ### Added
 - `Validate.Rules`:
@ -104,7 +243,7 @@ and this project adheres to
  `locations`.
 - Parsing comments in the front of definitions.
 - Some missing labels were added to the parsers, some labels were fixed to
-  refer to the AST nodes being parsed. 
+  refer to the AST nodes being parsed.

 ### Added
 - `AST` reexports `AST.Parser`.
@ -308,7 +447,6 @@ and this project adheres to
 - `Language.GraphQL.Schema`: `enum`, `enumA`, `wrappedEnum` and `wrappedEnumA`.
  Use `scalar`, `scalarA`, `wrappedScalar` and `wrappedScalarA` instead.

-
 ## [0.5.1.0] - 2019-10-22
 ### Deprecated
 - `Language.GraphQL.AST.Arguments`. Use `[Language.GraphQL.AST.Argument]`
@ -413,18 +551,31 @@ and this project adheres to
 ### Added
 - Data types for the GraphQL language.

-[0.11.1.0]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v0.11.1.0&rev_to=v0.11.0.0
-[0.11.0.0]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v0.11.0.0&rev_to=v0.10.0.0
-[0.10.0.0]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v0.10.0.0&rev_to=v0.9.0.0
-[0.9.0.0]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v0.9.0.0&rev_to=v0.8.0.0
-[0.8.0.0]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v0.8.0.0&rev_to=v0.7.0.0
-[0.7.0.0]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v0.7.0.0&rev_to=v0.6.1.0
-[0.6.1.0]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v0.6.1.0&rev_to=v0.6.0.0
-[0.6.0.0]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v0.6.0.0&rev_to=v0.5.1.0
-[0.5.1.0]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v0.5.1.0&rev_to=v0.5.0.1
-[0.5.0.1]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v0.5.0.1&rev_to=v0.5.0.0
-[0.5.0.0]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v0.5.0.0&rev_to=v0.4.0.0
-[0.4.0.0]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v0.4.0.0&rev_to=v0.3
-[0.3]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v0.3&rev_to=v0.2.1
-[0.2.1]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v0.2.1&rev_to=v0.2
-[0.2]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v0.2&rev_to=v0.1
+[1.5.0.1]: https://git.caraus.tech/OSS/graphql/compare/v1.5.0.0...v1.5.0.1
+[1.5.0.0]: https://git.caraus.tech/OSS/graphql/compare/v1.4.0.0...v1.5.0.0
+[1.4.0.0]: https://git.caraus.tech/OSS/graphql/compare/v1.3.0.0...v1.4.0.0
+[1.3.0.0]: https://git.caraus.tech/OSS/graphql/compare/v1.2.0.3...v1.3.0.0
+[1.2.0.3]: https://git.caraus.tech/OSS/graphql/compare/v1.2.0.2...v1.2.0.3
+[1.2.0.2]: https://git.caraus.tech/OSS/graphql/compare/v1.2.0.1...v1.2.0.2
+[1.2.0.1]: https://git.caraus.tech/OSS/graphql/compare/v1.2.0.0...v1.2.0.1
+[1.2.0.0]: https://git.caraus.tech/OSS/graphql/compare/v1.1.0.0...v1.2.0.0
+[1.1.0.0]: https://git.caraus.tech/OSS/graphql/compare/v1.0.3.0...v1.1.0.0
+[1.0.3.0]: https://git.caraus.tech/OSS/graphql/compare/v1.0.2.0...v1.0.3.0
+[1.0.2.0]: https://git.caraus.tech/OSS/graphql/compare/v1.0.1.0...v1.0.2.0
+[1.0.1.0]: https://git.caraus.tech/OSS/graphql/compare/v1.0.0.0...v1.0.1.0
+[1.0.0.0]: https://git.caraus.tech/OSS/graphql/compare/v0.11.1.0...v1.0.0.0
+[0.11.1.0]: https://git.caraus.tech/OSS/graphql/compare/v0.11.0.0...v0.11.1.0
+[0.11.0.0]: https://git.caraus.tech/OSS/graphql/compare/v0.10.0.0...v0.11.0.0
+[0.10.0.0]: https://git.caraus.tech/OSS/graphql/compare/v0.9.0.0...v0.10.0.0
+[0.9.0.0]: https://git.caraus.tech/OSS/graphql/compare/v0.8.0.0...v0.9.0.0
+[0.8.0.0]: https://git.caraus.tech/OSS/graphql/compare/v0.7.0.0...v0.8.0.0
+[0.7.0.0]: https://git.caraus.tech/OSS/graphql/compare/v0.6.1.0...v0.7.0.0
+[0.6.1.0]: https://git.caraus.tech/OSS/graphql/compare/v0.6.0.0...v0.6.1.0
+[0.6.0.0]: https://git.caraus.tech/OSS/graphql/compare/v0.5.1.0...v0.6.0.0
+[0.5.1.0]: https://git.caraus.tech/OSS/graphql/compare/v0.5.0.1...v0.5.1.0
+[0.5.0.1]: https://git.caraus.tech/OSS/graphql/compare/v0.5.0.0...v0.5.0.1
+[0.5.0.0]: https://git.caraus.tech/OSS/graphql/compare/v0.4.0.0...v0.5.0.0
+[0.4.0.0]: https://git.caraus.tech/OSS/graphql/compare/v0.3...v0.4.0.0
+[0.3]: https://git.caraus.tech/OSS/graphql/compare/v0.2.1...v0.3
+[0.2.1]: https://git.caraus.tech/OSS/graphql/compare/v0.2...v0.2.1
+[0.2]: https://git.caraus.tech/OSS/graphql/compare/v0.1...v0.2
--- a/README.md
+++ b/README.md
@ -1,15 +1,12 @@
 # GraphQL implementation in Haskell

-[![Simple Haskell](https://www.simplehaskell.org/badges/badge.svg)](https://www.simplehaskell.org)
-[![CI/CD](https://img.shields.io/badge/CI-CD-brightgreen)](https://build.caraus.tech/go/pipelines)
-
-See https://www.caraus.tech/projects/pub-graphql.
+See https://git.caraus.tech/OSS/graphql.

 Report issues on the
-[bug tracker](https://www.caraus.tech/projects/pub-graphql/issues).
+[bug tracker](https://git.caraus.tech/OSS/graphql/issues).

 API documentation is available through
 [Hackage](https://hackage.haskell.org/package/graphql).

 Further documentation will be made available in the
-[Wiki](https://www.caraus.tech/projects/pub-graphql/wiki).
+[Wiki](https://git.caraus.tech/OSS/graphql/wiki).
--- a/graphql.cabal
+++ b/graphql.cabal
@ -1,117 +1,108 @@
-cabal-version: 2.2
-
-- This file has been generated from package.yaml by hpack version 0.33.0.
--
-- see: https://github.com/sol/hpack
--
-- hash: 15a0880180192f918ba0bd3b3e955c57232f1efe8993745d505fcb6e1aab1451
+cabal-version: 3.0

 name:           graphql
-version:        0.11.1.0
+version:        1.5.0.1
 synopsis:       Haskell GraphQL implementation
 description:    Haskell <https://spec.graphql.org/June2018/ GraphQL> implementation.
 category:       Language
-homepage:       https://www.caraus.tech/projects/pub-graphql
-bug-reports:    https://www.caraus.tech/projects/pub-graphql/issues
+homepage:       https://git.caraus.tech/OSS/graphql
+bug-reports:    https://git.caraus.tech/OSS/graphql/issues
 author:         Danny Navarro <j@dannynavarro.net>,
                Matthías Páll Gissurarson <mpg@mpg.is>,
                Sólrún Halla Einarsdóttir <she@mpg.is>
 maintainer:     belka@caraus.de
-copyright:      (c) 2019-2021 Eugen Wissner,
+copyright:      (c) 2019-2025 Eugen Wissner,
                (c) 2015-2017 J. Daniel Navarro
 license:        MPL-2.0 AND BSD-3-Clause
 license-files:  LICENSE,
                LICENSE.MPL
 build-type:     Simple
 extra-source-files:
-    CHANGELOG.md
-    README.md
+  CHANGELOG.md
+  README.md
+tested-with:
+  GHC == 9.10.1

 source-repository head
  type: git
-  location: git://caraus.tech/pub/graphql.git
+  location: https://git.caraus.tech/OSS/graphql.git

 library
  exposed-modules:
-      Language.GraphQL
-      Language.GraphQL.AST
-      Language.GraphQL.AST.DirectiveLocation
-      Language.GraphQL.AST.Document
-      Language.GraphQL.AST.Encoder
-      Language.GraphQL.AST.Lexer
-      Language.GraphQL.AST.Parser
-      Language.GraphQL.Error
-      Language.GraphQL.Execute
-      Language.GraphQL.Execute.Coerce
-      Language.GraphQL.Type
-      Language.GraphQL.Type.In
-      Language.GraphQL.Type.Out
-      Language.GraphQL.Type.Schema
-      Language.GraphQL.Validate
-      Language.GraphQL.Validate.Validation
-      Test.Hspec.GraphQL
+    Language.GraphQL
+    Language.GraphQL.AST
+    Language.GraphQL.AST.DirectiveLocation
+    Language.GraphQL.AST.Document
+    Language.GraphQL.AST.Encoder
+    Language.GraphQL.AST.Lexer
+    Language.GraphQL.AST.Parser
+    Language.GraphQL.Error
+    Language.GraphQL.Execute
+    Language.GraphQL.Execute.Coerce
+    Language.GraphQL.Execute.OrderedMap
+    Language.GraphQL.Type
+    Language.GraphQL.Type.In
+    Language.GraphQL.Type.Out
+    Language.GraphQL.Type.Schema
+    Language.GraphQL.Validate
+    Language.GraphQL.Validate.Validation
  other-modules:
-      Language.GraphQL.Execute.Execution
-      Language.GraphQL.Execute.Subscribe
-      Language.GraphQL.Execute.Transform
-      Language.GraphQL.Type.Definition
-      Language.GraphQL.Type.Internal
-      Language.GraphQL.Validate.Rules
+    Language.GraphQL.Execute.Transform
+    Language.GraphQL.Type.Definition
+    Language.GraphQL.Type.Internal
+    Language.GraphQL.Validate.Rules
  hs-source-dirs:
-      src
+    src
+  ghc-options: -Wall
+
  build-depends:
-      aeson
-    , base >=4.7 && <5
-    , conduit
-    , containers
-    , exceptions
-    , hspec-expectations
-    , megaparsec
-    , parser-combinators
-    , scientific
-    , text
-    , transformers
-    , unordered-containers
+    base >= 4.15 && < 5,
+    conduit ^>= 1.3.4,
+    containers >= 0.6 && < 0.8,
+    exceptions ^>= 0.10.4,
+    megaparsec >= 9.0 && < 10,
+    parser-combinators >= 1.3 && < 2,
+    text >= 1.2 && < 3,
+    transformers >= 0.5.6 && < 0.7,
+    unordered-containers ^>= 0.2.14,
+    vector >= 0.12 && < 0.14
+
  default-language: Haskell2010

 test-suite graphql-test
  type: exitcode-stdio-1.0
  main-is: Spec.hs
  other-modules:
-      Language.GraphQL.AST.DocumentSpec
-      Language.GraphQL.AST.EncoderSpec
-      Language.GraphQL.AST.LexerSpec
-      Language.GraphQL.AST.ParserSpec
-      Language.GraphQL.ErrorSpec
-      Language.GraphQL.Execute.CoerceSpec
-      Language.GraphQL.ExecuteSpec
-      Language.GraphQL.Type.OutSpec
-      Language.GraphQL.Validate.RulesSpec
-      Test.DirectiveSpec
-      Test.FragmentSpec
-      Test.RootOperationSpec
-      Paths_graphql
-  autogen-modules:
-      Paths_graphql
+    Language.GraphQL.AST.DocumentSpec
+    Language.GraphQL.AST.EncoderSpec
+    Language.GraphQL.AST.LexerSpec
+    Language.GraphQL.AST.ParserSpec
+    Language.GraphQL.AST.Arbitrary
+    Language.GraphQL.ErrorSpec
+    Language.GraphQL.Execute.CoerceSpec
+    Language.GraphQL.Execute.OrderedMapSpec
+    Language.GraphQL.ExecuteSpec
+    Language.GraphQL.Type.OutSpec
+    Language.GraphQL.Validate.RulesSpec
+    Schemas.HeroSchema
  hs-source-dirs:
-      tests
-  ghc-options: -threaded -rtsopts -with-rtsopts=-N
+    tests
+  ghc-options: -threaded -rtsopts -with-rtsopts=-N -Wall
+
  build-depends:
-      QuickCheck
-    , aeson
-    , base >=4.7 && <5
-    , conduit
-    , containers
-    , exceptions
-    , graphql
-    , hspec
-    , hspec-expectations
-    , hspec-megaparsec
-    , megaparsec
-    , parser-combinators
-    , raw-strings-qq
-    , scientific
-    , text
-    , transformers
-    , unordered-containers
+    QuickCheck >= 2.14 && < 3,
+    base,
+    conduit,
+    exceptions,
+    graphql,
+    hspec >= 2.10.9 && < 2.12,
+    hspec-expectations ^>= 0.8.2,
+    hspec-megaparsec ^>= 2.2.0,
+    megaparsec,
+    text,
+    unordered-containers,
+    containers,
+    vector
+  build-tool-depends:
+    hspec-discover:hspec-discover
  default-language: Haskell2010
--- a/package.yaml
+++ b/package.yaml
@ -1,67 +0,0 @@
-name: graphql
-version: 0.11.1.0
-synopsis: Haskell GraphQL implementation
-description:
-  Haskell <https://spec.graphql.org/June2018/ GraphQL> implementation.
-maintainer: belka@caraus.de
-git: git://caraus.tech/pub/graphql.git
-homepage: https://www.caraus.tech/projects/pub-graphql
-bug-reports: https://www.caraus.tech/projects/pub-graphql/issues
-category: Language
-license: MPL-2.0 AND BSD-3-Clause
-
-copyright:
- (c) 2019-2021 Eugen Wissner
- (c) 2015-2017 J. Daniel Navarro
-author:
- Danny Navarro <j@dannynavarro.net>
- Matthías Páll Gissurarson <mpg@mpg.is>
- Sólrún Halla Einarsdóttir <she@mpg.is>
-
-license-file:
- LICENSE
- LICENSE.MPL
-extra-source-files:
- CHANGELOG.md
- README.md
-
-dependencies:
- aeson
- base >= 4.7 && < 5
- conduit
- containers
- exceptions
- hspec-expectations
- megaparsec
- parser-combinators
- scientific
- text
- transformers
- unordered-containers
-
-library:
-  source-dirs: src
-  other-modules:
-  - Language.GraphQL.Execute.Execution
-  - Language.GraphQL.Execute.Subscribe
-  - Language.GraphQL.Execute.Transform
-  - Language.GraphQL.Type.Definition
-  - Language.GraphQL.Type.Internal
-  - Language.GraphQL.Validate.Rules
-
-tests:
-  graphql-test:
-    main: Spec.hs
-    source-dirs: tests
-    ghc-options:
-    - -threaded
-    - -rtsopts
-    - -with-rtsopts=-N
-    dependencies:
-    - graphql
-    - hspec
-    - hspec-megaparsec
-    - QuickCheck
-    - raw-strings-qq
-    generated-other-modules:
-    - Paths_graphql
--- a/src/Language/GraphQL.hs
+++ b/src/Language/GraphQL.hs
@ -1,75 +1,46 @@
-{-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE RecordWildCards #-}

 -- | This module provides the functions to parse and execute @GraphQL@ queries.
 module Language.GraphQL
    ( graphql
-    , graphqlSubs
    ) where

 import Control.Monad.Catch (MonadCatch)
-import qualified Data.Aeson as Aeson
-import qualified Data.Aeson.Types as Aeson
-import qualified Data.HashMap.Strict as HashMap
-import Data.Maybe (catMaybes)
+import Data.HashMap.Strict (HashMap)
 import qualified Data.Sequence as Seq
 import Data.Text (Text)
-import Language.GraphQL.AST
+import qualified Data.Text as Text
+import qualified Language.GraphQL.AST as Full
 import Language.GraphQL.Error
 import Language.GraphQL.Execute
 import qualified Language.GraphQL.Validate as Validate
 import Language.GraphQL.Type.Schema (Schema)
+import Prelude hiding (null)
 import Text.Megaparsec (parse)

 -- | If the text parses correctly as a @GraphQL@ query the query is
 -- executed using the given 'Schema'.
-graphql :: MonadCatch m
-    => Schema m -- ^ Resolvers.
-    -> Text -- ^ Text representing a @GraphQL@ request document.
-    -> m (Either (ResponseEventStream m Aeson.Value) Aeson.Object) -- ^ Response.
-graphql schema = graphqlSubs schema mempty mempty
-
-- | If the text parses correctly as a @GraphQL@ query the substitution is
-- applied to the query and the query is then executed using to the given
-- 'Schema'.
-graphqlSubs :: MonadCatch m
+--
+-- An operation name can be given if the document contains multiple operations.
+graphql :: (MonadCatch m, VariableValue a, Serialize b)
    => Schema m -- ^ Resolvers.
    -> Maybe Text -- ^ Operation name.
-    -> Aeson.Object -- ^ Variable substitution function.
+    -> HashMap Full.Name a -- ^ Variable substitution function.
    -> Text -- ^ Text representing a @GraphQL@ request document.
-    -> m (Either (ResponseEventStream m Aeson.Value) Aeson.Object) -- ^ Response.
-graphqlSubs schema operationName variableValues document' =
-    case parse document "" document' of
-        Left errorBundle -> pure . formatResponse <$> parseError errorBundle
+    -> m (Either (ResponseEventStream m b) (Response b)) -- ^ Response.
+graphql schema operationName variableValues document' =
+    case parse Full.document "" document' of
+        Left errorBundle -> pure  <$> parseError errorBundle
        Right parsed ->
            case validate parsed of
-                Seq.Empty -> fmap formatResponse
-                    <$> execute schema operationName variableValues parsed
+                Seq.Empty -> execute schema operationName variableValues parsed
                errors -> pure $ pure
-                    $ HashMap.singleton "errors"
-                    $ Aeson.toJSON
+                    $ Response null
                    $ fromValidationError <$> errors
  where
    validate = Validate.document schema Validate.specifiedRules
-    formatResponse (Response data'' Seq.Empty) = HashMap.singleton "data" data''
-    formatResponse (Response data'' errors') = HashMap.fromList
-        [ ("data", data'')
-        , ("errors", Aeson.toJSON $ fromError <$> errors')
-        ]
-    fromError Error{..} = Aeson.object $ catMaybes
-        [ Just ("message", Aeson.toJSON message)
-        , toMaybe fromLocation "locations" locations
-        , toMaybe fromPath "path" path
-        ]
-    fromValidationError Validate.Error{..} = Aeson.object
-        [ ("message", Aeson.toJSON message)
-        , ("locations", Aeson.listValue fromLocation locations)
-        ]
-    toMaybe _ _ [] = Nothing
-    toMaybe f key xs = Just (key, Aeson.listValue f xs)
-    fromPath (Segment segment) = Aeson.String segment
-    fromPath (Index index) = Aeson.toJSON index
-    fromLocation Location{..} = Aeson.object
-        [ ("line", Aeson.toJSON line)
-        , ("column", Aeson.toJSON column)
-        ]
+    fromValidationError Validate.Error{..} = Error
+        { message = Text.pack message
+        , locations = locations
+        , path = []
+        }
--- a/src/Language/GraphQL/AST.hs
+++ b/src/Language/GraphQL/AST.hs
@ -1,6 +1,5 @@
-{- This Source Code Form is subject to the terms of the Mozilla Public License,
-   v. 2.0. If a copy of the MPL was not distributed with this file, You can
-   obtain one at https://mozilla.org/MPL/2.0/. -}
+{-# LANGUAGE NoGeneralizedNewtypeDeriving #-}
+{-# LANGUAGE Safe #-}

 -- | Target AST for parser.
 module Language.GraphQL.AST
--- a/src/Language/GraphQL/AST/DirectiveLocation.hs
+++ b/src/Language/GraphQL/AST/DirectiveLocation.hs
@ -1,10 +1,7 @@
-{- This Source Code Form is subject to the terms of the Mozilla Public License,
-   v. 2.0. If a copy of the MPL was not distributed with this file, You can
-   obtain one at https://mozilla.org/MPL/2.0/. -}
-
+{-# LANGUAGE NoGeneralizedNewtypeDeriving #-}
 {-# LANGUAGE Safe #-}

-- | Various parts of a GraphQL document can be annotated with directives. 
+-- | Various parts of a GraphQL document can be annotated with directives.
 -- This module describes locations in a document where directives can appear.
 module Language.GraphQL.AST.DirectiveLocation
    ( DirectiveLocation(..)
--- a/src/Language/GraphQL/AST/Document.hs
+++ b/src/Language/GraphQL/AST/Document.hs
@ -3,6 +3,7 @@
 {-# LANGUAGE NamedFieldPuns #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE RecordWildCards #-}
+{-# LANGUAGE NoGeneralizedNewtypeDeriving #-}
 {-# LANGUAGE Safe #-}

 -- | This module defines an abstract syntax tree for the @GraphQL@ language. It
@ -49,6 +50,8 @@ module Language.GraphQL.AST.Document
    , Value(..)
    , VariableDefinition(..)
    , escape
+    , showVariableName
+    , showVariable
    ) where

 import Data.Char (ord)
@ -226,6 +229,12 @@ type TypeCondition = Name

 -- ** Input Values

+-- | Escapes a single character according to the GraphQL escaping rules for
+-- double-quoted string values.
+--
+-- Characters, that should be escaped, are written as escaped characters with a
+-- backslash or Unicode with an \"\\u\". Other characters are returned as
+-- strings.
 escape :: Char -> String
 escape char'
    | char' == '\\' = "\\\\"
@ -257,7 +266,7 @@ data Value
    | Boolean Bool
    | Null
    | Enum Name
-    | List [Value]
+    | List [Node Value]
    | Object [ObjectField Value]
    deriving Eq

@ -281,7 +290,7 @@ data ConstValue
    | ConstBoolean Bool
    | ConstNull
    | ConstEnum Name
-    | ConstList [ConstValue]
+    | ConstList [Node ConstValue]
    | ConstObject [ObjectField ConstValue]
    deriving Eq

@ -318,13 +327,13 @@ instance Functor ObjectField where
 -- Each operation can include a list of variables:
 --
 -- @
-- query (protagonist: String = "Zarathustra") {
+-- query (protagonist: String = \"Zarathustra\") {
 --   getAuthor(protagonist: $protagonist)
 -- }
 -- @
 --
 -- This query defines an optional variable @protagonist@ of type @String@,
-- its default value is "Zarathustra". If no default value is defined and no
+-- its default value is \"Zarathustra\". If no default value is defined and no
 -- value is provided, a variable can still be @null@ if its type is nullable.
 --
 -- Variables are usually passed along with the query, but not in the query
@ -333,6 +342,12 @@ data VariableDefinition =
    VariableDefinition Name Type (Maybe (Node ConstValue)) Location
    deriving (Eq, Show)

+showVariableName :: VariableDefinition -> String
+showVariableName (VariableDefinition name _ _ _) = "$" <> Text.unpack name
+
+showVariable :: VariableDefinition -> String
+showVariable var@(VariableDefinition _ type' _ _) = showVariableName var <> ":" <> " " <> show type'
+
 -- ** Type References

 -- | Type representation.
@ -357,8 +372,8 @@ data NonNullType
    deriving Eq

 instance Show NonNullType where
-    show (NonNullTypeNamed typeName) = '!' : Text.unpack typeName
-    show (NonNullTypeList listType) =  concat ["![", show listType, "]"]
+    show (NonNullTypeNamed typeName) = Text.unpack $ typeName <> "!"
+    show (NonNullTypeList listType) = concat ["[", show listType, "]!"]

 -- ** Directives

@ -366,7 +381,11 @@ instance Show NonNullType where
 --
 -- Directives begin with "@", can accept arguments, and can be applied to the
 -- most GraphQL elements, providing additional information.
-data Directive = Directive Name [Argument] Location deriving (Eq, Show)
+data Directive = Directive
+    { name :: Name
+    , arguments :: [Argument]
+    , location :: Location
+    } deriving (Eq, Show)

 -- * Type System

@ -391,7 +410,7 @@ data TypeSystemDefinition
    = SchemaDefinition [Directive] (NonEmpty OperationTypeDefinition)
    | TypeDefinition TypeDefinition
    | DirectiveDefinition
-        Description Name ArgumentsDefinition (NonEmpty DirectiveLocation)
+        Description Name ArgumentsDefinition Bool (NonEmpty DirectiveLocation)
    deriving (Eq, Show)

 -- ** Type System Extensions
@ -450,18 +469,23 @@ data SchemaExtension
 newtype Description = Description (Maybe Text)
    deriving (Eq, Show)

+instance Semigroup Description
+  where
+    Description lhs <> Description rhs = Description $ lhs <> rhs
+
+instance Monoid Description
+  where
+    mempty = Description mempty
+
 -- ** Types

 -- | Type definitions describe various user-defined types.
 data TypeDefinition
    = ScalarTypeDefinition Description Name [Directive]
    | ObjectTypeDefinition
-        Description
-        Name
-        (ImplementsInterfaces [])
-        [Directive]
-        [FieldDefinition]
-    | InterfaceTypeDefinition Description Name [Directive] [FieldDefinition]
+        Description Name (ImplementsInterfaces []) [Directive] [FieldDefinition]
+    | InterfaceTypeDefinition
+        Description Name (ImplementsInterfaces []) [Directive] [FieldDefinition]
    | UnionTypeDefinition Description Name [Directive] (UnionMemberTypes [])
    | EnumTypeDefinition Description Name [Directive] [EnumValueDefinition]
    | InputObjectTypeDefinition
--- a/src/Language/GraphQL/AST/Encoder.hs
+++ b/src/Language/GraphQL/AST/Encoder.hs
@ -2,6 +2,7 @@
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE RecordWildCards #-}
+{-# LANGUAGE NoGeneralizedNewtypeDeriving #-}
 {-# LANGUAGE Safe #-}

 -- | This module defines a minifier and a printer for the @GraphQL@ language.
@ -11,12 +12,14 @@ module Language.GraphQL.AST.Encoder
    , directive
    , document
    , minified
+    , operationType
    , pretty
    , type'
+    , typeSystemDefinition
    , value
    ) where

-import Data.Foldable (fold)
+import Data.Foldable (fold, Foldable (..))
 import qualified Data.List.NonEmpty as NonEmpty
 import Data.Text (Text)
 import qualified Data.Text as Text
@ -27,6 +30,7 @@ import qualified Data.Text.Lazy.Builder as Builder
 import Data.Text.Lazy.Builder.Int (decimal)
 import Data.Text.Lazy.Builder.RealFloat (realFloat)
 import qualified Language.GraphQL.AST.Document as Full
+import qualified Language.GraphQL.AST.DirectiveLocation as DirectiveLocation

 -- | Instructs the encoder whether the GraphQL document should be minified or
 --   pretty printed.
@ -34,7 +38,7 @@ import qualified Language.GraphQL.AST.Document as Full
 --   Use 'pretty' or 'minified' to construct the formatter.
 data Formatter
    = Minified
-    | Pretty Word
+    | Pretty !Word

 -- | Constructs a formatter for pretty printing.
 pretty :: Formatter
@ -53,7 +57,248 @@ document formatter defs
    encodeDocument = foldr executableDefinition [] defs
    executableDefinition (Full.ExecutableDefinition executableDefinition') acc =
        definition formatter executableDefinition' : acc
-    executableDefinition _ acc = acc
+    executableDefinition (Full.TypeSystemDefinition typeSystemDefinition' _location) acc =
+        typeSystemDefinition formatter typeSystemDefinition' : acc
+    executableDefinition (Full.TypeSystemExtension typeSystemExtension' _location) acc =
+        typeSystemExtension formatter typeSystemExtension' : acc
+
+directiveLocation :: DirectiveLocation.DirectiveLocation -> Lazy.Text
+directiveLocation = Lazy.Text.pack . show
+
+withLineBreak :: Formatter -> Lazy.Text.Text -> Lazy.Text.Text
+withLineBreak formatter encodeDefinition
+    | Pretty _ <- formatter = Lazy.Text.snoc encodeDefinition '\n'
+    | Minified <- formatter = encodeDefinition
+
+typeSystemExtension :: Formatter -> Full.TypeSystemExtension -> Lazy.Text
+typeSystemExtension formatter = \case
+    Full.SchemaExtension schemaExtension' ->
+        schemaExtension formatter schemaExtension'
+    Full.TypeExtension typeExtension' -> typeExtension formatter typeExtension'
+
+schemaExtension :: Formatter -> Full.SchemaExtension -> Lazy.Text
+schemaExtension formatter = \case
+    Full.SchemaOperationExtension operationDirectives operationTypeDefinitions' ->
+        withLineBreak formatter
+            $ "extend schema "
+            <> optempty (directives formatter) operationDirectives
+            <> bracesList formatter (operationTypeDefinition formatter) (NonEmpty.toList operationTypeDefinitions')
+    Full.SchemaDirectivesExtension operationDirectives -> "extend schema "
+        <> optempty (directives formatter) (NonEmpty.toList operationDirectives)
+
+typeExtension :: Formatter -> Full.TypeExtension -> Lazy.Text
+typeExtension formatter = \case
+    Full.ScalarTypeExtension name' directives'
+        -> "extend scalar "
+        <> Lazy.Text.fromStrict name'
+        <> directives formatter (NonEmpty.toList directives')
+    Full.ObjectTypeFieldsDefinitionExtension name' ifaces' directives' fields'
+        -> "extend type "
+        <> Lazy.Text.fromStrict name'
+        <> optempty (" " <>) (implementsInterfaces ifaces')
+        <> optempty (directives formatter) directives'
+        <> eitherFormat formatter " " ""
+        <> bracesList formatter (fieldDefinition nextFormatter) (NonEmpty.toList fields')
+    Full.ObjectTypeDirectivesExtension name' ifaces' directives'
+        -> "extend type "
+        <> Lazy.Text.fromStrict name'
+        <> optempty (" " <>) (implementsInterfaces ifaces')
+        <> optempty (directives formatter) (NonEmpty.toList directives')
+    Full.ObjectTypeImplementsInterfacesExtension name' ifaces'
+        -> "extend type "
+        <> Lazy.Text.fromStrict name'
+        <> optempty (" " <>) (implementsInterfaces ifaces')
+    Full.InterfaceTypeFieldsDefinitionExtension name' directives' fields'
+        -> "extend interface "
+        <> Lazy.Text.fromStrict name'
+        <> optempty (directives formatter) directives'
+        <> eitherFormat formatter " " ""
+        <> bracesList formatter (fieldDefinition nextFormatter) (NonEmpty.toList fields')
+    Full.InterfaceTypeDirectivesExtension name' directives'
+        -> "extend interface "
+        <> Lazy.Text.fromStrict name'
+        <> optempty (directives formatter) (NonEmpty.toList directives')
+    Full.UnionTypeUnionMemberTypesExtension name' directives' members'
+        -> "extend union "
+        <> Lazy.Text.fromStrict name'
+        <> optempty (directives formatter) directives'
+        <> eitherFormat formatter " " ""
+        <> unionMemberTypes formatter members'
+    Full.UnionTypeDirectivesExtension name' directives'
+        -> "extend union "
+        <> Lazy.Text.fromStrict name'
+        <> optempty (directives formatter) (NonEmpty.toList directives')
+    Full.EnumTypeEnumValuesDefinitionExtension name' directives' members'
+        -> "extend enum "
+        <> Lazy.Text.fromStrict name'
+        <> optempty (directives formatter) directives'
+        <> eitherFormat formatter " " ""
+        <> bracesList formatter (enumValueDefinition formatter) (NonEmpty.toList members')
+    Full.EnumTypeDirectivesExtension name' directives'
+        -> "extend enum "
+        <> Lazy.Text.fromStrict name'
+        <> optempty (directives formatter) (NonEmpty.toList directives')
+    Full.InputObjectTypeInputFieldsDefinitionExtension name' directives' fields'
+        -> "extend input "
+        <> Lazy.Text.fromStrict name'
+        <> optempty (directives formatter) directives'
+        <> eitherFormat formatter " " ""
+        <> bracesList formatter (inputValueDefinition nextFormatter) (NonEmpty.toList fields')
+    Full.InputObjectTypeDirectivesExtension name' directives'
+        -> "extend input "
+        <> Lazy.Text.fromStrict name'
+        <> optempty (directives formatter) (NonEmpty.toList directives')
+  where
+    nextFormatter = incrementIndent formatter
+
+-- | Converts a t'Full.TypeSystemDefinition' into a string.
+typeSystemDefinition :: Formatter -> Full.TypeSystemDefinition -> Lazy.Text
+typeSystemDefinition formatter = \case
+    Full.SchemaDefinition operationDirectives operationTypeDefinitions' ->
+        withLineBreak formatter
+            $ "schema "
+            <> optempty (directives formatter) operationDirectives
+            <> bracesList formatter (operationTypeDefinition formatter) (NonEmpty.toList operationTypeDefinitions')
+    Full.TypeDefinition typeDefinition' -> typeDefinition formatter typeDefinition'
+    Full.DirectiveDefinition description' name' arguments' repeatable locations
+        -> description formatter description'
+        <> "@"
+        <> Lazy.Text.fromStrict name'
+        <> argumentsDefinition formatter arguments'
+        <> (if repeatable then " repeatable" else mempty)
+        <> " on"
+        <> pipeList formatter (directiveLocation <$> locations)
+
+operationTypeDefinition :: Formatter -> Full.OperationTypeDefinition -> Lazy.Text.Text
+operationTypeDefinition formatter (Full.OperationTypeDefinition operationType' namedType')
+    = indentLine (incrementIndent formatter)
+    <> operationType formatter operationType'
+    <> colon formatter
+    <> Lazy.Text.fromStrict namedType'
+
+fieldDefinition :: Formatter -> Full.FieldDefinition -> Lazy.Text.Text
+fieldDefinition formatter fieldDefinition' =
+    let Full.FieldDefinition description' name' arguments' type'' directives' = fieldDefinition'
+     in optempty (description formatter) description'
+            <> indentLine formatter
+            <> Lazy.Text.fromStrict name'
+            <> argumentsDefinition formatter arguments'
+            <> colon formatter
+            <> type' type''
+            <> optempty (directives formatter) directives'
+
+argumentsDefinition :: Formatter -> Full.ArgumentsDefinition -> Lazy.Text.Text
+argumentsDefinition formatter (Full.ArgumentsDefinition arguments') =
+    parensCommas formatter (argumentDefinition formatter) arguments'
+
+argumentDefinition :: Formatter -> Full.InputValueDefinition -> Lazy.Text.Text
+argumentDefinition formatter definition' =
+    let Full.InputValueDefinition description' name' type'' defaultValue' directives' = definition'
+     in optempty (description formatter) description'
+            <> Lazy.Text.fromStrict name'
+            <> colon formatter
+            <> type' type''
+            <> maybe mempty (defaultValue formatter . Full.node) defaultValue'
+            <> directives formatter directives'
+
+inputValueDefinition :: Formatter -> Full.InputValueDefinition -> Lazy.Text.Text
+inputValueDefinition formatter definition' =
+    let Full.InputValueDefinition description' name' type'' defaultValue' directives' = definition'
+     in optempty (description formatter) description'
+            <> indentLine formatter
+            <> Lazy.Text.fromStrict name'
+            <> colon formatter
+            <> type' type''
+            <> maybe mempty (defaultValue formatter . Full.node) defaultValue'
+            <> directives formatter directives'
+
+typeDefinition :: Formatter -> Full.TypeDefinition -> Lazy.Text
+typeDefinition formatter = \case
+    Full.ScalarTypeDefinition description' name' directives'
+        -> optempty (description formatter) description'
+        <> "scalar "
+        <> Lazy.Text.fromStrict name'
+        <> optempty (directives formatter) directives'
+    Full.ObjectTypeDefinition description' name' ifaces' directives' fields'
+        -> optempty (description formatter) description'
+        <> "type "
+        <> Lazy.Text.fromStrict name'
+        <> optempty (" " <>) (implementsInterfaces ifaces')
+        <> optempty (directives formatter) directives'
+        <> eitherFormat formatter " " ""
+        <> bracesList formatter (fieldDefinition nextFormatter) fields'
+    Full.InterfaceTypeDefinition description' name' ifaces' directives' fields'
+        -> optempty (description formatter) description'
+        <> "interface "
+        <> Lazy.Text.fromStrict name'
+        <> optempty (" " <>) (implementsInterfaces ifaces')
+        <> optempty (directives formatter) directives'
+        <> eitherFormat formatter " " ""
+        <> bracesList formatter (fieldDefinition nextFormatter) fields'
+    Full.UnionTypeDefinition description' name' directives' members'
+        -> optempty (description formatter) description'
+        <> "union "
+        <> Lazy.Text.fromStrict name'
+        <> optempty (directives formatter) directives'
+        <> eitherFormat formatter " " ""
+        <> unionMemberTypes formatter members'
+    Full.EnumTypeDefinition description' name' directives' members'
+        -> optempty (description formatter) description'
+        <> "enum "
+        <> Lazy.Text.fromStrict name'
+        <> optempty (directives formatter) directives'
+        <> eitherFormat formatter " " ""
+        <> bracesList formatter (enumValueDefinition formatter) members'
+    Full.InputObjectTypeDefinition description' name' directives' fields'
+        -> optempty (description formatter) description'
+        <> "input "
+        <> Lazy.Text.fromStrict name'
+        <> optempty (directives formatter) directives'
+        <> eitherFormat formatter " " ""
+        <> bracesList formatter (inputValueDefinition nextFormatter) fields'
+  where
+    nextFormatter = incrementIndent formatter
+
+implementsInterfaces :: Foldable t => Full.ImplementsInterfaces t -> Lazy.Text
+implementsInterfaces (Full.ImplementsInterfaces interfaces)
+    | null interfaces = mempty
+    | otherwise = Lazy.Text.fromStrict
+        $ Text.append "implements "
+        $ Text.intercalate " & "
+        $ toList interfaces
+
+unionMemberTypes :: Foldable t => Formatter -> Full.UnionMemberTypes t -> Lazy.Text
+unionMemberTypes formatter (Full.UnionMemberTypes memberTypes)
+    | null memberTypes = mempty
+    | otherwise = Lazy.Text.append "="
+        $ pipeList formatter
+        $ Lazy.Text.fromStrict
+        <$> toList memberTypes
+
+pipeList :: Foldable t => Formatter -> t Lazy.Text -> Lazy.Text
+pipeList Minified =  (" " <>) . Lazy.Text.intercalate " | " . toList
+pipeList (Pretty _) =  Lazy.Text.concat
+    . fmap (("\n" <> indentSymbol <> "| ") <>)
+    . toList
+
+enumValueDefinition :: Formatter -> Full.EnumValueDefinition -> Lazy.Text
+enumValueDefinition (Pretty _) enumValue =
+    let Full.EnumValueDefinition description' name' directives' = enumValue
+        formatter = Pretty 1
+     in description formatter description'
+        <> indentLine formatter
+        <> Lazy.Text.fromStrict name'
+        <> directives formatter directives'
+enumValueDefinition Minified enumValue =
+    let Full.EnumValueDefinition description' name' directives' = enumValue
+     in description Minified description'
+        <> Lazy.Text.fromStrict name'
+        <> directives Minified directives'
+
+description :: Formatter -> Full.Description -> Lazy.Text.Text
+description _formatter (Full.Description Nothing) = ""
+description formatter (Full.Description (Just description')) =
+    stringValue formatter description'

 -- | Converts a t'Full.ExecutableDefinition' into a string.
 definition :: Formatter -> Full.ExecutableDefinition -> Lazy.Text
@ -99,9 +344,9 @@ variableDefinition formatter variableDefinition' =
    let Full.VariableDefinition variableName variableType defaultValue' _ =
            variableDefinition'
     in variable variableName
-    <> eitherFormat formatter ": " ":"
+    <> colon formatter
    <> type' variableType
-    <> maybe mempty (defaultValue formatter) (Full.node <$> defaultValue')
+    <> maybe mempty (defaultValue formatter . Full.node) defaultValue'

 defaultValue :: Formatter -> Full.ConstValue -> Lazy.Text
 defaultValue formatter val
@ -126,20 +371,26 @@ indent :: (Integral a) => a -> Lazy.Text
 indent indentation = Lazy.Text.replicate (fromIntegral indentation) indentSymbol

 selection :: Formatter -> Full.Selection -> Lazy.Text
-selection formatter = Lazy.Text.append indent' . encodeSelection
+selection formatter = Lazy.Text.append (indentLine formatter')
+    . encodeSelection
  where
    encodeSelection (Full.FieldSelection fieldSelection) =
-        field incrementIndent fieldSelection
+        field formatter' fieldSelection
    encodeSelection (Full.InlineFragmentSelection fragmentSelection) =
-        inlineFragment incrementIndent fragmentSelection
+        inlineFragment formatter' fragmentSelection
    encodeSelection (Full.FragmentSpreadSelection fragmentSelection) =
-        fragmentSpread incrementIndent fragmentSelection
-    incrementIndent
-        | Pretty indentation <- formatter = Pretty $ indentation + 1
-        | otherwise = Minified
-    indent'
-        | Pretty indentation <- formatter = indent $ indentation + 1
-        | otherwise = ""
+        fragmentSpread formatter' fragmentSelection
+    formatter' = incrementIndent formatter
+
+indentLine :: Formatter -> Lazy.Text
+indentLine formatter
+    | Pretty indentation <- formatter = indent indentation
+    | otherwise = ""
+
+incrementIndent :: Formatter -> Formatter
+incrementIndent formatter
+    | Pretty indentation <- formatter = Pretty $ indentation + 1
+    | otherwise = Minified

 colon :: Formatter -> Lazy.Text
 colon formatter = eitherFormat formatter ": " ":"
@ -197,8 +448,10 @@ directive formatter (Full.Directive name args _)
    = "@" <> Lazy.Text.fromStrict name <> optempty (arguments formatter) args

 directives :: Formatter -> [Full.Directive] -> Lazy.Text
-directives Minified = spaces (directive Minified)
-directives formatter = Lazy.Text.cons ' ' . spaces (directive formatter)
+directives Minified values = spaces (directive Minified) values
+directives formatter values
+    | null values = ""
+    | otherwise = Lazy.Text.cons ' ' $ spaces (directive formatter) values

 -- | Converts a 'Full.Value' into a string.
 value :: Formatter -> Full.Value -> Lazy.Text
@ -219,7 +472,7 @@ fromConstValue (Full.ConstBoolean  x) = Full.Boolean x
 fromConstValue Full.ConstNull = Full.Null
 fromConstValue (Full.ConstString string) = Full.String string
 fromConstValue (Full.ConstEnum x) = Full.Enum x
-fromConstValue (Full.ConstList x) = Full.List $ fromConstValue <$> x
+fromConstValue (Full.ConstList x) = Full.List $ fmap fromConstValue <$> x
 fromConstValue (Full.ConstObject x) = Full.Object $ fromConstObjectField <$> x
  where
    fromConstObjectField Full.ObjectField{value = value', ..} =
@ -266,8 +519,8 @@ stringValue (Pretty indentation) string =
            = Builder.fromLazyText (indent (indentation + 1))
            <> line' <> newline <> acc

-listValue :: Formatter -> [Full.Value] -> Lazy.Text
-listValue formatter = bracketsCommas formatter $ value formatter
+listValue :: Formatter -> [Full.Node Full.Value] -> Lazy.Text
+listValue formatter = bracketsCommas formatter $ value formatter . Full.node

 objectValue :: Formatter -> [Full.ObjectField Full.Value] -> Lazy.Text
 objectValue formatter = intercalate $ objectField formatter
@ -294,6 +547,12 @@ nonNullType :: Full.NonNullType -> Lazy.Text
 nonNullType (Full.NonNullTypeNamed x) = Lazy.Text.fromStrict x <> "!"
 nonNullType (Full.NonNullTypeList x) = listType x <> "!"

+-- | Produces lowercase operation type: query, mutation or subscription.
+operationType :: Formatter -> Full.OperationType -> Lazy.Text
+operationType _formatter Full.Query = "query"
+operationType _formatter Full.Mutation = "mutation"
+operationType _formatter Full.Subscription = "subscription"
+
 -- * Internal

 between :: Char -> Char -> Lazy.Text -> Lazy.Text
--- a/src/Language/GraphQL/AST/Lexer.hs
+++ b/src/Language/GraphQL/AST/Lexer.hs
@ -1,5 +1,7 @@
 {-# LANGUAGE ExplicitForAll #-}
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE NoGeneralizedNewtypeDeriving #-}
+{-# LANGUAGE Safe #-}

 -- | This module defines a bunch of small parsers used to parse individual
 --   lexemes.
@ -29,7 +31,8 @@ module Language.GraphQL.AST.Lexer
    , unicodeBOM
    ) where

-import Control.Applicative (Alternative(..), liftA2)
+import Control.Applicative (Alternative(..))
+import qualified Control.Applicative.Combinators.NonEmpty as NonEmpty
 import Data.Char (chr, digitToInt, isAsciiLower, isAsciiUpper, ord)
 import Data.Foldable (foldl')
 import Data.List (dropWhileEnd)
@ -37,27 +40,28 @@ import qualified Data.List.NonEmpty as NonEmpty
 import Data.List.NonEmpty (NonEmpty(..))
 import Data.Proxy (Proxy(..))
 import Data.Void (Void)
-import Text.Megaparsec ( Parsec
-                       , (<?>)
-                       , between
-                       , chunk
-                       , chunkToTokens
-                       , notFollowedBy
-                       , oneOf
-                       , option
-                       , optional
-                       , satisfy
-                       , sepBy
-                       , skipSome
-                       , takeP
-                       , takeWhile1P
-                       , try
-                       )
+import Text.Megaparsec
+    ( Parsec
+    , (<?>)
+    , between
+    , chunk
+    , chunkToTokens
+    , notFollowedBy
+    , oneOf
+    , option
+    , optional
+    , satisfy
+    , skipSome
+    , takeP
+    , takeWhile1P
+    , try
+    )
 import Text.Megaparsec.Char (char, digitChar, space1)
 import qualified Text.Megaparsec.Char.Lexer as Lexer
 import Data.Text (Text)
 import qualified Data.Text as T
 import qualified Data.Text.Lazy as TL
+import Control.Monad (void)

 -- | Standard parser.
 -- Accepts the type of the parsed token.
@ -93,7 +97,7 @@ dollar = symbol "$"

 -- | Parser for "@".
 at :: Parser ()
-at = symbol "@" >> pure ()
+at = void $ symbol "@"

 -- | Parser for "&".
 amp :: Parser T.Text
@ -101,7 +105,7 @@ amp = symbol "&"

 -- | Parser for ":".
 colon :: Parser ()
-colon = symbol ":" >> pure ()
+colon = void $ symbol ":"

 -- | Parser for "=".
 equals :: Parser T.Text
@ -141,12 +145,13 @@ blockString :: Parser T.Text
 blockString = between "\"\"\"" "\"\"\"" stringValue <* spaceConsumer
  where
    stringValue = do
-        byLine <- sepBy (many blockStringCharacter) lineTerminator
-        let indentSize = foldr countIndent 0 $ tail byLine
-            withoutIndent = head byLine : (removeIndent indentSize <$> tail byLine)
+        byLine <- NonEmpty.sepBy1 (many blockStringCharacter) lineTerminator
+        let indentSize = foldr countIndent 0 $ NonEmpty.tail byLine
+            withoutIndent = NonEmpty.head byLine
+                : (removeIndent indentSize <$> NonEmpty.tail byLine)
            withoutEmptyLines = liftA2 (.) dropWhile dropWhileEnd removeEmptyLine withoutIndent

-        return $ T.intercalate "\n" $ T.concat <$> withoutEmptyLines
+        pure $ T.intercalate "\n" $ T.concat <$> withoutEmptyLines
    removeEmptyLine [] = True
    removeEmptyLine [x] = T.null x || isWhiteSpace (T.head x)
    removeEmptyLine _ = False
@ -179,10 +184,10 @@ name :: Parser T.Text
 name = do
    firstLetter <- nameFirstLetter
    rest <- many $ nameFirstLetter <|> digitChar
-    _ <- spaceConsumer
-    return $ TL.toStrict $ TL.cons firstLetter $ TL.pack rest
-      where
-        nameFirstLetter = satisfy isAsciiUpper <|> satisfy isAsciiLower <|> char '_'
+    void spaceConsumer
+    pure $ TL.toStrict $ TL.cons firstLetter $ TL.pack rest
+  where
+    nameFirstLetter = satisfy isAsciiUpper <|> satisfy isAsciiLower <|> char '_'

 isChunkDelimiter :: Char -> Bool
 isChunkDelimiter = flip notElem ['"', '\\', '\n', '\r']
@ -196,31 +201,31 @@ lineTerminator = chunk "\r\n" <|> chunk "\n" <|> chunk "\r"
 isSourceCharacter :: Char -> Bool
 isSourceCharacter = isSourceCharacter' . ord
  where
-    isSourceCharacter' code = code >= 0x0020
-                           || code == 0x0009
-                           || code == 0x000a
-                           || code == 0x000d
+    isSourceCharacter' code
+        = code >= 0x0020
+        || elem code [0x0009, 0x000a, 0x000d]

 escapeSequence :: Parser Char
 escapeSequence = do
-    _ <- char '\\'
+    void $ char '\\'
    escaped <- oneOf ['"', '\\', '/', 'b', 'f', 'n', 'r', 't', 'u']
    case escaped of
-        'b' -> return '\b'
-        'f' -> return '\f'
-        'n' -> return '\n'
-        'r' -> return '\r'
-        't' -> return '\t'
-        'u' -> chr . foldl' step 0
-                   . chunkToTokens (Proxy :: Proxy T.Text)
-                 <$> takeP Nothing 4
-        _ -> return escaped
+        'b' -> pure '\b'
+        'f' -> pure '\f'
+        'n' -> pure '\n'
+        'r' -> pure '\r'
+        't' -> pure '\t'
+        'u' -> chr
+            . foldl' step 0
+            . chunkToTokens (Proxy :: Proxy T.Text)
+            <$> takeP Nothing 4
+        _ -> pure escaped
  where
    step accumulator = (accumulator * 16 +) . digitToInt

 -- | Parser for the "Byte Order Mark".
 unicodeBOM :: Parser ()
-unicodeBOM = optional (char '\xfeff') >> pure ()
+unicodeBOM = void $ optional $ char '\xfeff'

 -- | Parses "extend" followed by a 'symbol'. It is used by schema extensions.
 extend :: forall a. Text -> String -> NonEmpty (Parser a) -> Parser a
--- a/src/Language/GraphQL/AST/Parser.hs
+++ b/src/Language/GraphQL/AST/Parser.hs
@ -2,23 +2,21 @@
 {-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE RecordWildCards #-}
+{-# LANGUAGE NoGeneralizedNewtypeDeriving #-}
+{-# LANGUAGE Safe #-}

 -- | @GraphQL@ document parser.
 module Language.GraphQL.AST.Parser
    ( document
    ) where

-import Control.Applicative (Alternative(..), liftA2, optional)
+import Control.Applicative (Alternative(..), optional)
 import Control.Applicative.Combinators (sepBy1)
 import qualified Control.Applicative.Combinators.NonEmpty as NonEmpty
 import Data.List.NonEmpty (NonEmpty(..))
 import Data.Text (Text)
 import qualified Language.GraphQL.AST.DirectiveLocation as Directive
-import Language.GraphQL.AST.DirectiveLocation
-    ( DirectiveLocation
-    , ExecutableDirectiveLocation
-    , TypeSystemDirectiveLocation
-    )
+import Language.GraphQL.AST.DirectiveLocation (DirectiveLocation)
 import qualified Language.GraphQL.AST.Document as Full
 import Language.GraphQL.AST.Lexer
 import Text.Megaparsec
@ -31,6 +29,7 @@ import Text.Megaparsec
    , unPos
    , (<?>)
    )
+import Data.Maybe (isJust)

 -- | Parser for the GraphQL documents.
 document :: Parser Full.Document
@ -86,6 +85,7 @@ directiveDefinition description' = Full.DirectiveDefinition description'
    <* at
    <*> name
    <*> argumentsDefinition
+    <*> (isJust <$> optional (symbol "repeatable"))
    <* symbol "on"
    <*> directiveLocations
    <?> "DirectiveDefinition"
@ -96,34 +96,28 @@ directiveLocations = optional pipe
    <?> "DirectiveLocations"

 directiveLocation :: Parser DirectiveLocation
-directiveLocation
-    = Directive.ExecutableDirectiveLocation <$> executableDirectiveLocation
-    <|> Directive.TypeSystemDirectiveLocation <$> typeSystemDirectiveLocation
+directiveLocation = e (Directive.Query <$ symbol "QUERY")
+    <|> e (Directive.Mutation <$ symbol "MUTATION")
+    <|> e (Directive.Subscription <$ symbol "SUBSCRIPTION")
+    <|> t (Directive.FieldDefinition <$ symbol "FIELD_DEFINITION")
+    <|> e (Directive.Field <$ symbol "FIELD")
+    <|> e (Directive.FragmentDefinition <$ "FRAGMENT_DEFINITION")
+    <|> e (Directive.FragmentSpread <$ "FRAGMENT_SPREAD")
+    <|> e (Directive.InlineFragment <$ "INLINE_FRAGMENT")
+    <|> t (Directive.Schema <$ symbol "SCHEMA")
+    <|> t (Directive.Scalar <$ symbol "SCALAR")
+    <|> t (Directive.Object <$ symbol "OBJECT")
+    <|> t (Directive.ArgumentDefinition <$ symbol "ARGUMENT_DEFINITION")
+    <|> t (Directive.Interface <$ symbol "INTERFACE")
+    <|> t (Directive.Union <$ symbol "UNION")
+    <|> t (Directive.EnumValue <$ symbol "ENUM_VALUE")
+    <|> t (Directive.Enum <$ symbol "ENUM")
+    <|> t (Directive.InputObject <$ symbol "INPUT_OBJECT")
+    <|> t (Directive.InputFieldDefinition <$ symbol "INPUT_FIELD_DEFINITION")
    <?> "DirectiveLocation"
-
-executableDirectiveLocation :: Parser ExecutableDirectiveLocation
-executableDirectiveLocation = Directive.Query <$ symbol "QUERY"
-    <|> Directive.Mutation <$ symbol "MUTATION"
-    <|> Directive.Subscription <$ symbol "SUBSCRIPTION"
-    <|> Directive.Field <$ symbol "FIELD"
-    <|> Directive.FragmentDefinition <$ "FRAGMENT_DEFINITION"
-    <|> Directive.FragmentSpread <$ "FRAGMENT_SPREAD"
-    <|> Directive.InlineFragment <$ "INLINE_FRAGMENT"
-    <?> "ExecutableDirectiveLocation"
-
-typeSystemDirectiveLocation :: Parser TypeSystemDirectiveLocation
-typeSystemDirectiveLocation = Directive.Schema <$ symbol "SCHEMA"
-    <|> Directive.Scalar <$ symbol "SCALAR"
-    <|> Directive.Object <$ symbol "OBJECT"
-    <|> Directive.FieldDefinition <$ symbol "FIELD_DEFINITION"
-    <|> Directive.ArgumentDefinition <$ symbol "ARGUMENT_DEFINITION"
-    <|> Directive.Interface <$ symbol "INTERFACE"
-    <|> Directive.Union <$ symbol "UNION"
-    <|> Directive.Enum <$ symbol "ENUM"
-    <|> Directive.EnumValue <$ symbol "ENUM_VALUE"
-    <|> Directive.InputObject <$ symbol "INPUT_OBJECT"
-    <|> Directive.InputFieldDefinition <$ symbol "INPUT_FIELD_DEFINITION"
-    <?> "TypeSystemDirectiveLocation"
+  where
+    e = fmap Directive.ExecutableDirectiveLocation
+    t = fmap Directive.TypeSystemDirectiveLocation

 typeDefinition :: Full.Description -> Parser Full.TypeDefinition
 typeDefinition description' = scalarTypeDefinition description'
@ -222,6 +216,7 @@ interfaceTypeDefinition :: Full.Description -> Parser Full.TypeDefinition
 interfaceTypeDefinition description' = Full.InterfaceTypeDefinition description'
    <$ symbol "interface"
    <*> name
+    <*> option (Full.ImplementsInterfaces []) (implementsInterfaces sepBy1)
    <*> directives
    <*> braces (many fieldDefinition)
    <?> "InterfaceTypeDefinition"
@ -460,8 +455,8 @@ value = Full.Variable <$> variable
    <|> Full.Null <$  nullValue
    <|> Full.String <$> stringValue
    <|> Full.Enum <$> try enumValue
-    <|> Full.List <$> brackets (some value)
-    <|> Full.Object <$> braces (some $ objectField $ valueNode value)
+    <|> Full.List <$> brackets (many $ valueNode value)
+    <|> Full.Object <$> braces (many $ objectField $ valueNode value)
    <?> "Value"

 constValue :: Parser Full.ConstValue
@ -471,8 +466,8 @@ constValue = Full.ConstFloat <$> try float
    <|> Full.ConstNull <$ nullValue
    <|> Full.ConstString <$> stringValue
    <|> Full.ConstEnum <$> try enumValue
-    <|> Full.ConstList <$> brackets (some constValue)
-    <|> Full.ConstObject <$> braces (some $ objectField $ valueNode constValue)
+    <|> Full.ConstList <$> brackets (many $ valueNode constValue)
+    <|> Full.ConstObject <$> braces (many $ objectField $ valueNode constValue)
    <?> "Value"

 booleanValue :: Parser Bool
--- a/src/Language/GraphQL/Error.hs
+++ b/src/Language/GraphQL/Error.hs
@ -1,35 +1,29 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 {-# LANGUAGE DuplicateRecordFields #-}
 {-# LANGUAGE ExistentialQuantification #-}
-{-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE RecordWildCards #-}

 -- | Error handling.
 module Language.GraphQL.Error
-    ( CollectErrsT
-    , Error(..)
+    ( Error(..)
    , Path(..)
-    , Resolution(..)
    , ResolverException(..)
    , Response(..)
    , ResponseEventStream
-    , addErr
-    , addErrMsg
    , parseError
-    , runCollectErrs
-    , singleError
    ) where

 import Conduit
 import Control.Exception (Exception(..))
-import Control.Monad.Trans.State (StateT, modify, runStateT)
-import Data.HashMap.Strict (HashMap)
 import Data.Sequence (Seq(..), (|>))
 import qualified Data.Sequence as Seq
 import Data.Text (Text)
 import qualified Data.Text as Text
-import Language.GraphQL.AST (Location(..), Name)
+import Language.GraphQL.AST (Location(..))
 import Language.GraphQL.Execute.Coerce
-import qualified Language.GraphQL.Type.Schema as Schema
 import Prelude hiding (null)
 import Text.Megaparsec
    ( ParseErrorBundle(..)
@ -41,12 +35,6 @@ import Text.Megaparsec
    , unPos
    )

-- | Executor context.
-data Resolution m = Resolution
-    { errors :: Seq Error
-    , types :: HashMap Name (Schema.Type m)
-    }
-
 -- | Wraps a parse error into a list of errors.
 parseError :: (Applicative f, Serialize a)
    => ParseErrorBundle Text Void
@ -66,28 +54,6 @@ parseError ParseErrorBundle{..}  =
            sourcePosition = pstateSourcePos newState
         in (result |> errorObject x sourcePosition, newState)

-- | A wrapper to pass error messages around.
-type CollectErrsT m = StateT (Resolution m) m
-
-- | Adds an error to the list of errors.
-addErr :: Monad m => Error -> CollectErrsT m ()
-addErr v = modify appender
-  where
-    appender :: Monad m => Resolution m -> Resolution m
-    appender resolution@Resolution{..} = resolution{ errors = errors |> v }
-
-makeErrorMessage :: Text -> Error
-makeErrorMessage s = Error s [] []
-
-- | Constructs a response object containing only the error with the given
-- message.
-singleError :: Serialize a => Text -> Response a
-singleError message = Response null $ Seq.singleton $ makeErrorMessage message
-
-- | Convenience function for just wrapping an error message.
-addErrMsg :: (Monad m, Serialize a) => Text -> CollectErrsT m a
-addErrMsg errorMessage = (addErr . makeErrorMessage) errorMessage >> pure null
-
 -- | If an error can be associated to a particular field in the GraphQL result,
 -- it must contain an entry with the key path that details the path of the
 -- response field which experienced the error. This allows clients to identify
@ -125,14 +91,3 @@ instance Show ResolverException where
    show (ResolverException e) = show e

 instance Exception ResolverException
-
-- | Runs the given query computation, but collects the errors into an error
-- list, which is then sent back with the data.
-runCollectErrs :: (Monad m, Serialize a)
-    => HashMap Name (Schema.Type m)
-    -> CollectErrsT m a
-    -> m (Response a)
-runCollectErrs types' res = do
-    (dat, Resolution{..}) <- runStateT res
-        $ Resolution{ errors = Seq.empty, types = types' }
-    pure $ Response dat errors
--- a/src/Language/GraphQL/Execute.hs
+++ b/src/Language/GraphQL/Execute.hs
@ -1,4 +1,13 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE ExistentialQuantification #-}
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE RecordWildCards #-}
+{-# LANGUAGE ViewPatterns #-}
+{-# LANGUAGE TypeApplications #-}

 -- | This module provides functions to execute a @GraphQL@ request.
 module Language.GraphQL.Execute
@ -6,19 +15,223 @@ module Language.GraphQL.Execute
    , module Language.GraphQL.Execute.Coerce
    ) where

-import Control.Monad.Catch (MonadCatch)
+import Conduit (mapMC, (.|))
+import Control.Arrow (left)
+import Control.Monad.Catch
+     ( Exception(..)
+     , Handler(..)
+     , MonadCatch(..)
+     , MonadThrow(..)
+     , SomeException(..)
+     , catches
+     )
+import Control.Monad.Trans.Class (MonadTrans(..))
+import Control.Monad.Trans.Reader (ReaderT(..), ask, runReaderT)
+import Control.Monad.Trans.Writer (WriterT(..), runWriterT)
+import qualified Control.Monad.Trans.Writer as Writer
+import Control.Monad (foldM)
+import qualified Language.GraphQL.AST.Document as Full
+import Data.Foldable (find)
 import Data.HashMap.Strict (HashMap)
-import Data.Sequence (Seq(..))
+import qualified Data.HashMap.Strict as HashMap
+import Data.List.NonEmpty (NonEmpty(..))
+import qualified Data.List.NonEmpty as NonEmpty
+import Data.Maybe (fromMaybe)
+import Data.Sequence (Seq)
+import qualified Data.Sequence as Seq
+import qualified Data.Vector as Vector
 import Data.Text (Text)
-import Language.GraphQL.AST.Document (Document, Name)
+import qualified Data.Text as Text
+import Data.Typeable (cast)
+import GHC.Records (HasField(..))
 import Language.GraphQL.Execute.Coerce
-import Language.GraphQL.Execute.Execution
+import Language.GraphQL.Execute.OrderedMap (OrderedMap)
+import qualified Language.GraphQL.Execute.OrderedMap as OrderedMap
 import qualified Language.GraphQL.Execute.Transform as Transform
-import qualified Language.GraphQL.Execute.Subscribe as Subscribe
-import Language.GraphQL.Error
-import qualified Language.GraphQL.Type.Definition as Definition
+import qualified Language.GraphQL.Type.In as In
 import qualified Language.GraphQL.Type.Out as Out
-import Language.GraphQL.Type.Schema
+import qualified Language.GraphQL.Type as Type
+import qualified Language.GraphQL.Type.Internal as Type.Internal
+import Language.GraphQL.Type.Schema (Schema, Type)
+import qualified Language.GraphQL.Type.Schema as Schema
+import Language.GraphQL.Error
+    ( Error(..)
+    , Response(..)
+    , Path(..)
+    , ResolverException(..)
+    , ResponseEventStream
+    )
+import Prelude hiding (null)
+import Language.GraphQL.AST.Document (showVariableName)
+
+newtype ExecutorT m a = ExecutorT
+    { runExecutorT :: ReaderT (HashMap Full.Name (Type m)) (WriterT (Seq Error) m) a
+    }
+
+instance Functor m => Functor (ExecutorT m) where
+    fmap f = ExecutorT . fmap f . runExecutorT
+
+instance Applicative m => Applicative (ExecutorT m) where
+    pure = ExecutorT . pure
+    ExecutorT f <*> ExecutorT x = ExecutorT $ f <*> x
+
+instance Monad m => Monad (ExecutorT m) where
+    ExecutorT x >>= f = ExecutorT $ x >>= runExecutorT . f
+
+instance MonadTrans ExecutorT where
+    lift = ExecutorT . lift . lift
+
+instance MonadThrow m => MonadThrow (ExecutorT m) where
+    throwM = lift . throwM
+
+instance MonadCatch m => MonadCatch (ExecutorT m) where
+  catch (ExecutorT stack) handler =
+      ExecutorT $ catch stack $ runExecutorT . handler
+
+data GraphQLException = forall e. Exception e => GraphQLException e
+
+instance Show GraphQLException where
+    show (GraphQLException e) = show e
+
+instance Exception GraphQLException
+
+graphQLExceptionToException :: Exception e => e -> SomeException
+graphQLExceptionToException = toException . GraphQLException
+
+graphQLExceptionFromException :: Exception e => SomeException -> Maybe e
+graphQLExceptionFromException e = do
+    GraphQLException graphqlException <- fromException e
+    cast graphqlException
+
+data ResultException = forall e. Exception e => ResultException e
+
+instance Show ResultException where
+    show (ResultException e) = show e
+
+instance Exception ResultException where
+    toException = graphQLExceptionToException
+    fromException = graphQLExceptionFromException
+
+resultExceptionToException :: Exception e => e -> SomeException
+resultExceptionToException = toException . ResultException
+
+resultExceptionFromException :: Exception e => SomeException -> Maybe e
+resultExceptionFromException e = do
+    ResultException resultException <- fromException e
+    cast resultException
+
+data FieldException = forall e. Exception e => FieldException Full.Location [Path] e
+
+instance Show FieldException where
+    show (FieldException _ _ e) = displayException e
+
+instance Exception FieldException where
+    toException = graphQLExceptionToException
+    fromException = graphQLExceptionFromException
+
+data ValueCompletionException = ValueCompletionException String Type.Value
+
+instance Show ValueCompletionException where
+    show (ValueCompletionException typeRepresentation found) = concat
+        [ "Value completion error. Expected type "
+        , typeRepresentation
+        , ", found: "
+        , show found
+        , "."
+        ]
+
+instance Exception ValueCompletionException where
+    toException = resultExceptionToException
+    fromException = resultExceptionFromException
+
+data InputCoercionException =
+    InputCoercionException String In.Type (Maybe (Full.Node Transform.Input))
+
+instance Show InputCoercionException where
+    show (InputCoercionException argumentName argumentType Nothing) = concat
+        [ "Required argument \""
+        , argumentName
+        , "\" of type "
+        , show argumentType
+        , " not specified."
+        ]
+    show (InputCoercionException argumentName argumentType (Just givenValue)) = concat
+        [ "Argument \""
+        , argumentName
+        , "\" has invalid type. Expected type "
+        , show argumentType
+        , ", found: "
+        , show givenValue
+        , "."
+        ]
+
+instance Exception InputCoercionException where
+    toException = graphQLExceptionToException
+    fromException = graphQLExceptionFromException
+
+newtype ResultCoercionException = ResultCoercionException String
+
+instance Show ResultCoercionException where
+    show (ResultCoercionException typeRepresentation) = concat
+        [ "Unable to coerce result to "
+        , typeRepresentation
+        , "."
+        ]
+
+instance Exception ResultCoercionException where
+    toException = resultExceptionToException
+    fromException = resultExceptionFromException
+
+-- | Query error types.
+data QueryError
+    = OperationNameRequired
+    | OperationNotFound String
+    | CoercionError Full.VariableDefinition
+    | UnknownInputType Full.VariableDefinition
+
+type ExecuteHandler m a e = e -> ExecutorT m a
+
+tell :: Monad m => Seq Error -> ExecutorT m ()
+tell = ExecutorT . lift . Writer.tell
+
+operationNameErrorText :: Text
+operationNameErrorText = Text.unlines
+    [ "Named operations must be provided with the name of the desired operation."
+    , "See https://spec.graphql.org/June2018/#sec-Language.Document description."
+    ]
+
+queryError :: QueryError -> Error
+queryError OperationNameRequired =
+    let queryErrorMessage = "Operation name is required. " <> operationNameErrorText
+    in Error{ message = queryErrorMessage, locations = [], path = [] }
+queryError (OperationNotFound operationName) =
+    let queryErrorMessage = Text.unlines
+            [ Text.concat
+              [ "Operation \""
+              , Text.pack operationName
+              , "\" is not found in the named operations you've provided. "
+              ]
+            , operationNameErrorText
+            ]
+     in Error{ message = queryErrorMessage, locations = [], path = [] }
+queryError (CoercionError variableDefinition) =
+    let (Full.VariableDefinition _ _ _ location) = variableDefinition
+        queryErrorMessage = Text.concat
+            [ "Failed to coerce the variable "
+            , Text.pack $ Full.showVariable variableDefinition
+            , "."
+            ]
+     in Error{ message = queryErrorMessage, locations = [location], path = [] }
+queryError (UnknownInputType variableDefinition) =
+    let Full.VariableDefinition _ variableTypeName _ location = variableDefinition
+        queryErrorMessage = Text.concat
+            [ "Variable "
+            , Text.pack $ showVariableName variableDefinition
+            , " has unknown type "
+            , Text.pack $ show variableTypeName
+            , "."
+            ]
+     in Error{ message = queryErrorMessage, locations = [location], path = [] }

 -- | The substitution is applied to the document, and the resolvers are applied
 -- to the resulting fields. The operation name can be used if the document
@ -29,35 +242,483 @@ import Language.GraphQL.Type.Schema
 execute :: (MonadCatch m, VariableValue a, Serialize b)
    => Schema m -- ^ Resolvers.
    -> Maybe Text -- ^ Operation name.
-    -> HashMap Name a -- ^ Variable substitution function.
-    -> Document -- @GraphQL@ document.
+    -> HashMap Full.Name a -- ^ Variable substitution function.
+    -> Full.Document -- @GraphQL@ document.
    -> m (Either (ResponseEventStream m b) (Response b))
-execute schema' operationName subs document =
-    case Transform.document schema' operationName subs document of
-        Left queryError -> pure
-            $ Right
-            $ singleError
-            $ Transform.queryError queryError
-        Right transformed -> executeRequest transformed
+execute schema' operationName subs document' =
+    executeRequest schema' document' (Text.unpack <$> operationName) subs

-executeRequest :: (MonadCatch m, Serialize a)
-    => Transform.Document m
+executeRequest :: (MonadCatch m, Serialize a, VariableValue b)
+    => Schema m
+    -> Full.Document
+    -> Maybe String
+    -> HashMap Full.Name b
    -> m (Either (ResponseEventStream m a) (Response a))
-executeRequest (Transform.Document types' rootObjectType operation)
-    | (Transform.Query _ fields) <- operation =
-        Right <$> executeOperation types' rootObjectType fields
-    | (Transform.Mutation _ fields) <- operation =
-        Right <$> executeOperation types' rootObjectType fields
-    | (Transform.Subscription _ fields) <- operation
-        = either (Right . singleError) Left
-        <$> Subscribe.subscribe types' rootObjectType fields
+executeRequest schema sourceDocument operationName variableValues = do
+    operationAndVariables <- sequence buildOperation
+    case operationAndVariables of
+        Left queryError' -> pure
+            $ Right
+            $ Response null $ pure $ queryError queryError'
+        Right operation
+            | Transform.Operation Full.Query topSelections _operationLocation <- operation ->
+                 Right <$> executeQuery topSelections schema
+            | Transform.Operation Full.Mutation topSelections operationLocation <- operation ->
+                Right <$> executeMutation topSelections schema operationLocation
+            | Transform.Operation Full.Subscription topSelections operationLocation <- operation ->
+                either rightErrorResponse Left <$> subscribe topSelections schema operationLocation
+  where
+    schemaTypes = Schema.types schema
+    (operationDefinitions, fragmentDefinitions') =
+        Transform.document sourceDocument
+    buildOperation = do
+        operationDefinition <- getOperation operationDefinitions operationName
+        coercedVariableValues <- coerceVariableValues
+            schemaTypes
+            operationDefinition
+            variableValues
+        let replacement = Transform.Replacement
+                { variableValues = coercedVariableValues
+                , fragmentDefinitions = fragmentDefinitions'
+                , visitedFragments = mempty
+                , types = schemaTypes
+                }
+        pure $ flip runReaderT replacement
+            $ Transform.runTransformT
+            $ Transform.transform operationDefinition

-- This is actually executeMutation, but we don't distinguish between queries
-- and mutations yet.
-executeOperation :: (MonadCatch m, Serialize a)
-    => HashMap Name (Type m)
+rightErrorResponse :: Serialize b => forall a. Error -> Either a (Response b)
+rightErrorResponse = Right . Response null . pure
+
+getOperation :: [Full.OperationDefinition] -> Maybe String -> Either QueryError Full.OperationDefinition
+getOperation [operation] Nothing = Right operation
+getOperation operations (Just givenOperationName)
+    = maybe (Left $ OperationNotFound givenOperationName) Right
+    $ find findOperationByName operations
+  where
+    findOperationByName (Full.OperationDefinition _ (Just operationName) _ _ _ _) =
+        givenOperationName == Text.unpack operationName
+    findOperationByName _ = False
+getOperation _ _ = Left OperationNameRequired
+
+executeQuery :: (MonadCatch m, Serialize a)
+    => Seq (Transform.Selection m)
+    -> Schema m
+    -> m (Response a)
+executeQuery topSelections schema = do
+    let queryType = Schema.query schema
+    (data', errors) <- runWriterT
+        $ flip runReaderT (Schema.types schema)
+        $ runExecutorT
+        $ catch (executeSelectionSet topSelections queryType Type.Null [])
+        handleException
+    pure $ Response data' errors
+
+handleException :: (MonadCatch m, Serialize a)
+    => ExecuteHandler m a FieldException
+handleException (FieldException fieldLocation errorPath next) =
+    let newError = constructError next fieldLocation errorPath
+     in tell (Seq.singleton newError) >> pure null
+
+constructError :: Exception e => e -> Full.Location -> [Path] -> Error
+constructError e fieldLocation errorPath = Error
+    { message = Text.pack (displayException e)
+    , path = reverse errorPath
+    , locations = [fieldLocation]
+    }
+
+executeMutation :: (MonadCatch m, Serialize a)
+    => Seq (Transform.Selection m)
+    -> Schema m
+    -> Full.Location
+    -> m (Response a)
+executeMutation topSelections schema operationLocation
+    | Just mutationType <- Schema.mutation schema = do
+        (data', errors) <- runWriterT
+            $ flip runReaderT (Schema.types schema)
+            $ runExecutorT
+            $ catch (executeSelectionSet topSelections mutationType Type.Null [])
+            handleException
+        pure $ Response data' errors
+    | otherwise = pure
+        $ Response null
+        $ Seq.singleton
+        $ Error "Schema doesn't support mutations." [operationLocation] []
+
+executeSelectionSet :: (MonadCatch m, Serialize a)
+    => Seq (Transform.Selection m)
+    -> Out.ObjectType m
+    -> Type.Value
+    -> [Path]
+    -> ExecutorT m a
+executeSelectionSet selections objectType objectValue errorPath = do
+    let groupedFieldSet = collectFields objectType selections
+    resolvedValues <- OrderedMap.traverseMaybe go groupedFieldSet
+    coerceResult (Out.NonNullObjectType objectType) $ Object resolvedValues
+  where
+    executeField' fields resolver =
+        executeField objectValue fields resolver errorPath
+    Out.ObjectType _ _ _ resolvers = objectType
+    go fields@(Transform.Field _ fieldName _ _ _ :| _) =
+        traverse (executeField' fields) $ HashMap.lookup fieldName resolvers
+
+fieldsSegment :: forall m. NonEmpty (Transform.Field m) -> Path
+fieldsSegment (Transform.Field alias fieldName _ _ _ :| _) =
+    Segment (fromMaybe fieldName alias)
+
+viewResolver :: Out.Resolver m -> (Out.Field m, Out.Resolve m)
+viewResolver (Out.ValueResolver resolverField' resolveFunction) =
+    (resolverField', resolveFunction)
+viewResolver (Out.EventStreamResolver resolverField' resolveFunction _) =
+    (resolverField', resolveFunction)
+
+executeField :: forall m a
+    . (MonadCatch m, Serialize a)
+    => Type.Value
+    -> NonEmpty (Transform.Field m)
+    -> Out.Resolver m
+    -> [Path]
+    -> ExecutorT m a
+executeField objectValue fields (viewResolver -> resolverPair) errorPath =
+    let Transform.Field _ fieldName inputArguments _ fieldLocation :| _ = fields
+     in catches (go fieldName inputArguments)
+        [ Handler nullResultHandler
+        , Handler (inputCoercionHandler fieldLocation)
+        , Handler (resultHandler fieldLocation)
+        , Handler (resolverHandler fieldLocation)
+        ]
+  where
+    fieldErrorPath = fieldsSegment fields : errorPath
+    inputCoercionHandler :: (MonadCatch m, Serialize a)
+        => Full.Location
+        -> ExecuteHandler m a InputCoercionException
+    inputCoercionHandler _ e@(InputCoercionException _ _ (Just valueNode)) =
+        let argumentLocation = getField @"location" valueNode
+         in exceptionHandler argumentLocation e
+    inputCoercionHandler fieldLocation e = exceptionHandler fieldLocation e
+    resultHandler :: (MonadCatch m, Serialize a)
+        => Full.Location
+        -> ExecuteHandler m a ResultException
+    resultHandler = exceptionHandler
+    resolverHandler :: (MonadCatch m, Serialize a)
+        => Full.Location
+        -> ExecuteHandler m a ResolverException
+    resolverHandler = exceptionHandler
+    nullResultHandler :: (MonadCatch m, Serialize a) => ExecuteHandler m a FieldException
+    nullResultHandler e@(FieldException fieldLocation errorPath' next) =
+        let newError = constructError next fieldLocation errorPath'
+         in if Out.isNonNullType fieldType
+             then throwM e
+             else returnError newError
+    exceptionHandler :: (Exception e, MonadCatch m, Serialize a)
+        => Full.Location
+        -> ExecuteHandler m a e
+    exceptionHandler errorLocation e =
+        let newError = constructError e errorLocation fieldErrorPath
+         in if Out.isNonNullType fieldType
+             then throwM $ FieldException errorLocation fieldErrorPath e
+             else returnError newError
+    returnError newError = tell (Seq.singleton newError) >> pure null
+    go fieldName inputArguments = do
+        argumentValues <- coerceArgumentValues argumentTypes inputArguments
+        resolvedValue <-
+           resolveFieldValue resolveFunction objectValue fieldName argumentValues
+        completeValue fieldType fields fieldErrorPath resolvedValue
+    (resolverField, resolveFunction) = resolverPair
+    Out.Field _ fieldType argumentTypes = resolverField
+
+resolveFieldValue :: MonadCatch m
+    => Out.Resolve m
+    -> Type.Value
+    -> Full.Name
+    -> Type.Subs
+    -> ExecutorT m Type.Value
+resolveFieldValue resolver objectValue _fieldName argumentValues =
+    lift $ runReaderT resolver context
+  where
+    context = Type.Context
+        { Type.arguments = Type.Arguments argumentValues
+        , Type.values = objectValue
+        }
+
+resolveAbstractType :: Monad m
+    => Type.Internal.AbstractType m
+    -> Type.Subs
+    -> ExecutorT m (Maybe (Out.ObjectType m))
+resolveAbstractType abstractType values'
+    | Just (Type.String typeName) <- HashMap.lookup "__typename" values' = do
+        types' <- ExecutorT ask
+        case HashMap.lookup typeName types' of
+            Just (Type.Internal.ObjectType objectType) ->
+                if Type.Internal.instanceOf objectType abstractType
+                    then pure $ Just objectType
+                    else pure Nothing
+            _ -> pure Nothing
+    | otherwise = pure Nothing
+
+-- https://spec.graphql.org/October2021/#sec-Value-Completion
+completeValue :: (MonadCatch m, Serialize a)
+    => Out.Type m
+    -> NonEmpty (Transform.Field m)
+    -> [Path]
+    -> Type.Value
+    -> ExecutorT m a
+completeValue (Out.isNonNullType -> False) _ _ Type.Null =
+    pure null
+completeValue outputType@(Out.ListBaseType listType) fields errorPath (Type.List list)
+    = foldM go Vector.empty list >>= coerceResult outputType . List . Vector.toList
+  where
+    go accumulator listItem =
+        let updatedPath = Index (Vector.length accumulator) : errorPath
+         in Vector.snoc accumulator
+            <$> completeValue listType fields updatedPath listItem
+completeValue outputType@(Out.ScalarBaseType _) _ _ (Type.Int int) =
+    coerceResult outputType $ Int int
+completeValue outputType@(Out.ScalarBaseType _) _ _ (Type.Boolean boolean) =
+    coerceResult outputType $ Boolean boolean
+completeValue outputType@(Out.ScalarBaseType _) _ _ (Type.Float float) =
+    coerceResult outputType $ Float float
+completeValue outputType@(Out.ScalarBaseType _) _ _ (Type.String string) =
+    coerceResult outputType $ String string
+completeValue outputType@(Out.EnumBaseType enumType) _ _ (Type.Enum enum) =
+    let Type.EnumType _ _ enumMembers = enumType
+     in if HashMap.member enum enumMembers
+        then coerceResult outputType $ Enum enum
+        else throwM
+            $ ValueCompletionException (show outputType)
+            $ Type.Enum enum
+completeValue (Out.ObjectBaseType objectType) fields errorPath result
+    = executeSelectionSet (mergeSelectionSets fields) objectType result errorPath
+completeValue outputType@(Out.InterfaceBaseType interfaceType) fields errorPath result
+    | Type.Object objectMap <- result = do
+        let abstractType = Type.Internal.AbstractInterfaceType interfaceType
+        concreteType <- resolveAbstractType abstractType objectMap
+        case concreteType of
+            Just objectType
+                -> executeSelectionSet (mergeSelectionSets fields) objectType result
+                $ fieldsSegment fields : errorPath
+            Nothing -> throwM
+                $ ValueCompletionException (show outputType) result
+completeValue outputType@(Out.UnionBaseType unionType) fields errorPath result
+    | Type.Object objectMap <- result = do
+        let abstractType = Type.Internal.AbstractUnionType unionType
+        concreteType <- resolveAbstractType abstractType objectMap
+        case concreteType of
+            Just objectType
+                -> executeSelectionSet (mergeSelectionSets fields) objectType result
+                $ fieldsSegment fields : errorPath
+            Nothing -> throwM
+                $ ValueCompletionException (show outputType) result
+completeValue outputType _ _ result =
+    throwM $ ValueCompletionException (show outputType) result
+
+coerceResult :: (MonadCatch m, Serialize a)
+    => Out.Type m
+    -> Output a
+    -> ExecutorT m a
+coerceResult outputType result
+    | Just serialized <- serialize outputType result = pure serialized
+    | otherwise = throwM $ ResultCoercionException $ show outputType
+
+mergeSelectionSets :: MonadCatch m
+    => NonEmpty (Transform.Field m)
+    -> Seq (Transform.Selection m)
+mergeSelectionSets = foldr forEach mempty
+  where
+    forEach (Transform.Field _ _ _ fieldSelectionSet _) selectionSet' =
+        selectionSet' <> fieldSelectionSet
+
+coerceArgumentValues :: MonadCatch m
+    => HashMap Full.Name In.Argument
+    -> HashMap Full.Name (Full.Node Transform.Input)
+    -> m Type.Subs
+coerceArgumentValues argumentDefinitions argumentValues =
+    HashMap.foldrWithKey c pure argumentDefinitions mempty
+  where
+    c argumentName argumentType pure' resultMap =
+        forEach argumentName argumentType resultMap >>= pure'
+    forEach :: MonadCatch m
+         => Full.Name
+         -> In.Argument
+         -> Type.Subs
+         -> m Type.Subs
+    forEach argumentName (In.Argument _ variableType defaultValue) resultMap = do
+        let matchedMap
+                = matchFieldValues' argumentName variableType defaultValue
+                $ Just resultMap
+         in case matchedMap of
+            Just matchedValues -> pure matchedValues
+            Nothing
+                | Just inputValue <- HashMap.lookup argumentName argumentValues
+                    -> throwM
+                    $ InputCoercionException (Text.unpack argumentName) variableType
+                    $ Just inputValue
+                | otherwise -> throwM
+                    $ InputCoercionException (Text.unpack argumentName) variableType Nothing
+
+    matchFieldValues' = matchFieldValues coerceArgumentValue
+        $ Full.node <$> argumentValues
+
+    coerceArgumentValue inputType transform =
+      coerceInputLiteral inputType $ extractArgumentValue transform
+
+    extractArgumentValue (Transform.Int integer) = Type.Int integer
+    extractArgumentValue (Transform.Boolean boolean) = Type.Boolean boolean
+    extractArgumentValue (Transform.String string) = Type.String string
+    extractArgumentValue (Transform.Float float) = Type.Float float
+    extractArgumentValue (Transform.Enum enum) = Type.Enum enum
+    extractArgumentValue Transform.Null = Type.Null
+    extractArgumentValue (Transform.List list) =
+      Type.List $ extractArgumentValue <$> list
+    extractArgumentValue (Transform.Object object) =
+      Type.Object $ extractArgumentValue <$> object
+    extractArgumentValue (Transform.Variable variable) = variable
+
+collectFields :: Monad m
+    => Out.ObjectType m
+    -> Seq (Transform.Selection m)
+    -> OrderedMap (NonEmpty (Transform.Field m))
+collectFields objectType = foldl forEach OrderedMap.empty
+  where
+    forEach groupedFields (Transform.FieldSelection fieldSelection) =
+        let Transform.Field maybeAlias fieldName _ _ _ = fieldSelection
+            responseKey = fromMaybe fieldName maybeAlias
+         in OrderedMap.insert responseKey (fieldSelection :| []) groupedFields
+    forEach groupedFields (Transform.FragmentSelection selectionFragment)
+        | Transform.Fragment fragmentType fragmentSelectionSet _ <- selectionFragment
+        , Type.Internal.doesFragmentTypeApply fragmentType objectType =
+            let fragmentGroupedFieldSet =
+                    collectFields objectType fragmentSelectionSet
+             in groupedFields <> fragmentGroupedFieldSet
+        | otherwise = groupedFields
+
+coerceVariableValues :: (Monad m, VariableValue b)
+    => HashMap Full.Name (Schema.Type m)
+    -> Full.OperationDefinition
+    -> HashMap Full.Name b
+    -> Either QueryError Type.Subs
+coerceVariableValues types operationDefinition' variableValues
+    | Full.OperationDefinition _ _ variableDefinitions _ _ _ <-
+        operationDefinition'
+    = foldr forEach (Right HashMap.empty) variableDefinitions
+    | otherwise = pure mempty
+  where
+    forEach variableDefinition (Right coercedValues) =
+        let Full.VariableDefinition variableName variableTypeName defaultValue _ =
+                variableDefinition
+            defaultValue' = constValue . Full.node <$> defaultValue
+         in case Type.Internal.lookupInputType variableTypeName types of
+            Just variableType ->
+                maybe (Left $ CoercionError variableDefinition) Right
+                    $ matchFieldValues
+                        coerceVariableValue'
+                        variableValues
+                        variableName
+                        variableType
+                        defaultValue'
+                    $ Just coercedValues
+            Nothing -> Left $ UnknownInputType variableDefinition
+    forEach _ coercedValuesOrError = coercedValuesOrError
+    coerceVariableValue' variableType value'
+        = coerceVariableValue variableType value'
+        >>= coerceInputLiteral variableType
+
+constValue :: Full.ConstValue -> Type.Value
+constValue (Full.ConstInt i) = Type.Int i
+constValue (Full.ConstFloat f) = Type.Float f
+constValue (Full.ConstString x) = Type.String x
+constValue (Full.ConstBoolean b) = Type.Boolean b
+constValue Full.ConstNull = Type.Null
+constValue (Full.ConstEnum e) = Type.Enum e
+constValue (Full.ConstList list) = Type.List $ constValue . Full.node <$> list
+constValue (Full.ConstObject o) =
+    Type.Object $ HashMap.fromList $ constObjectField <$> o
+  where
+    constObjectField Full.ObjectField{value = value', ..} =
+        (name, constValue $ Full.node value')
+
+subscribe :: (MonadCatch m, Serialize a)
+    => Seq (Transform.Selection m)
+    -> Schema m
+    -> Full.Location
+    -> m (Either Error (ResponseEventStream m a))
+subscribe fields schema objectLocation
+    | Just objectType <- Schema.subscription schema = do
+        let types' = Schema.types schema
+        sourceStream <-
+            createSourceEventStream types' objectType objectLocation fields
+        let traverser =
+                mapSourceToResponseEvent types' objectType fields
+        traverse traverser sourceStream
+    | otherwise = pure $ Left
+        $ Error "Schema doesn't support subscriptions." [] []
+
+mapSourceToResponseEvent :: (MonadCatch m, Serialize a)
+    => HashMap Full.Name (Type m)
    -> Out.ObjectType m
    -> Seq (Transform.Selection m)
+    -> Out.SourceEventStream m
+    -> m (ResponseEventStream m a)
+mapSourceToResponseEvent types' subscriptionType fields sourceStream
+    = pure
+    $ sourceStream
+    .| mapMC (executeSubscriptionEvent types' subscriptionType fields)
+
+createSourceEventStream :: MonadCatch m
+    => HashMap Full.Name (Type m)
+    -> Out.ObjectType m
+    -> Full.Location
+    -> Seq (Transform.Selection m)
+    -> m (Either Error (Out.SourceEventStream m))
+createSourceEventStream _types subscriptionType objectLocation fields
+    | [fieldGroup] <- OrderedMap.elems groupedFieldSet
+    , Transform.Field _ fieldName arguments' _ errorLocation <-
+        NonEmpty.head fieldGroup
+    , Out.ObjectType _ _ _ fieldTypes <- subscriptionType
+    , resolverT <- fieldTypes HashMap.! fieldName
+    , Out.EventStreamResolver fieldDefinition _ resolver <- resolverT
+    , Out.Field _ _fieldType argumentDefinitions <- fieldDefinition =
+        case coerceArgumentValues argumentDefinitions arguments' of
+            Left _ -> pure
+                $ Left
+                $ Error "Argument coercion failed." [errorLocation] []
+            Right  argumentValues -> left (singleError [errorLocation])
+                <$> resolveFieldEventStream Type.Null argumentValues resolver
+    | otherwise = pure
+        $ Left
+        $ Error "Subscription contains more than one field." [objectLocation] []
+  where
+    groupedFieldSet = collectFields subscriptionType fields
+    singleError :: [Full.Location] -> String -> Error
+    singleError errorLocations message = Error (Text.pack message) errorLocations []
+
+resolveFieldEventStream :: MonadCatch m
+    => Type.Value
+    -> Type.Subs
+    -> Out.Subscribe m
+    -> m (Either String (Out.SourceEventStream m))
+resolveFieldEventStream result args resolver =
+    catch (Right <$> runReaderT resolver context) handleEventStreamError
+  where
+    handleEventStreamError :: MonadCatch m
+        => ResolverException
+        -> m (Either String (Out.SourceEventStream m))
+    handleEventStreamError = pure . Left . displayException
+    context = Type.Context
+        { Type.arguments = Type.Arguments args
+        , Type.values = result
+        }
+
+executeSubscriptionEvent :: (MonadCatch m, Serialize a)
+    => HashMap Full.Name (Type m)
+    -> Out.ObjectType m
+    -> Seq (Transform.Selection m)
+    -> Type.Value
    -> m (Response a)
-executeOperation types' objectType fields =
-    runCollectErrs types' $ executeSelectionSet Definition.Null objectType fields
+executeSubscriptionEvent types' objectType fields initialValue = do
+    (data', errors) <- runWriterT
+        $ flip runReaderT types'
+        $ runExecutorT
+        $ catch (executeSelectionSet fields objectType initialValue [])
+        handleException
+    pure $ Response data' errors
--- a/src/Language/GraphQL/Execute/Coerce.hs
+++ b/src/Language/GraphQL/Execute/Coerce.hs
@ -15,18 +15,17 @@ module Language.GraphQL.Execute.Coerce
    , matchFieldValues
    ) where

-import qualified Data.Aeson as Aeson
 import Data.Int (Int32)
 import Data.HashMap.Strict (HashMap)
 import qualified Data.HashMap.Strict as HashMap
-import Data.Map.Strict (Map)
 import Data.String (IsString(..))
 import Data.Text (Text)
 import qualified Data.Text.Lazy as Text.Lazy
 import qualified Data.Text.Lazy.Builder as Text.Builder
 import qualified Data.Text.Lazy.Builder.Int as Text.Builder
-import Data.Scientific (toBoundedInteger, toRealFloat)
 import Language.GraphQL.AST (Name)
+import Language.GraphQL.Execute.OrderedMap (OrderedMap)
+import qualified Language.GraphQL.Execute.OrderedMap as OrderedMap
 import qualified Language.GraphQL.Type as Type
 import qualified Language.GraphQL.Type.In as In
 import qualified Language.GraphQL.Type.Out as Out
@ -60,20 +59,13 @@ class VariableValue a where
        -> a -- ^ Variable value being coerced.
        -> Maybe Type.Value -- ^ Coerced value on success, 'Nothing' otherwise.

-instance VariableValue Aeson.Value where
-    coerceVariableValue _ Aeson.Null = Just Type.Null
-    coerceVariableValue (In.ScalarBaseType scalarType) value
-        | (Aeson.String stringValue) <- value = Just $ Type.String stringValue
-        | (Aeson.Bool booleanValue) <- value = Just $ Type.Boolean booleanValue
-        | (Aeson.Number numberValue) <- value
-        , (Type.ScalarType "Float" _) <- scalarType =
-            Just $ Type.Float $ toRealFloat numberValue
-        | (Aeson.Number numberValue) <- value = -- ID or Int
-            Type.Int <$> toBoundedInteger numberValue
-    coerceVariableValue (In.EnumBaseType _) (Aeson.String stringValue) =
+instance VariableValue Type.Value where
+    coerceVariableValue _ Type.Null = Just Type.Null
+    coerceVariableValue (In.ScalarBaseType _) value = Just value
+    coerceVariableValue (In.EnumBaseType _) (Type.Enum stringValue) =
        Just $ Type.Enum stringValue
    coerceVariableValue (In.InputObjectBaseType objectType) value
-        | (Aeson.Object objectValue) <- value = do
+        | (Type.Object objectValue) <- value = do
            let (In.InputObjectType _ _ inputFields) = objectType
            (newObjectValue, resultMap) <- foldWithKey objectValue inputFields
            if HashMap.null newObjectValue
@ -93,14 +85,9 @@ instance VariableValue Aeson.Value where
                        pure (newObjectValue, insert coerced)
                    Nothing -> Just (objectValue, resultMap)
    coerceVariableValue (In.ListBaseType listType) value
-        | (Aeson.Array arrayValue) <- value =
-            Type.List <$> foldr foldVector (Just []) arrayValue
+        | (Type.List arrayValue) <- value =
+            Type.List <$> traverse (coerceVariableValue listType) arrayValue
        | otherwise = coerceVariableValue listType value
-      where
-        foldVector _ Nothing = Nothing
-        foldVector variableValue (Just list) = do
-            coerced <- coerceVariableValue listType variableValue
-            pure $ coerced : list 
    coerceVariableValue _ _ = Nothing

 -- | Looks up a value by name in the given map, coerces it and inserts into the
@ -160,7 +147,7 @@ coerceInputLiteral (In.EnumBaseType type') (Type.Enum enumValue)
    | member enumValue type' = Just $ Type.Enum enumValue
  where
    member value (Type.EnumType _ _ members) = HashMap.member value members
-coerceInputLiteral (In.InputObjectBaseType type') (Type.Object values) = 
+coerceInputLiteral (In.InputObjectBaseType type') (Type.Object values) =
    let (In.InputObjectType _ _ inputFields) = type'
     in Type.Object
            <$> HashMap.foldrWithKey (matchFieldValues' values) (Just HashMap.empty) inputFields
@ -209,26 +196,29 @@ data Output a
    | Boolean Bool
    | Enum Name
    | List [a]
-    | Object (Map Name a)
+    | Object (OrderedMap a)
    deriving (Eq, Show)

 instance forall a. IsString (Output a) where
    fromString = String . fromString

-instance Serialize Aeson.Value where
+instance Serialize Type.Value where
+    null = Type.Null
    serialize (Out.ScalarBaseType scalarType) value
        | Type.ScalarType "Int" _ <- scalarType
-        , Int int <- value = Just $ Aeson.toJSON int
+        , Int int <- value = Just $ Type.Int int
        | Type.ScalarType "Float" _ <- scalarType
-        , Float float <- value = Just $ Aeson.toJSON float
+        , Float float <- value = Just $ Type.Float float
        | Type.ScalarType "String" _ <- scalarType
-        , String string <- value = Just $ Aeson.String string
+        , String string <- value = Just $ Type.String string
        | Type.ScalarType "ID" _ <- scalarType
-        , String string <- value = Just $ Aeson.String string
+        , String string <- value = Just $ Type.String string
        | Type.ScalarType "Boolean" _ <- scalarType
-        , Boolean boolean <- value = Just $ Aeson.Bool boolean
-    serialize _ (Enum enum) = Just $ Aeson.String enum
-    serialize _ (List list) = Just $ Aeson.toJSON list
-    serialize _ (Object object) = Just $ Aeson.toJSON object
+        , Boolean boolean <- value = Just $ Type.Boolean boolean
+    serialize _ (Enum enum) = Just $ Type.Enum enum
+    serialize _ (List list) = Just $ Type.List list
+    serialize _ (Object object) = Just
+        $ Type.Object
+        $ HashMap.fromList
+        $ OrderedMap.toList object
    serialize _ _ = Nothing
-    null = Aeson.Null
--- a/src/Language/GraphQL/Execute/Execution.hs
+++ b/src/Language/GraphQL/Execute/Execution.hs
@ -1,216 +0,0 @@
-{-# LANGUAGE ExplicitForAll #-}
-{-# LANGUAGE OverloadedStrings #-}
-{-# LANGUAGE ViewPatterns #-}
-
-module Language.GraphQL.Execute.Execution
-    ( coerceArgumentValues
-    , collectFields
-    , executeSelectionSet
-    ) where
-
-import Control.Monad.Catch (Exception(..), MonadCatch(..))
-import Control.Monad.Trans.Class (lift)
-import Control.Monad.Trans.Reader (runReaderT)
-import Control.Monad.Trans.State (gets)
-import Data.List.NonEmpty (NonEmpty(..))
-import Data.Map.Strict (Map)
-import Data.HashMap.Strict (HashMap)
-import qualified Data.HashMap.Strict as HashMap
-import qualified Data.Map.Strict as Map
-import Data.Maybe (fromMaybe)
-import Data.Sequence (Seq(..))
-import qualified Data.Text as Text
-import Language.GraphQL.AST (Name)
-import Language.GraphQL.Error
-import Language.GraphQL.Execute.Coerce
-import qualified Language.GraphQL.Execute.Transform as Transform
-import qualified Language.GraphQL.Type as Type
-import qualified Language.GraphQL.Type.In as In
-import qualified Language.GraphQL.Type.Out as Out
-import qualified Language.GraphQL.Type.Internal as Internal
-import Prelude hiding (null)
-
-resolveFieldValue :: MonadCatch m
-    => Type.Value
-    -> Type.Subs
-    -> Type.Resolve m
-    -> CollectErrsT m Type.Value
-resolveFieldValue result args resolver =
-    catch (lift $ runReaderT resolver context) handleFieldError
-  where
-    handleFieldError :: MonadCatch m
-        => ResolverException
-        -> CollectErrsT m Type.Value
-    handleFieldError e =
-        addErr (Error (Text.pack $ displayException e) [] []) >> pure Type.Null
-    context = Type.Context
-        { Type.arguments = Type.Arguments args
-        , Type.values = result
-        }
-
-collectFields :: Monad m
-    => Out.ObjectType m
-    -> Seq (Transform.Selection m)
-    -> Map Name (NonEmpty (Transform.Field m))
-collectFields objectType = foldl forEach Map.empty
-  where
-    forEach groupedFields (Transform.SelectionField field) =
-        let responseKey = aliasOrName field
-         in Map.insertWith (<>) responseKey (field :| []) groupedFields
-    forEach groupedFields (Transform.SelectionFragment selectionFragment)
-        | Transform.Fragment fragmentType fragmentSelectionSet <- selectionFragment
-        , Internal.doesFragmentTypeApply fragmentType objectType =
-            let fragmentGroupedFieldSet = collectFields objectType fragmentSelectionSet
-             in Map.unionWith (<>) groupedFields fragmentGroupedFieldSet
-        | otherwise = groupedFields
-
-aliasOrName :: forall m. Transform.Field m -> Name
-aliasOrName (Transform.Field alias name _ _) = fromMaybe name alias
-
-resolveAbstractType :: Monad m
-    => Internal.AbstractType m
-    -> Type.Subs
-    -> CollectErrsT m (Maybe (Out.ObjectType m))
-resolveAbstractType abstractType values'
-    | Just (Type.String typeName) <- HashMap.lookup "__typename" values' = do
-        types' <- gets types
-        case HashMap.lookup typeName types' of
-            Just (Internal.ObjectType objectType) ->
-                if Internal.instanceOf objectType abstractType
-                    then pure $ Just objectType
-                    else pure Nothing
-            _ -> pure Nothing
-    | otherwise = pure Nothing
-
-executeField :: (MonadCatch m, Serialize a)
-    => Out.Resolver m
-    -> Type.Value
-    -> NonEmpty (Transform.Field m)
-    -> CollectErrsT m a
-executeField fieldResolver prev fields
-    | Out.ValueResolver fieldDefinition resolver <- fieldResolver =
-        executeField' fieldDefinition resolver
-    | Out.EventStreamResolver fieldDefinition resolver _ <- fieldResolver =
-        executeField' fieldDefinition resolver
-  where
-    executeField' fieldDefinition resolver = do
-        let Out.Field _ fieldType argumentDefinitions = fieldDefinition
-        let (Transform.Field _ _ arguments' _ :| []) = fields
-        case coerceArgumentValues argumentDefinitions arguments' of
-            Nothing -> addErrMsg "Argument coercing failed."
-            Just argumentValues -> do
-                answer <- resolveFieldValue prev argumentValues resolver
-                completeValue fieldType fields answer
-
-completeValue :: (MonadCatch m, Serialize a)
-    => Out.Type m
-    -> NonEmpty (Transform.Field m)
-    -> Type.Value
-    -> CollectErrsT m a
-completeValue (Out.isNonNullType -> False) _ Type.Null = pure null
-completeValue outputType@(Out.ListBaseType listType) fields (Type.List list)
-    = traverse (completeValue listType fields) list
-    >>= coerceResult outputType . List
-completeValue outputType@(Out.ScalarBaseType _) _ (Type.Int int) =
-    coerceResult outputType $ Int int
-completeValue outputType@(Out.ScalarBaseType _) _ (Type.Boolean boolean) =
-    coerceResult outputType $ Boolean boolean
-completeValue outputType@(Out.ScalarBaseType _) _ (Type.Float float) =
-    coerceResult outputType $ Float float
-completeValue outputType@(Out.ScalarBaseType _) _ (Type.String string) =
-    coerceResult outputType $ String string
-completeValue outputType@(Out.EnumBaseType enumType) _ (Type.Enum enum) =
-    let Type.EnumType _ _ enumMembers = enumType
-     in if HashMap.member enum enumMembers
-        then coerceResult outputType $ Enum enum
-        else addErrMsg "Enum value completion failed."
-completeValue (Out.ObjectBaseType objectType) fields result =
-    executeSelectionSet result objectType $ mergeSelectionSets fields
-completeValue (Out.InterfaceBaseType interfaceType) fields result
-    | Type.Object objectMap <- result = do
-        let abstractType = Internal.AbstractInterfaceType interfaceType
-        concreteType <- resolveAbstractType abstractType objectMap
-        case concreteType of
-            Just objectType -> executeSelectionSet result objectType
-                $ mergeSelectionSets fields
-            Nothing -> addErrMsg "Interface value completion failed."
-completeValue (Out.UnionBaseType unionType) fields result
-    | Type.Object objectMap <- result = do
-        let abstractType = Internal.AbstractUnionType unionType
-        concreteType <- resolveAbstractType abstractType objectMap
-        case concreteType of
-            Just objectType -> executeSelectionSet result objectType
-                $ mergeSelectionSets fields
-            Nothing -> addErrMsg "Union value completion failed."
-completeValue _ _ _ = addErrMsg "Value completion failed."
-
-mergeSelectionSets :: MonadCatch m
-    => NonEmpty (Transform.Field m)
-    -> Seq (Transform.Selection m)
-mergeSelectionSets = foldr forEach mempty
-  where
-    forEach (Transform.Field _ _ _ fieldSelectionSet) selectionSet =
-        selectionSet <> fieldSelectionSet
-
-coerceResult :: (MonadCatch m, Serialize a)
-    => Out.Type m
-    -> Output a
-    -> CollectErrsT m a
-coerceResult outputType result
-    | Just serialized <- serialize outputType result = pure serialized
-    | otherwise = addErrMsg "Result coercion failed."
-
-- | Takes an 'Out.ObjectType' and a list of 'Transform.Selection's and applies
-- each field to each 'Transform.Selection'. Resolves into a value containing
-- the resolved 'Transform.Selection', or a null value and error information.
-executeSelectionSet :: (MonadCatch m, Serialize a)
-    => Type.Value
-    -> Out.ObjectType m
-    -> Seq (Transform.Selection m)
-    -> CollectErrsT m a
-executeSelectionSet result objectType@(Out.ObjectType _ _ _ resolvers) selectionSet = do
-    let fields = collectFields objectType selectionSet
-    resolvedValues <- Map.traverseMaybeWithKey forEach fields
-    coerceResult (Out.NonNullObjectType objectType) $ Object resolvedValues
-  where
-    forEach _ fields@(field :| _) =
-        let Transform.Field _ name _ _ = field
-         in traverse (tryResolver fields) $ lookupResolver name
-    lookupResolver = flip HashMap.lookup resolvers
-    tryResolver fields resolver =
-        executeField resolver result fields >>= lift . pure
-
-coerceArgumentValues
-    :: HashMap Name In.Argument
-    -> HashMap Name Transform.Input
-    -> Maybe Type.Subs
-coerceArgumentValues argumentDefinitions argumentValues =
-    HashMap.foldrWithKey forEach (pure mempty) argumentDefinitions
-  where
-    forEach variableName (In.Argument _ variableType defaultValue) =
-        matchFieldValues coerceArgumentValue argumentValues variableName variableType defaultValue
-    coerceArgumentValue inputType (Transform.Int integer) =
-        coerceInputLiteral inputType (Type.Int integer)
-    coerceArgumentValue inputType (Transform.Boolean boolean) =
-        coerceInputLiteral inputType (Type.Boolean boolean)
-    coerceArgumentValue inputType (Transform.String string) =
-        coerceInputLiteral inputType (Type.String string)
-    coerceArgumentValue inputType (Transform.Float float) =
-        coerceInputLiteral inputType (Type.Float float)
-    coerceArgumentValue inputType (Transform.Enum enum) =
-        coerceInputLiteral inputType (Type.Enum enum)
-    coerceArgumentValue inputType Transform.Null
-        | In.isNonNullType inputType = Nothing
-        | otherwise = coerceInputLiteral inputType Type.Null
-    coerceArgumentValue (In.ListBaseType inputType) (Transform.List list) =
-        let coerceItem = coerceInputLiteral inputType
-         in Type.List <$> traverse coerceItem list
-    coerceArgumentValue (In.InputObjectBaseType inputType) (Transform.Object object)
-        | In.InputObjectType _ _ inputFields <- inputType = 
-            let go = forEachField object
-                resultMap = HashMap.foldrWithKey go (pure mempty) inputFields
-             in Type.Object <$> resultMap
-    coerceArgumentValue _ (Transform.Variable variable) = pure variable
-    coerceArgumentValue _ _ = Nothing
-    forEachField object variableName (In.InputField _ variableType defaultValue) =
-        matchFieldValues coerceArgumentValue object variableName variableType defaultValue
--- a/src/Language/GraphQL/Execute/OrderedMap.hs
+++ b/src/Language/GraphQL/Execute/OrderedMap.hs
@ -0,0 +1,148 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
+{-# LANGUAGE ExplicitForAll #-}
+
+-- | This module contains a map data structure, that preserves insertion order.
+-- Some definitions conflict with functions from prelude, so this module should
+-- probably be imported qualified.
+module Language.GraphQL.Execute.OrderedMap
+    ( OrderedMap
+    , elems
+    , empty
+    , insert
+    , foldlWithKey'
+    , keys
+    , lookup
+    , replace
+    , singleton
+    , size
+    , toList
+    , traverseMaybe
+    ) where
+
+import qualified Data.Foldable as Foldable
+import Data.HashMap.Strict (HashMap, (!))
+import qualified Data.HashMap.Strict as HashMap
+import Data.Text (Text)
+import Data.Vector (Vector)
+import qualified Data.Vector as Vector
+import Prelude hiding (filter, lookup)
+
+-- | This map associates values with the given text keys. Insertion order is
+-- preserved. When inserting a value with a key, that is already available in
+-- the map, the existing value isn't overridden, but combined with the new value
+-- using its 'Semigroup' instance.
+--
+-- Internally this map uses an array with keys to preserve the order and an
+-- unorded map with key-value pairs.
+data OrderedMap v = OrderedMap (Vector Text) (HashMap Text v)
+    deriving (Eq)
+
+instance Functor OrderedMap where
+    fmap f (OrderedMap vector hashMap) = OrderedMap vector $ fmap f hashMap
+
+instance Foldable OrderedMap where
+    foldr f = foldrWithKey $ const f
+    null (OrderedMap vector _) = Vector.null vector
+
+instance Semigroup v => Semigroup (OrderedMap v) where
+    (<>) = foldlWithKey'
+        $ \accumulator key value -> insert key value accumulator
+
+instance Semigroup v => Monoid (OrderedMap v) where
+    mempty = empty
+
+instance Traversable OrderedMap where
+    traverse f (OrderedMap vector hashMap) = OrderedMap vector
+        <$> traverse f hashMap
+
+instance Show v => Show (OrderedMap v) where
+    showsPrec precedence map' = showParen (precedence > 10)
+        $ showString "fromList " . shows (toList map')
+
+-- * Construction
+
+-- | Constructs a map with a single element.
+singleton :: forall v. Text -> v -> OrderedMap v
+singleton key value = OrderedMap (Vector.singleton key)
+    $ HashMap.singleton key value
+
+-- | Constructs an empty map.
+empty :: forall v. OrderedMap v
+empty = OrderedMap mempty mempty
+
+-- * Traversal
+
+-- | Reduces this map by applying a binary operator from right to left to all
+-- elements, using the given starting value.
+foldrWithKey :: forall v a. (Text -> v -> a -> a) -> a -> OrderedMap v -> a
+foldrWithKey f initial (OrderedMap vector hashMap) = foldr go initial vector
+  where
+    go key = f key (hashMap ! key)
+
+-- | Reduces this map by applying a binary operator from left to right to all
+-- elements, using the given starting value.
+foldlWithKey' :: forall v a. (a -> Text -> v -> a) -> a -> OrderedMap v -> a
+foldlWithKey' f initial (OrderedMap vector hashMap) =
+    Vector.foldl' go initial vector
+  where
+    go accumulator key = f accumulator key (hashMap ! key)
+
+-- | Traverse over the elements and collect the 'Just' results.
+traverseMaybe
+    :: Applicative f
+    => forall a
+    . (a -> f (Maybe b))
+    -> OrderedMap a
+    -> f (OrderedMap b)
+traverseMaybe f orderedMap = foldlWithKey' filter empty
+    <$> traverse f orderedMap
+  where
+    filter accumulator key (Just value) = replace key value accumulator
+    filter accumulator _ Nothing = accumulator
+
+-- * Lists
+
+-- | Converts this map to the list of key-value pairs.
+toList :: forall v. OrderedMap v -> [(Text, v)]
+toList = foldrWithKey ((.) (:) . (,)) []
+
+-- | Returns a list with all keys in this map.
+keys :: forall v. OrderedMap v -> [Text]
+keys (OrderedMap vector _) = Foldable.toList vector
+
+-- | Returns a list with all elements in this map.
+elems :: forall v. OrderedMap v -> [v]
+elems = fmap snd . toList
+
+-- * Basic interface
+
+-- | Associates the specified value with the specified key in this map. If this
+-- map previously contained a mapping for the key, the existing and new values
+-- are combined.
+insert :: Semigroup v => Text -> v -> OrderedMap v -> OrderedMap v
+insert key value (OrderedMap vector hashMap)
+    | Just available <- HashMap.lookup key hashMap = OrderedMap vector
+        $ HashMap.insert key (available <> value) hashMap
+    | otherwise = OrderedMap (Vector.snoc vector key)
+        $ HashMap.insert key value hashMap
+
+-- | Associates the specified value with the specified key in this map. If this
+-- map previously contained a mapping for the key, the existing value is
+-- replaced by the new one.
+replace :: Text -> v -> OrderedMap v -> OrderedMap v
+replace key value (OrderedMap vector hashMap)
+    | HashMap.member key hashMap = OrderedMap vector
+        $ HashMap.insert key value hashMap
+    | otherwise = OrderedMap (Vector.snoc vector key)
+        $ HashMap.insert key value hashMap
+
+-- | Gives the size of this map, i.e. number of elements in it.
+size :: forall v. OrderedMap v -> Int
+size (OrderedMap vector _) = Vector.length vector
+
+-- | Looks up a value in this map by key.
+lookup :: forall v. Text -> OrderedMap v -> Maybe v
+lookup key (OrderedMap _ hashMap) = HashMap.lookup key hashMap
--- a/src/Language/GraphQL/Execute/Subscribe.hs
+++ b/src/Language/GraphQL/Execute/Subscribe.hs
@ -1,97 +0,0 @@
-{- This Source Code Form is subject to the terms of the Mozilla Public License,
-   v. 2.0. If a copy of the MPL was not distributed with this file, You can
-   obtain one at https://mozilla.org/MPL/2.0/. -}
-
-{-# LANGUAGE ExplicitForAll #-}
-{-# LANGUAGE OverloadedStrings #-}
-module Language.GraphQL.Execute.Subscribe
-    ( subscribe
-    ) where
-
-import Conduit
-import Control.Monad.Catch (Exception(..), MonadCatch(..))
-import Control.Monad.Trans.Reader (ReaderT(..), runReaderT)
-import Data.HashMap.Strict (HashMap)
-import qualified Data.HashMap.Strict as HashMap
-import qualified Data.Map.Strict as Map
-import qualified Data.List.NonEmpty as NonEmpty
-import Data.Sequence (Seq(..))
-import Data.Text (Text)
-import qualified Data.Text as Text
-import Language.GraphQL.AST (Name)
-import Language.GraphQL.Execute.Coerce
-import Language.GraphQL.Execute.Execution
-import qualified Language.GraphQL.Execute.Transform as Transform
-import Language.GraphQL.Error
-import qualified Language.GraphQL.Type.Definition as Definition
-import qualified Language.GraphQL.Type as Type
-import qualified Language.GraphQL.Type.Out as Out
-import Language.GraphQL.Type.Schema
-
-- This is actually executeMutation, but we don't distinguish between queries
-- and mutations yet.
-subscribe :: (MonadCatch m, Serialize a)
-    => HashMap Name (Type m)
-    -> Out.ObjectType m
-    -> Seq (Transform.Selection m)
-    -> m (Either Text (ResponseEventStream m a))
-subscribe types' objectType fields = do
-    sourceStream <- createSourceEventStream types' objectType fields
-    traverse (mapSourceToResponseEvent types' objectType fields) sourceStream
-
-mapSourceToResponseEvent :: (MonadCatch m, Serialize a)
-    => HashMap Name (Type m)
-    -> Out.ObjectType m
-    -> Seq (Transform.Selection m)
-    -> Out.SourceEventStream m
-    -> m (ResponseEventStream m a)
-mapSourceToResponseEvent types' subscriptionType fields sourceStream = pure
-    $ sourceStream
-    .| mapMC (executeSubscriptionEvent types' subscriptionType fields)
-
-createSourceEventStream :: MonadCatch m
-    => HashMap Name (Type m)
-    -> Out.ObjectType m
-    -> Seq (Transform.Selection m)
-    -> m (Either Text (Out.SourceEventStream m))
-createSourceEventStream _types subscriptionType@(Out.ObjectType _ _ _ fieldTypes) fields
-    | [fieldGroup] <- Map.elems groupedFieldSet
-    , Transform.Field _ fieldName arguments' _ <- NonEmpty.head fieldGroup
-    , resolverT <- fieldTypes HashMap.! fieldName
-    , Out.EventStreamResolver fieldDefinition _ resolver <- resolverT
-    , Out.Field _ _fieldType argumentDefinitions <- fieldDefinition =
-        case coerceArgumentValues argumentDefinitions arguments' of
-            Nothing -> pure $ Left "Argument coercion failed."
-            Just argumentValues ->
-                resolveFieldEventStream Type.Null argumentValues resolver
-    | otherwise = pure $ Left "Subscription contains more than one field."
-  where
-    groupedFieldSet = collectFields subscriptionType fields
-
-resolveFieldEventStream :: MonadCatch m
-    => Type.Value
-    -> Type.Subs
-    -> Out.Subscribe m
-    -> m (Either Text (Out.SourceEventStream m))
-resolveFieldEventStream result args resolver =
-    catch (Right <$> runReaderT resolver context) handleEventStreamError
-  where
-    handleEventStreamError :: MonadCatch m
-        => ResolverException
-        -> m (Either Text (Out.SourceEventStream m))
-    handleEventStreamError = pure . Left . Text.pack . displayException
-    context = Type.Context
-        { Type.arguments = Type.Arguments args
-        , Type.values = result
-        }
-
-- This is actually executeMutation, but we don't distinguish between queries
-- and mutations yet.
-executeSubscriptionEvent :: (MonadCatch m, Serialize a)
-    => HashMap Name (Type m)
-    -> Out.ObjectType m
-    -> Seq (Transform.Selection m)
-    -> Definition.Value
-    -> m (Response a)
-executeSubscriptionEvent types' objectType fields initialValue =
-    runCollectErrs types' $ executeSelectionSet initialValue objectType fields
--- a/src/Language/GraphQL/Execute/Transform.hs
+++ b/src/Language/GraphQL/Execute/Transform.hs
@ -1,8 +1,12 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 {-# LANGUAGE ExplicitForAll #-}
 {-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE RecordWildCards #-}
-{-# LANGUAGE TupleSections #-}
+{-# LANGUAGE NamedFieldPuns #-}

 -- | After the document is parsed, before getting executed, the AST is
 -- transformed into a similar, simpler AST. Performed transformations include:
@ -17,400 +21,305 @@
 -- This module is also responsible for smaller rewrites that touch only parts of
 -- the original AST.
 module Language.GraphQL.Execute.Transform
-    ( Document(..)
-    , Field(..)
+    ( Field(..)
    , Fragment(..)
    , Input(..)
    , Operation(..)
-    , QueryError(..)
+    , Replacement(..)
    , Selection(..)
+    , TransformT(..)
    , document
-    , queryError
+    , transform
    ) where

-import Control.Monad (foldM, unless)
-import Control.Monad.Trans.Class (lift)
-import Control.Monad.Trans.State (State, evalStateT, gets, modify)
-import Data.Foldable (find)
-import Data.Functor.Identity (Identity(..))
+import Control.Monad (foldM)
+import Control.Monad.Catch (MonadCatch(..), MonadThrow(..))
+import Control.Monad.Trans.Class (MonadTrans(..))
+import Control.Monad.Trans.Reader (ReaderT(..), local)
+import qualified Control.Monad.Trans.Reader as Reader
+import Data.Bifunctor (first)
+import Data.Functor ((<&>))
 import Data.HashMap.Strict (HashMap)
 import qualified Data.HashMap.Strict as HashMap
+import Data.HashSet (HashSet)
+import qualified Data.HashSet as HashSet
 import Data.Int (Int32)
-import Data.Maybe (fromMaybe)
-import Data.List.NonEmpty (NonEmpty(..))
+import Data.List (intercalate)
 import qualified Data.List.NonEmpty as NonEmpty
-import Data.Sequence (Seq, (<|), (><))
+import Data.Maybe (fromMaybe, isJust)
+import Data.Sequence (Seq, (><))
+import qualified Data.Sequence as Seq
 import Data.Text (Text)
 import qualified Data.Text as Text
-import qualified Language.GraphQL.AST as Full
-import Language.GraphQL.AST (Name)
-import qualified Language.GraphQL.Execute.Coerce as Coerce
-import qualified Language.GraphQL.Type.Definition as Definition
+import qualified Language.GraphQL.AST.Document as Full
+import Language.GraphQL.Type.Schema (Type)
 import qualified Language.GraphQL.Type as Type
+import qualified Language.GraphQL.Type.Definition as Definition
 import qualified Language.GraphQL.Type.Internal as Type
-import qualified Language.GraphQL.Type.Out as Out
-import qualified Language.GraphQL.Type.Schema as Schema
+import Numeric (showFloat)

 -- | Associates a fragment name with a list of 'Field's.
 data Replacement m = Replacement
-    { fragments :: HashMap Full.Name (Fragment m)
-    , fragmentDefinitions :: FragmentDefinitions
-    , variableValues :: Type.Subs
-    , types :: HashMap Full.Name (Schema.Type m)
+    { variableValues :: Type.Subs
+    , fragmentDefinitions :: HashMap Full.Name Full.FragmentDefinition
+    , visitedFragments :: HashSet Full.Name
+    , types :: HashMap Full.Name (Type m)
    }

-type FragmentDefinitions = HashMap Full.Name Full.FragmentDefinition
+newtype TransformT m a = TransformT
+    { runTransformT :: ReaderT (Replacement m) m a
+    }

-- | Represents fragments and inline fragments.
-data Fragment m
-    = Fragment (Type.CompositeType m) (Seq (Selection m))
+instance Functor m => Functor (TransformT m) where
+    fmap f = TransformT . fmap f . runTransformT

-- | Single selection element.
-data Selection m
-    = SelectionFragment (Fragment m)
-    | SelectionField (Field m)
+instance Applicative m => Applicative (TransformT m) where
+    pure = TransformT . pure
+    TransformT f <*> TransformT x = TransformT $ f <*> x
+
+instance Monad m => Monad (TransformT m) where
+    TransformT x >>= f = TransformT $ x >>= runTransformT . f
+
+instance MonadTrans TransformT where
+    lift = TransformT . lift
+
+instance MonadThrow m => MonadThrow (TransformT m) where
+    throwM = lift . throwM
+
+instance MonadCatch m => MonadCatch (TransformT m) where
+  catch (TransformT stack) handler =
+      TransformT $ catch stack $ runTransformT . handler
+
+asks :: Monad m => forall a. (Replacement m -> a) -> TransformT m a
+asks = TransformT . Reader.asks

 -- | GraphQL has 3 operation types: queries, mutations and subscribtions.
--
-- Currently only queries and mutations are supported.
 data Operation m
-    = Query (Maybe Text) (Seq (Selection m))
-    | Mutation (Maybe Text) (Seq (Selection m))
-    | Subscription (Maybe Text) (Seq (Selection m))
+    = Operation Full.OperationType (Seq (Selection m)) Full.Location
+
+-- | Field or inlined fragment.
+data Selection m
+    = FieldSelection (Field m)
+    | FragmentSelection (Fragment m)

-- | Single GraphQL field.
 data Field m = Field
-    (Maybe Full.Name) Full.Name (HashMap Full.Name Input) (Seq (Selection m))
-
-- | Contains the operation to be executed along with its root type.
-data Document m = Document
-    (HashMap Full.Name (Schema.Type m)) (Out.ObjectType m) (Operation m)
-
-data OperationDefinition = OperationDefinition
-    Full.OperationType
    (Maybe Full.Name)
-    [Full.VariableDefinition]
-    [Full.Directive]
-    Full.SelectionSet
+    Full.Name
+    (HashMap Full.Name (Full.Node Input))
+    (Seq (Selection m))
+    Full.Location

-- | Query error types.
-data QueryError
-    = OperationNotFound Text
-    | OperationNameRequired
-    | CoercionError
-    | TransformationError
-    | EmptyDocument
-    | UnsupportedRootOperation
+data Fragment m = Fragment
+    (Type.CompositeType m) (Seq (Selection m)) Full.Location

 data Input
-    = Int Int32
+    = Variable Type.Value
+    | Int Int32
    | Float Double
    | String Text
    | Boolean Bool
    | Null
-    | Enum Name
-    | List [Type.Value]
-    | Object (HashMap Name Input)
-    | Variable Type.Value
-    deriving (Eq, Show)
+    | Enum Full.Name
+    | List [Input]
+    | Object (HashMap Full.Name Input)
+    deriving Eq

-queryError :: QueryError -> Text
-queryError (OperationNotFound operationName) = Text.unwords
-    ["Operation", operationName, "couldn't be found in the document."]
-queryError OperationNameRequired = "Missing operation name."
-queryError CoercionError = "Coercion error."
-queryError TransformationError = "Schema transformation error."
-queryError EmptyDocument =
-    "The document doesn't contain any executable operations."
-queryError UnsupportedRootOperation =
-    "Root operation type couldn't be found in the schema."
+instance Show Input where
+    showList = mappend . showList'
+      where
+        showList' list = "[" ++ intercalate ", " (show <$> list) ++ "]"
+    show (Int integer) = show integer
+    show (Float float') = showFloat float' mempty
+    show (String text) = "\"" <> Text.foldr (mappend . Full.escape) "\"" text
+    show (Boolean boolean') = show boolean'
+    show Null = "null"
+    show (Enum name) = Text.unpack name
+    show (List list) = show list
+    show (Object fields) = unwords
+        [ "{"
+        , intercalate ", " (HashMap.foldrWithKey showObject [] fields)
+        , "}"
+        ]
+      where
+        showObject key value accumulator =
+            concat [Text.unpack key, ": ", show value] : accumulator
+    show variableValue = show variableValue

-getOperation
-    :: Maybe Full.Name
-    -> NonEmpty OperationDefinition
-    -> Either QueryError OperationDefinition
-getOperation Nothing (operation' :| []) = pure operation'
-getOperation Nothing _ = Left OperationNameRequired
-getOperation (Just operationName) operations
-    | Just operation' <- find matchingName operations = pure operation'
-    | otherwise = Left $ OperationNotFound operationName
+-- | Extracts operations and fragment definitions of the document.
+document :: Full.Document
+    -> ([Full.OperationDefinition], HashMap Full.Name Full.FragmentDefinition)
+document = foldr filterOperation ([], HashMap.empty)
  where
-    matchingName (OperationDefinition _ name _ _ _) =
-        name == Just operationName
-
-coerceVariableValues :: Coerce.VariableValue a
-    => forall m
-    . HashMap Full.Name (Schema.Type m)
-    -> OperationDefinition
-    -> HashMap.HashMap Full.Name a
-    -> Either QueryError Type.Subs
-coerceVariableValues types operationDefinition variableValues =
-    let OperationDefinition _ _ variableDefinitions _ _ = operationDefinition
-     in maybe (Left CoercionError) Right
-        $ foldr forEach (Just HashMap.empty) variableDefinitions
-  where
-    forEach variableDefinition coercedValues = do
-        let Full.VariableDefinition variableName variableTypeName defaultValue _ =
-                variableDefinition
-        let defaultValue' = constValue . Full.node <$> defaultValue
-        variableType <- Type.lookupInputType variableTypeName types
-
-        Coerce.matchFieldValues
-            coerceVariableValue'
-            variableValues
-            variableName
-            variableType
-            defaultValue'
-            coercedValues
-    coerceVariableValue' variableType value'
-        = Coerce.coerceVariableValue variableType value'
-        >>= Coerce.coerceInputLiteral variableType
-
-constValue :: Full.ConstValue -> Type.Value
-constValue (Full.ConstInt i) = Type.Int i
-constValue (Full.ConstFloat f) = Type.Float f
-constValue (Full.ConstString x) = Type.String x
-constValue (Full.ConstBoolean b) = Type.Boolean b
-constValue Full.ConstNull = Type.Null
-constValue (Full.ConstEnum e) = Type.Enum e
-constValue (Full.ConstList l) = Type.List $ constValue <$> l
-constValue (Full.ConstObject o) =
-    Type.Object $ HashMap.fromList $ constObjectField <$> o
-  where
-    constObjectField Full.ObjectField{value = value', ..} =
-        (name, constValue $ Full.node value')
+    filterOperation (Full.ExecutableDefinition executableDefinition) accumulator
+        | Full.DefinitionOperation operationDefinition' <- executableDefinition =
+            first (operationDefinition' :) accumulator
+        | Full.DefinitionFragment fragmentDefinition <- executableDefinition
+        , Full.FragmentDefinition fragmentName _ _ _ _ <- fragmentDefinition =
+            HashMap.insert fragmentName fragmentDefinition <$> accumulator
+    filterOperation _ accumulator = accumulator -- Type system definitions.

 -- | Rewrites the original syntax tree into an intermediate representation used
-- for query execution.
-document :: Coerce.VariableValue a
-    => forall m
-    . Type.Schema m
-    -> Maybe Full.Name
+-- for the query execution.
+transform :: Monad m => Full.OperationDefinition -> TransformT m (Operation m)
+transform (Full.OperationDefinition operationType _ _ _ selectionSet' operationLocation) = do
+    transformedSelections <- selectionSet selectionSet'
+    pure $ Operation operationType transformedSelections operationLocation
+transform (Full.SelectionSet selectionSet' operationLocation) = do
+    transformedSelections <- selectionSet selectionSet'
+    pure $ Operation Full.Query transformedSelections operationLocation
+
+selectionSet :: Monad m => Full.SelectionSet -> TransformT m (Seq (Selection m))
+selectionSet = selectionSetOpt . NonEmpty.toList
+
+selectionSetOpt :: Monad m => Full.SelectionSetOpt -> TransformT m (Seq (Selection m))
+selectionSetOpt = foldM go Seq.empty
+  where
+    go accumulatedSelections currentSelection =
+        selection currentSelection <&> (accumulatedSelections ><)
+
+selection :: Monad m => Full.Selection -> TransformT m (Seq (Selection m))
+selection (Full.FieldSelection field') =
+    maybeToSelectionSet FieldSelection $ field field'
+selection (Full.FragmentSpreadSelection fragmentSpread') =
+    maybeToSelectionSet FragmentSelection $ fragmentSpread fragmentSpread'
+selection (Full.InlineFragmentSelection inlineFragment') =
+    either id (pure . FragmentSelection) <$> inlineFragment inlineFragment'
+
+maybeToSelectionSet :: Monad m
+    => forall a
+    . (a -> Selection m)
+    -> TransformT m (Maybe a)
+    -> TransformT m (Seq (Selection m))
+maybeToSelectionSet selectionType = fmap (maybe Seq.empty $ pure . selectionType)
+
+directives :: Monad m => [Full.Directive] -> TransformT m (Maybe [Definition.Directive])
+directives = fmap Type.selection . traverse directive
+
+inlineFragment :: Monad m
+    => Full.InlineFragment
+    -> TransformT m (Either (Seq (Selection m)) (Fragment m))
+inlineFragment (Full.InlineFragment maybeCondition directives' selectionSet' location)
+    | Just typeCondition <- maybeCondition = do
+        transformedSelections <- selectionSet selectionSet'
+        transformedDirectives <- directives directives'
+        maybeFragmentType <- asks
+            $ Type.lookupTypeCondition typeCondition
+            . types
+        pure $ case transformedDirectives >> maybeFragmentType of
+            Just fragmentType -> Right
+                $ Fragment fragmentType transformedSelections location
+            Nothing -> Left Seq.empty
+    | otherwise = do
+        transformedSelections <- selectionSet selectionSet'
+        transformedDirectives <- directives directives'
+        pure $ if isJust transformedDirectives
+            then Left transformedSelections
+            else Left Seq.empty
+
+fragmentSpread :: Monad m => Full.FragmentSpread -> TransformT m (Maybe (Fragment m))
+fragmentSpread (Full.FragmentSpread spreadName directives' location) = do
+    transformedDirectives <- directives directives'
+    visitedFragment <- asks $ HashSet.member spreadName . visitedFragments
+    possibleFragmentDefinition <- asks
+        $ HashMap.lookup spreadName
+        . fragmentDefinitions
+    case transformedDirectives >> possibleFragmentDefinition of
+        Just (Full.FragmentDefinition _ typeCondition _ selections _)
+            | visitedFragment -> pure Nothing
+            | otherwise -> do
+                fragmentType <- asks
+                    $ Type.lookupTypeCondition typeCondition
+                    . types
+                traverse (traverseSelections selections) fragmentType
+        Nothing -> pure Nothing
+  where
+    traverseSelections selections typeCondition = do
+        transformedSelections <- TransformT
+            $ local fragmentInserter
+            $ runTransformT
+            $ selectionSet selections
+        pure $ Fragment typeCondition transformedSelections location
+    fragmentInserter replacement@Replacement{ visitedFragments } = replacement
+        { visitedFragments = HashSet.insert spreadName visitedFragments }
+
+field :: Monad m => Full.Field -> TransformT m (Maybe (Field m))
+field (Full.Field alias' name' arguments' directives' selectionSet' location') = do
+    transformedSelections <- selectionSetOpt selectionSet'
+    transformedDirectives <- directives directives'
+    transformedArguments <- arguments arguments'
+    let transformedField = Field
+            alias'
+            name'
+            transformedArguments
+            transformedSelections
+            location'
+    pure $ transformedDirectives >> pure transformedField
+
+arguments :: Monad m => [Full.Argument] -> TransformT m (HashMap Full.Name (Full.Node Input))
+arguments = foldM go HashMap.empty
+  where
+    go accumulator (Full.Argument name' valueNode argumentLocation) = do
+        let replaceLocation = flip Full.Node argumentLocation . Full.node
+        argumentValue <- fmap replaceLocation <$> node valueNode
+        pure $ insertIfGiven name' argumentValue accumulator
+
+directive :: Monad m => Full.Directive -> TransformT m Definition.Directive
+directive (Full.Directive name' arguments' _)
+    = Definition.Directive name'
+    . Type.Arguments
+    <$> foldM go HashMap.empty arguments'
+  where
+    go accumulator (Full.Argument argumentName Full.Node{ node = node' } _) = do
+        transformedValue <- directiveValue node'
+        pure $ HashMap.insert argumentName transformedValue accumulator
+
+directiveValue :: Monad m => Full.Value -> TransformT m Type.Value
+directiveValue = \case
+    (Full.Variable name') -> asks
+        $ HashMap.lookupDefault Type.Null name'
+        . variableValues
+    (Full.Int integer) -> pure $ Type.Int integer
+    (Full.Float double) -> pure $ Type.Float double
+    (Full.String string) -> pure $ Type.String string
+    (Full.Boolean boolean) -> pure $ Type.Boolean boolean
+    Full.Null -> pure Type.Null
+    (Full.Enum enum) -> pure $ Type.Enum enum
+    (Full.List list) -> Type.List <$> traverse directiveNode list
+    (Full.Object objectFields) ->
+        Type.Object <$> foldM objectField HashMap.empty objectFields
+  where
+    directiveNode Full.Node{ node = node'} = directiveValue node'
+    objectField accumulator Full.ObjectField{ name, value } = do
+        transformedValue <- directiveNode value
+        pure $ HashMap.insert name transformedValue accumulator
+
+input :: Monad m => Full.Value -> TransformT m (Maybe Input)
+input (Full.Variable name') =
+    asks (HashMap.lookup name' . variableValues) <&> fmap Variable
+input (Full.Int integer) = pure $ Just $ Int integer
+input (Full.Float double) = pure $ Just $ Float double
+input (Full.String string) = pure $ Just $ String string
+input (Full.Boolean boolean) = pure $ Just $ Boolean boolean
+input Full.Null = pure $ Just Null
+input (Full.Enum enum) = pure $ Just $ Enum enum
+input (Full.List list) = Just . List
+    <$> traverse (fmap (fromMaybe Null) . input . Full.node) list
+input (Full.Object objectFields) = Just . Object
+    <$> foldM objectField HashMap.empty objectFields
+  where
+    objectField accumulator Full.ObjectField{..} = do
+        objectFieldValue <- fmap Full.node <$> node value
+        pure $ insertIfGiven name objectFieldValue accumulator
+
+insertIfGiven :: forall a
+    . Full.Name
+    -> Maybe a
    -> HashMap Full.Name a
-    -> Full.Document
-    -> Either QueryError (Document m)
-document schema operationName subs ast = do
-    let referencedTypes = Schema.types schema
+    -> HashMap Full.Name a
+insertIfGiven name (Just v) = HashMap.insert name v
+insertIfGiven _ _ = id

-    (operations, fragmentTable) <- defragment ast
-    chosenOperation <- getOperation operationName operations
-    coercedValues <- coerceVariableValues referencedTypes chosenOperation subs
+node :: Monad m => Full.Node Full.Value -> TransformT m (Maybe (Full.Node Input))
+node Full.Node{node = node', ..} =
+    traverse Full.Node <$> input node' <*> pure location

-    let replacement = Replacement
-            { fragments = HashMap.empty
-            , fragmentDefinitions = fragmentTable
-            , variableValues = coercedValues
-            , types = referencedTypes
-            }
-    case chosenOperation of
-        OperationDefinition Full.Query _ _ _ _ ->
-            pure $ Document referencedTypes (Schema.query schema)
-                $ operation chosenOperation replacement
-        OperationDefinition Full.Mutation _ _ _ _
-            | Just mutationType <- Schema.mutation schema ->
-                pure $ Document referencedTypes mutationType
-                    $ operation chosenOperation replacement
-        OperationDefinition Full.Subscription _ _ _ _
-            | Just subscriptionType <- Schema.subscription schema ->
-                pure $ Document referencedTypes subscriptionType
-                    $ operation chosenOperation replacement
-        _ -> Left UnsupportedRootOperation
-
-defragment
-    :: Full.Document
-    -> Either QueryError (NonEmpty OperationDefinition, FragmentDefinitions)
-defragment ast =
-    let (operations, fragmentTable) = foldr defragment' ([], HashMap.empty) ast
-        nonEmptyOperations = NonEmpty.nonEmpty operations
-        emptyDocument = Left EmptyDocument
-     in (, fragmentTable) <$> maybe emptyDocument Right nonEmptyOperations
-  where
-    defragment' definition (operations, fragments')
-        | (Full.ExecutableDefinition executable) <- definition
-        , (Full.DefinitionOperation operation') <- executable =
-            (transform operation' : operations, fragments')
-        | (Full.ExecutableDefinition executable) <- definition
-        , (Full.DefinitionFragment fragment) <- executable
-        , (Full.FragmentDefinition name _ _ _ _) <- fragment =
-            (operations, HashMap.insert name fragment fragments')
-    defragment' _ acc = acc
-    transform = \case
-        Full.OperationDefinition type' name variables directives' selections _ ->
-            OperationDefinition type' name variables directives' selections
-        Full.SelectionSet selectionSet _ ->
-            OperationDefinition Full.Query Nothing mempty mempty selectionSet
-
-- * Operation
-
-operation :: OperationDefinition -> Replacement m -> Operation m
-operation operationDefinition replacement
-    = runIdentity
-    $ evalStateT (collectFragments >> transform operationDefinition) replacement
-  where
-    transform (OperationDefinition Full.Query name _ _ sels) =
-        Query name <$> appendSelection sels
-    transform (OperationDefinition Full.Mutation name _ _ sels) =
-        Mutation name <$> appendSelection sels
-    transform (OperationDefinition Full.Subscription name _ _ sels) =
-        Subscription name <$> appendSelection sels
-
-- * Selection
-
-selection
-    :: Full.Selection
-    ->  State (Replacement m) (Either (Seq (Selection m)) (Selection m))
-selection (Full.FieldSelection fieldSelection) =
-    maybe (Left mempty) (Right . SelectionField) <$> field fieldSelection
-selection (Full.FragmentSpreadSelection fragmentSelection)
-    = maybe (Left mempty) (Right . SelectionFragment)
-    <$> fragmentSpread fragmentSelection
-selection (Full.InlineFragmentSelection fragmentSelection) =
-    inlineFragment fragmentSelection
-
-field :: Full.Field ->  State (Replacement m) (Maybe (Field m))
-field (Full.Field alias name arguments' directives' selections _) = do
-    fieldArguments <- foldM go HashMap.empty arguments'
-    fieldSelections <- appendSelection selections
-    fieldDirectives <- Definition.selection <$> directives directives'
-    let field' = Field alias name fieldArguments fieldSelections
-    pure $ field' <$ fieldDirectives
-  where
-    go arguments (Full.Argument name' (Full.Node value' _) _) =
-        inputField arguments name' value'
-
-fragmentSpread
-    :: Full.FragmentSpread
-    -> State (Replacement m) (Maybe (Fragment m))
-fragmentSpread (Full.FragmentSpread name directives' _) = do
-    spreadDirectives <- Definition.selection <$> directives directives'
-    fragments' <- gets fragments
-
-    fragmentDefinitions' <- gets fragmentDefinitions
-    case HashMap.lookup name fragments' of
-        Just definition -> lift $ pure $ definition <$ spreadDirectives
-        Nothing
-            | Just definition <- HashMap.lookup name fragmentDefinitions' -> do
-                fragDef <- fragmentDefinition definition
-                case fragDef of
-                    Just fragment -> lift $ pure $ fragment <$ spreadDirectives
-                    _ -> lift $ pure  Nothing
-            | otherwise -> lift $ pure  Nothing
-
-inlineFragment
-    :: Full.InlineFragment
-    ->  State (Replacement m) (Either (Seq (Selection m)) (Selection m))
-inlineFragment (Full.InlineFragment type' directives' selections _) = do
-    fragmentDirectives <- Definition.selection <$> directives directives'
-    case fragmentDirectives of
-        Nothing -> pure $ Left mempty
-        _ -> do
-            fragmentSelectionSet <- appendSelection selections
-
-            case type' of
-                Nothing -> pure $ Left fragmentSelectionSet
-                Just typeName -> do
-                    types' <- gets types
-                    case Type.lookupTypeCondition typeName types' of
-                        Just typeCondition -> pure $
-                            selectionFragment typeCondition fragmentSelectionSet
-                        Nothing -> pure $ Left mempty
-  where
-    selectionFragment typeName = Right
-        . SelectionFragment
-        . Fragment typeName
-
-appendSelection :: Traversable t
-    => t Full.Selection
-    ->  State (Replacement m) (Seq (Selection m))
-appendSelection = foldM go mempty
-  where
-    go acc sel = append acc <$> selection sel
-    append acc (Left list) = list >< acc
-    append acc (Right one) = one <| acc
-
-directives :: [Full.Directive] ->  State (Replacement m) [Definition.Directive]
-directives = traverse directive
-  where
-    directive (Full.Directive directiveName directiveArguments _)
-        = Definition.Directive directiveName . Type.Arguments
-        <$> foldM go HashMap.empty directiveArguments
-    go arguments (Full.Argument name (Full.Node value' _) _) = do
-        substitutedValue <- value value'
-        return $ HashMap.insert name substitutedValue arguments
-
-- * Fragment replacement
-
-- | Extract fragment definitions into a single 'HashMap'.
-collectFragments ::  State (Replacement m) ()
-collectFragments = do
-    fragDefs <- gets fragmentDefinitions
-    let nextValue = head $ HashMap.elems fragDefs
-    unless (HashMap.null fragDefs) $ do
-        _ <- fragmentDefinition nextValue
-        collectFragments
-
-fragmentDefinition
-    :: Full.FragmentDefinition
-    ->  State (Replacement m) (Maybe (Fragment m))
-fragmentDefinition (Full.FragmentDefinition name type' _ selections _) = do
-    modify deleteFragmentDefinition
-    fragmentSelection <- appendSelection selections
-    types' <- gets types
-
-    case Type.lookupTypeCondition type' types' of
-        Just compositeType -> do
-            let newValue = Fragment compositeType fragmentSelection
-            modify $ insertFragment newValue
-            lift $ pure $ Just newValue
-        _ -> lift $ pure Nothing
-  where
-    deleteFragmentDefinition replacement@Replacement{..} =
-        let newDefinitions = HashMap.delete name fragmentDefinitions
-         in replacement{ fragmentDefinitions = newDefinitions }
-    insertFragment newValue replacement@Replacement{..} =
-        let newFragments = HashMap.insert name newValue fragments
-         in replacement{ fragments = newFragments }
-
-value :: forall m. Full.Value -> State (Replacement m) Type.Value
-value (Full.Variable name) =
-    gets (fromMaybe Type.Null . HashMap.lookup name . variableValues)
-value (Full.Int int) = pure $ Type.Int int
-value (Full.Float float) = pure $ Type.Float float
-value (Full.String string) = pure $ Type.String string
-value (Full.Boolean boolean) = pure $ Type.Boolean boolean
-value Full.Null = pure Type.Null
-value (Full.Enum enum) = pure $ Type.Enum enum
-value (Full.List list) = Type.List <$> traverse value list
-value (Full.Object object) =
-    Type.Object . HashMap.fromList <$> traverse objectField object
-  where
-    objectField Full.ObjectField{value = value', ..} =
-        (name,) <$> value (Full.node value')
-
-input :: forall m. Full.Value -> State (Replacement m) (Maybe Input)
-input (Full.Variable name) =
-    gets (fmap Variable . HashMap.lookup name . variableValues)
-input (Full.Int int) = pure $ pure $ Int int
-input (Full.Float float) = pure $ pure $ Float float
-input (Full.String string) = pure $ pure $ String string
-input (Full.Boolean boolean) = pure $ pure $ Boolean boolean
-input Full.Null = pure $ pure Null
-input (Full.Enum enum) = pure $ pure $ Enum enum
-input (Full.List list) = pure . List <$> traverse value list
-input (Full.Object object) = do
-    objectFields <- foldM objectField HashMap.empty object
-    pure $ pure $ Object objectFields
-  where
-    objectField resultMap Full.ObjectField{value = value', ..} =
-        inputField resultMap name $ Full.node value'
-
-inputField :: forall m
-    . HashMap Full.Name Input
-    -> Full.Name
-    -> Full.Value
-    -> State (Replacement m) (HashMap Full.Name Input)
-inputField resultMap name value' = do
-    objectFieldValue <-  input value'
-    case objectFieldValue of
-        Just fieldValue -> pure $ HashMap.insert name fieldValue resultMap
-        Nothing -> pure resultMap
--- a/src/Language/GraphQL/Type.hs
+++ b/src/Language/GraphQL/Type.hs
@ -21,6 +21,6 @@ module Language.GraphQL.Type
    ) where

 import Language.GraphQL.Type.Definition
-import Language.GraphQL.Type.Schema (Schema, schema)
+import Language.GraphQL.Type.Schema (Schema, schema, schemaWithTypes)
 import qualified Language.GraphQL.Type.In as In
 import qualified Language.GraphQL.Type.Out as Out
--- a/src/Language/GraphQL/Type/Definition.hs
+++ b/src/Language/GraphQL/Type/Definition.hs
@ -1,4 +1,10 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE NoGeneralizedNewtypeDeriving #-}
+{-# LANGUAGE Safe #-}

 -- | Types that can be used as both input and output types.
 module Language.GraphQL.Type.Definition
@ -13,6 +19,8 @@ module Language.GraphQL.Type.Definition
    , float
    , id
    , int
+    , showNonNullType
+    , showNonNullListType
    , selection
    , string
    ) where
@ -20,10 +28,12 @@ module Language.GraphQL.Type.Definition
 import Data.Int (Int32)
 import Data.HashMap.Strict (HashMap)
 import qualified Data.HashMap.Strict as HashMap
+import Data.List (intercalate)
 import Data.String (IsString(..))
 import Data.Text (Text)
 import qualified Data.Text as Text
-import Language.GraphQL.AST (Name)
+import Language.GraphQL.AST (Name, escape)
+import Numeric (showFloat)
 import Prelude hiding (id)

 -- | Represents accordingly typed GraphQL values.
@ -36,7 +46,27 @@ data Value
    | Enum Name
    | List [Value] -- ^ Arbitrary nested list.
    | Object (HashMap Name Value)
-    deriving (Eq, Show)
+    deriving Eq
+
+instance Show Value where
+    showList = mappend . showList'
+      where
+        showList' list = "[" ++ intercalate ", " (show <$> list) ++ "]"
+    show (Int integer) = show integer
+    show (Float float') = showFloat float' mempty
+    show (String text) = "\"" <> Text.foldr (mappend . escape) "\"" text
+    show (Boolean boolean') = show boolean'
+    show Null = "null"
+    show (Enum name) = Text.unpack name
+    show (List list) = show list
+    show (Object fields) = unwords
+        [ "{"
+        , intercalate ", " (HashMap.foldrWithKey showObject [] fields)
+        , "}"
+        ]
+      where
+        showObject key value accumulator =
+            concat [Text.unpack key, ": ", show value] : accumulator

 instance IsString Value where
    fromString = String . fromString
@ -180,3 +210,11 @@ include = handle include'
            (Just (Boolean True)) -> Include directive'
            _ -> Skip
    include' directive' = Continue directive'
+
+showNonNullType :: Show a => a -> String
+showNonNullType = (++ "!") . show
+
+showNonNullListType :: Show a => a -> String
+showNonNullListType listType =
+    let representation = show listType
+     in concat ["[", representation, "]!"]
--- a/src/Language/GraphQL/Type/In.hs
+++ b/src/Language/GraphQL/Type/In.hs
@ -3,6 +3,8 @@
   obtain one at https://mozilla.org/MPL/2.0/. -}

 {-# LANGUAGE PatternSynonyms #-}
+{-# LANGUAGE NoGeneralizedNewtypeDeriving #-}
+{-# LANGUAGE Safe #-}
 {-# LANGUAGE ViewPatterns #-}

 -- | Input types and values.
@ -65,13 +67,15 @@ instance Show Type where
    show (NamedEnumType enumType) = show enumType
    show (NamedInputObjectType inputObjectType) = show inputObjectType
    show (ListType baseType) = concat ["[", show baseType, "]"]
-    show (NonNullScalarType scalarType) = '!' : show scalarType
-    show (NonNullEnumType enumType) = '!' : show enumType
-    show (NonNullInputObjectType inputObjectType) = '!' : show inputObjectType
-    show (NonNullListType baseType) = concat ["![", show baseType, "]"]
+    show (NonNullScalarType scalarType) = Definition.showNonNullType scalarType
+    show (NonNullEnumType enumType) = Definition.showNonNullType enumType
+    show (NonNullInputObjectType inputObjectType) =
+        Definition.showNonNullType inputObjectType
+    show (NonNullListType baseType) = Definition.showNonNullListType baseType

 -- | Field argument definition.
 data Argument = Argument (Maybe Text) Type (Maybe Definition.Value)
+    deriving Eq

 -- | Field argument definitions.
 type Arguments = HashMap Name Argument
--- a/src/Language/GraphQL/Type/Internal.hs
+++ b/src/Language/GraphQL/Type/Internal.hs
@ -12,6 +12,7 @@ module Language.GraphQL.Type.Internal
    , Directives
    , Schema(..)
    , Type(..)
+    , description
    , directives
    , doesFragmentTypeApply
    , implementations
@ -47,7 +48,11 @@ data Type m
    deriving Eq

 -- | Directive definition.
-data Directive = Directive (Maybe Text) [DirectiveLocation] In.Arguments
+--
+-- A definition consists of an optional description, arguments, whether the
+-- directive is repeatable, and the allowed directive locations.
+data Directive = Directive (Maybe Text) In.Arguments Bool [DirectiveLocation]
+    deriving Eq

 -- | Directive definitions.
 type Directives = HashMap Full.Name Directive
@ -55,41 +60,43 @@ type Directives = HashMap Full.Name Directive
 -- | A Schema is created by supplying the root types of each type of operation,
 --   query and mutation (optional). A schema definition is then supplied to the
 --   validator and executor.
--
--   __Note:__ When the schema is constructed, by default only the types that
--   are reachable by traversing the root types are included, other types must
--   be explicitly referenced.
 data Schema m = Schema
-    (Out.ObjectType m)
-    (Maybe (Out.ObjectType m))
-    (Maybe (Out.ObjectType m))
-    Directives
-    (HashMap Full.Name (Type m))
+    (Maybe Text) -- ^ Description.
+    (Out.ObjectType m) -- ^ Query.
+    (Maybe (Out.ObjectType m)) -- ^ Mutation.
+    (Maybe (Out.ObjectType m)) -- ^ Subscription.
+    Directives -- ^ Directives
+    (HashMap Full.Name (Type m)) -- ^ Types.
+    -- Interface implementations (used only for faster access).
    (HashMap Full.Name [Type m])

+-- | Schema description.
+description :: forall m. Schema m -> Maybe Text
+description (Schema description' _ _ _ _ _ _) = description'
+
 -- | Schema query type.
 query :: forall m. Schema m -> Out.ObjectType m
-query (Schema query' _ _ _ _ _) = query'
+query (Schema _ query' _ _ _ _ _) = query'

 -- | Schema mutation type.
 mutation :: forall m. Schema m -> Maybe (Out.ObjectType m)
-mutation (Schema _ mutation' _ _ _ _) = mutation'
+mutation (Schema _ _ mutation' _ _ _ _) = mutation'

 -- | Schema subscription type.
 subscription :: forall m. Schema m -> Maybe (Out.ObjectType m)
-subscription (Schema _ _ subscription' _ _ _) = subscription'
+subscription (Schema _ _ _ subscription' _ _ _) = subscription'

 -- | Schema directive definitions.
 directives :: forall m. Schema m -> Directives
-directives (Schema _ _ _ directives' _ _) = directives'
+directives (Schema _ _ _ _ directives' _ _) = directives'

 -- | Types referenced by the schema.
 types :: forall m. Schema m -> HashMap Full.Name (Type m)
-types (Schema _ _ _ _ types' _) = types'
+types (Schema _ _ _ _ _ types' _) = types'

 -- | Interface implementations.
 implementations :: forall m. Schema m -> HashMap Full.Name [Type m]
-implementations (Schema _ _ _ _ _ implementations') = implementations'
+implementations (Schema _ _ _ _ _ _ implementations') = implementations'

 -- | These types may describe the parent context of a selection set.
 data CompositeType m
--- a/src/Language/GraphQL/Type/Out.hs
+++ b/src/Language/GraphQL/Type/Out.hs
@ -115,12 +115,12 @@ instance forall a. Show (Type a) where
    show (NamedInterfaceType interfaceType) = show interfaceType
    show (NamedUnionType unionType) = show unionType
    show (ListType baseType) = concat ["[", show baseType, "]"]
-    show (NonNullScalarType scalarType) = '!' : show scalarType
-    show (NonNullEnumType enumType) = '!' : show enumType
-    show (NonNullObjectType inputObjectType) = '!' : show inputObjectType
-    show (NonNullInterfaceType interfaceType) = '!' : show interfaceType
-    show (NonNullUnionType unionType) = '!' : show unionType
-    show (NonNullListType baseType) = concat ["![", show baseType, "]"]
+    show (NonNullScalarType scalarType) = showNonNullType scalarType
+    show (NonNullEnumType enumType) = showNonNullType enumType
+    show (NonNullObjectType inputObjectType) = showNonNullType inputObjectType
+    show (NonNullInterfaceType interfaceType) = showNonNullType interfaceType
+    show (NonNullUnionType unionType) = showNonNullType unionType
+    show (NonNullListType baseType) = showNonNullListType baseType

 -- | Matches either 'NamedScalarType' or 'NonNullScalarType'.
 pattern ScalarBaseType :: forall m. ScalarType -> Type m
--- a/src/Language/GraphQL/Type/Schema.hs
+++ b/src/Language/GraphQL/Type/Schema.hs
@ -9,11 +9,13 @@
 -- functions for defining and manipulating schemas.
 module Language.GraphQL.Type.Schema
    ( schema
+    , schemaWithTypes
    , module Language.GraphQL.Type.Internal
    ) where

 import Data.HashMap.Strict (HashMap)
 import qualified Data.HashMap.Strict as HashMap
+import Data.Text (Text)
 import Language.GraphQL.AST.DirectiveLocation (DirectiveLocation(..))
 import qualified Language.GraphQL.AST.DirectiveLocation as DirectiveLocation
 import qualified Language.GraphQL.AST as Full
@ -22,6 +24,7 @@ import Language.GraphQL.Type.Internal
    , Directives
    , Schema
    , Type(..)
+    , description
    , directives
    , implementations
    , mutation
@ -35,32 +38,63 @@ import qualified Language.GraphQL.Type.In as In
 import qualified Language.GraphQL.Type.Out as Out

 -- | Schema constructor.
+--
+-- __Note:__ When the schema is constructed, by default only the types that
+-- are reachable by traversing the root types are included, other types must
+-- be explicitly referenced using 'schemaWithTypes' instead.
 schema :: forall m
    . Out.ObjectType m -- ^ Query type.
    -> Maybe (Out.ObjectType m) -- ^ Mutation type.
    -> Maybe (Out.ObjectType m) -- ^ Subscription type.
    -> Directives -- ^ Directive definitions.
    -> Schema m -- ^ Schema.
-schema queryRoot mutationRoot subscriptionRoot directiveDefinitions =
-    Internal.Schema queryRoot mutationRoot subscriptionRoot
+schema queryRoot mutationRoot subscriptionRoot =
+    schemaWithTypes Nothing queryRoot mutationRoot subscriptionRoot mempty
+
+-- | Constructs a complete schema, including user-defined types not referenced
+-- in the schema directly (for example interface implementations).
+schemaWithTypes :: forall m
+    . Maybe Text -- ^ Schema description
+    -> Out.ObjectType m -- ^ Query type.
+    -> Maybe (Out.ObjectType m) -- ^ Mutation type.
+    -> Maybe (Out.ObjectType m) -- ^ Subscription type.
+    -> [Type m] -- ^ Additional types.
+    -> Directives -- ^ Directive definitions.
+    -> Schema m -- ^ Schema.
+schemaWithTypes description' queryRoot mutationRoot subscriptionRoot types' directiveDefinitions =
+    Internal.Schema description' queryRoot mutationRoot subscriptionRoot
        allDirectives collectedTypes collectedImplementations
  where
-    collectedTypes = collectReferencedTypes queryRoot mutationRoot subscriptionRoot
+    allTypes = foldr addTypeDefinition HashMap.empty types'
+    addTypeDefinition type'@(ScalarType (Definition.ScalarType typeName _)) accumulator =
+        HashMap.insert typeName type' accumulator
+    addTypeDefinition type'@(EnumType (Definition.EnumType typeName _ _)) accumulator =
+        HashMap.insert typeName type' accumulator
+    addTypeDefinition type'@(ObjectType (Out.ObjectType typeName _ _ _)) accumulator =
+        HashMap.insert typeName type' accumulator
+    addTypeDefinition type'@(InputObjectType (In.InputObjectType typeName _ _)) accumulator =
+        HashMap.insert typeName type' accumulator
+    addTypeDefinition type'@(InterfaceType (Out.InterfaceType typeName _ _ _)) accumulator =
+        HashMap.insert typeName type' accumulator
+    addTypeDefinition type'@(UnionType (Out.UnionType typeName _ _)) accumulator =
+        HashMap.insert typeName type' accumulator
+    collectedTypes = collectReferencedTypes queryRoot mutationRoot subscriptionRoot allTypes
    collectedImplementations = collectImplementations collectedTypes
    allDirectives = HashMap.union directiveDefinitions defaultDirectives
    defaultDirectives = HashMap.fromList
        [ ("skip", skipDirective)
        , ("include", includeDirective)
        , ("deprecated", deprecatedDirective)
+        , ("specifiedBy", specifiedByDirective)
        ]
    includeDirective =
-        Directive includeDescription skipIncludeLocations includeArguments
+        Directive includeDescription includeArguments False skipIncludeLocations
    includeArguments = HashMap.singleton "if"
        $ In.Argument (Just "Included when true.") ifType Nothing
    includeDescription = Just
        "Directs the executor to include this field or fragment only when the \
        \`if` argument is true."
-    skipDirective = Directive skipDescription skipIncludeLocations skipArguments
+    skipDirective = Directive skipDescription skipArguments False skipIncludeLocations
    skipArguments = HashMap.singleton "if"
        $ In.Argument (Just "skipped when true.") ifType Nothing
    ifType = In.NonNullScalarType Definition.boolean
@ -73,16 +107,15 @@ schema queryRoot mutationRoot subscriptionRoot directiveDefinitions =
        , ExecutableDirectiveLocation DirectiveLocation.InlineFragment
        ]
    deprecatedDirective =
-        Directive deprecatedDescription deprecatedLocations deprecatedArguments
+        Directive deprecatedDescription deprecatedArguments False deprecatedLocations
    reasonDescription = Just
        "Explains why this element was deprecated, usually also including a \
        \suggestion for how to access supported similar data. Formatted using \
        \the Markdown syntax, as specified by \
        \[CommonMark](https://commonmark.org/).'"
    deprecatedArguments = HashMap.singleton "reason"
-        $ In.Argument reasonDescription reasonType
+        $ In.Argument reasonDescription (In.NamedScalarType Definition.string)
        $ Just "No longer supported"
-    reasonType = In.NamedScalarType Definition.string
    deprecatedDescription = Just
        "Marks an element of a GraphQL schema as no longer supported."
    deprecatedLocations =
@ -91,6 +124,16 @@ schema queryRoot mutationRoot subscriptionRoot directiveDefinitions =
        , TypeSystemDirectiveLocation DirectiveLocation.InputFieldDefinition
        , TypeSystemDirectiveLocation DirectiveLocation.EnumValue
        ]
+    specifiedByDirective =
+        Directive specifiedByDescription specifiedByArguments False specifiedByLocations
+    urlDescription = Just
+        "The URL that specifies the behavior of this scalar."
+    specifiedByArguments = HashMap.singleton "url"
+        $ In.Argument urlDescription (In.NonNullScalarType Definition.string) Nothing
+    specifiedByDescription = Just
+        "Exposes a URL that specifies the behavior of this scalar."
+    specifiedByLocations =
+        [TypeSystemDirectiveLocation DirectiveLocation.Scalar]

 -- | Traverses the schema and finds all referenced types.
 collectReferencedTypes :: forall m
@ -98,11 +141,12 @@ collectReferencedTypes :: forall m
    -> Maybe (Out.ObjectType m)
    -> Maybe (Out.ObjectType m)
    -> HashMap Full.Name (Type m)
-collectReferencedTypes queryRoot mutationRoot subscriptionRoot =
-    let queryTypes = traverseObjectType queryRoot HashMap.empty
+    -> HashMap Full.Name (Type m)
+collectReferencedTypes queryRoot mutationRoot subscriptionRoot extraTypes =
+    let queryTypes = traverseObjectType queryRoot extraTypes
        mutationTypes = maybe queryTypes (`traverseObjectType` queryTypes)
            mutationRoot
-     in maybe mutationTypes (`traverseObjectType` queryTypes) subscriptionRoot
+     in maybe mutationTypes (`traverseObjectType` mutationTypes) subscriptionRoot
  where
    collect traverser typeName element foundTypes
        | HashMap.member typeName foundTypes = foundTypes
@ -171,5 +215,5 @@ collectImplementations = HashMap.foldr go HashMap.empty
        let Out.ObjectType _ _ interfaces _ = objectType
         in foldr (add implementation) accumulator interfaces
    go _ accumulator = accumulator
-    add implementation (Out.InterfaceType typeName _ _ _) accumulator =
-        HashMap.insertWith (++) typeName [implementation] accumulator
+    add implementation (Out.InterfaceType typeName _ _ _) =
+        HashMap.insertWith (++) typeName [implementation]
--- a/src/Language/GraphQL/Validate.hs
+++ b/src/Language/GraphQL/Validate.hs
@ -4,7 +4,6 @@

 {-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE RecordWildCards #-}
-{-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE ScopedTypeVariables #-}

 -- | GraphQL validator.
@ -201,7 +200,7 @@ typeSystemDefinition context rule = \case
        directives context rule schemaLocation directives'
    Full.TypeDefinition typeDefinition' ->
        typeDefinition context rule typeDefinition'
-    Full.DirectiveDefinition _ _ arguments' _ ->
+    Full.DirectiveDefinition _ _ arguments' _ _ ->
        argumentsDefinition context rule arguments'

 typeDefinition :: forall m. Validation m -> ApplyRule m Full.TypeDefinition
@ -211,7 +210,7 @@ typeDefinition context rule = \case
    Full.ObjectTypeDefinition _ _ _ directives' fields
        -> directives context rule objectLocation directives'
        >< foldMap (fieldDefinition context rule) fields
-    Full.InterfaceTypeDefinition _ _ directives' fields
+    Full.InterfaceTypeDefinition _ _ _ directives' fields
        -> directives context rule interfaceLocation directives'
        >< foldMap (fieldDefinition context rule) fields
    Full.UnionTypeDefinition _ _ directives' _ ->
@ -284,7 +283,7 @@ operationDefinition rule context operation
    schema' = Validation.schema context
    queryRoot = Just $ Out.NamedObjectType $ Schema.query schema'
    types' = Schema.types schema'
-        
+
 typeToOut :: forall m. Schema.Type m -> Maybe (Out.Type m)
 typeToOut (Schema.ObjectType objectType) =
    Just $ Out.NamedObjectType objectType
@ -315,9 +314,6 @@ constValue (Validation.ValueRule _ rule) valueType = go valueType
    go inputObjectType value'@(Full.Node (Full.ConstObject fields) _)
        = foldMap (forEach inputObjectType) (Seq.fromList fields)
        |> rule inputObjectType value'
-    go listType value'@(Full.Node (Full.ConstList values) location')
-        = embedListLocation go listType values location'
-        |> rule listType value'
    go anotherValue value' = pure $ rule anotherValue value'
    forEach inputObjectType Full.ObjectField{value = value', ..} =
        go (valueTypeByName name inputObjectType) value'
@ -407,7 +403,7 @@ arguments :: forall m
    -> Seq (Validation.RuleT m)
 arguments rule argumentTypes = foldMap forEach . Seq.fromList
  where
-    forEach argument'@(Full.Argument argumentName _ _) = 
+    forEach argument'@(Full.Argument argumentName _ _) =
       let argumentType = HashMap.lookup argumentName argumentTypes
        in argument rule argumentType argument'

@ -421,20 +417,6 @@ argument rule argumentType (Full.Argument _ value' _) =
  where
    valueType (In.Argument _ valueType' _) = valueType'

-- valueTypeFromList :: Maybe In.Type -> Maybe In.Type
-embedListLocation :: forall a m
-    . (Maybe In.Type -> Full.Node a -> Seq m)
-    -> Maybe In.Type
-    -> [a]
-    -> Full.Location
-    -> Seq m
-embedListLocation go listType values location'
-    = foldMap (go $ valueTypeFromList listType) 
-    $ flip Full.Node location' <$> Seq.fromList values
-  where
-    valueTypeFromList (Just (In.ListBaseType baseType)) = Just baseType
-    valueTypeFromList _ = Nothing
-
 value :: forall m
    . Validation.Rule m
    -> Maybe In.Type
@ -445,9 +427,6 @@ value (Validation.ValueRule rule _) valueType = go valueType
    go inputObjectType value'@(Full.Node (Full.Object fields) _)
        = foldMap (forEach inputObjectType) (Seq.fromList fields)
        |> rule inputObjectType value'
-    go listType value'@(Full.Node (Full.List values) location')
-        = embedListLocation go listType values location'
-        |> rule listType value'
    go anotherValue value' = pure $ rule anotherValue value'
    forEach inputObjectType Full.ObjectField{value = value', ..} =
        go (valueTypeByName name inputObjectType) value'
@ -503,4 +482,4 @@ directive context rule (Full.Directive directiveName arguments' _) =
            $ Validation.schema context
     in arguments rule argumentTypes arguments'
  where
-    directiveArguments (Schema.Directive _ _ argumentTypes) = argumentTypes
+    directiveArguments (Schema.Directive _ argumentTypes _ _) = argumentTypes
--- a/src/Language/GraphQL/Validate/Rules.hs
+++ b/src/Language/GraphQL/Validate/Rules.hs
@ -2,11 +2,13 @@
   v. 2.0. If a copy of the MPL was not distributed with this file, You can
   obtain one at https://mozilla.org/MPL/2.0/. -}

+{-# LANGUAGE DataKinds #-}
 {-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE NamedFieldPuns #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE RecordWildCards #-}
 {-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE ViewPatterns #-}

 -- | This module contains default rules defined in the GraphQL specification.
@ -48,19 +50,21 @@ import Control.Monad.Trans.Class (MonadTrans(..))
 import Control.Monad.Trans.Reader (ReaderT(..), ask, asks, mapReaderT)
 import Control.Monad.Trans.State (StateT, evalStateT, gets, modify)
 import Data.Bifunctor (first)
-import Data.Foldable (find, fold, foldl', toList)
+import Data.Foldable (Foldable(..), find)
 import qualified Data.HashMap.Strict as HashMap
 import Data.HashMap.Strict (HashMap)
 import Data.HashSet (HashSet)
 import qualified Data.HashSet as HashSet
-import Data.List (groupBy, sortBy, sortOn)
-import Data.Maybe (catMaybes, fromMaybe, isJust, isNothing, mapMaybe)
+import Data.List (sortBy)
+import Data.Maybe (fromMaybe, isJust, isNothing, mapMaybe)
 import Data.List.NonEmpty (NonEmpty(..))
+import qualified Data.List.NonEmpty as NonEmpty
 import Data.Ord (comparing)
 import Data.Sequence (Seq(..), (|>))
 import qualified Data.Sequence as Seq
 import Data.Text (Text)
 import qualified Data.Text as Text
+import GHC.Records (HasField(..))
 import qualified Language.GraphQL.AST.Document as Full
 import qualified Language.GraphQL.Type.Definition as Definition
 import qualified Language.GraphQL.Type.Internal as Type
@ -133,26 +137,29 @@ singleFieldSubscriptionsRule :: forall m. Rule m
 singleFieldSubscriptionsRule = OperationDefinitionRule $ \case
    Full.OperationDefinition Full.Subscription name' _ _ rootFields location' -> do
        groupedFieldSet <- evalStateT (collectFields rootFields) HashSet.empty
-        case HashSet.size groupedFieldSet of
-            1 -> lift mempty
-            _
-                | Just name <- name' -> pure $ Error
-                    { message = concat
-                        [ "Subscription \""
-                        , Text.unpack name
-                        , "\" must select only one top level field."
-                        ]
-                    , locations = [location']
-                    }
-                | otherwise -> pure $ Error
-                    { message = errorMessage
-                    , locations = [location']
-                    }
+        case HashSet.toList groupedFieldSet of
+            [rootName]
+                | Text.isPrefixOf "__" rootName -> makeError location' name'
+                    "exactly one top level field, which must not be an introspection field."
+                | otherwise -> lift mempty
+            [] -> makeError location' name' "exactly one top level field."
+            _ -> makeError location' name' "only one top level field."
    _ -> lift mempty
  where
-    errorMessage =
-        "Anonymous Subscription must select only one top level field."
-    collectFields selectionSet = foldM forEach HashSet.empty selectionSet
+    makeError location' (Just operationName) errorLine = pure $ Error
+        { message = concat
+            [ "Subscription \""
+            , Text.unpack operationName
+            , "\" must select "
+            , errorLine
+            ]
+        , locations = [location']
+        }
+    makeError location' Nothing errorLine = pure $ Error
+        { message = "Anonymous Subscription must select " <> errorLine
+        , locations = [location']
+        }
+    collectFields = foldM forEach HashSet.empty
    forEach accumulator = \case
        Full.FieldSelection fieldSelection -> forField accumulator fieldSelection
        Full.FragmentSpreadSelection fragmentSelection ->
@ -250,14 +257,16 @@ findDuplicates :: (Full.Definition -> [Full.Location] -> [Full.Location])
    -> Full.Location
    -> String
    -> RuleT m
-findDuplicates filterByName thisLocation errorMessage = do
-    ast' <- asks ast
-    let locations' = foldr filterByName [] ast'
-    if length locations' > 1 && head locations' == thisLocation
-        then pure $ error' locations'
-        else lift mempty
+findDuplicates filterByName thisLocation errorMessage =
+    asks ast >>= go . foldr filterByName []
  where
-    error' locations' = Error 
+    go locations' =
+        case locations' of
+            headLocation : otherLocations -- length locations' > 1
+                | not $ null otherLocations
+                , headLocation == thisLocation -> pure $ makeError locations'
+            _ -> lift mempty
+    makeError locations' = Error
        { message = errorMessage
        , locations = locations'
        }
@ -472,7 +481,7 @@ noFragmentCyclesRule = FragmentDefinitionRule $ \case
    collectCycles :: Traversable t
        => t Full.Selection
        -> StateT (Int, Full.Name) (ReaderT (Validation m) Seq) (HashMap Full.Name Int)
-    collectCycles selectionSet = foldM forEach HashMap.empty selectionSet
+    collectCycles = foldM forEach HashMap.empty
    forEach accumulator = \case
        Full.FieldSelection fieldSelection -> forField accumulator fieldSelection
        Full.InlineFragmentSelection fragmentSelection ->
@ -527,16 +536,20 @@ uniqueArgumentNamesRule = ArgumentsRule fieldRule directiveRule
 -- used, the expected metadata or behavior becomes ambiguous, therefore only one
 -- of each directive is allowed per location.
 uniqueDirectiveNamesRule :: forall m. Rule m
-uniqueDirectiveNamesRule = DirectivesRule
-    $ const $ lift . filterDuplicates extract "directive"
+uniqueDirectiveNamesRule = DirectivesRule $ const $ \directives' -> do
+    definitions' <- asks $ Schema.directives . schema
+    let filterNonRepeatable = flip HashSet.member nonRepeatableSet
+            . getField @"name"
+        nonRepeatableSet =
+            HashMap.foldlWithKey foldNonRepeatable HashSet.empty definitions'
+    lift $ filterDuplicates extract "directive"
+        $ filter filterNonRepeatable directives'
  where
-    extract (Full.Directive directiveName _ location') =
-        (directiveName, location')
-
-groupSorted :: forall a. (a -> Text) -> [a] -> [[a]]
-groupSorted getName = groupBy equalByName . sortOn getName
-  where
-    equalByName lhs rhs = getName lhs == getName rhs
+    foldNonRepeatable  hashSet directiveName' (Schema.Directive _ _ False _) =
+        HashSet.insert directiveName' hashSet
+    foldNonRepeatable hashSet _ _ = hashSet
+    extract (Full.Directive directiveName' _ location') =
+        (directiveName', location')

 filterDuplicates :: forall a
    . (a -> (Text, Full.Location))
@ -546,12 +559,12 @@ filterDuplicates :: forall a
 filterDuplicates extract nodeType = Seq.fromList
    . fmap makeError
    . filter ((> 1) . length)
-    . groupSorted getName
+    . NonEmpty.groupAllWith getName
  where
    getName = fst . extract
    makeError directives' = Error
-        { message = makeMessage $ head directives'
-        , locations = snd . extract <$> directives'
+        { message = makeMessage $ NonEmpty.head directives'
+        , locations = snd . extract <$> toList directives'
        }
    makeMessage directive = concat
        [ "There can be only one "
@ -618,6 +631,10 @@ noUndefinedVariablesRule =
        , "\"."
        ]

+-- Used to find the difference between defined and used variables. The first
+-- argument are variables defined in the operation, the second argument are
+-- variables used in the query. It should return the difference between these
+-- 2 sets.
 type UsageDifference
    = HashMap Full.Name [Full.Location]
    -> HashMap Full.Name [Full.Location]
@ -664,11 +681,17 @@ variableUsageDifference difference errorMessage = OperationDefinitionRule $ \cas
        = filterSelections' selections
        >>= lift . mapReaderT (<> mapDirectives directives') . pure
    findDirectiveVariables (Full.Directive _ arguments _) = mapArguments arguments
-    mapArguments = Seq.fromList . mapMaybe findArgumentVariables
+    mapArguments = Seq.fromList . (>>= findArgumentVariables)
    mapDirectives = foldMap findDirectiveVariables
-    findArgumentVariables (Full.Argument _ Full.Node{ node = Full.Variable value', ..} _) =
-        Just (value', [location])
-    findArgumentVariables _ = Nothing
+
+    findArgumentVariables (Full.Argument _ value _) = findNodeVariables value
+    findNodeVariables Full.Node{ node = value, ..} = findValueVariables location value
+
+    findValueVariables location (Full.Variable value') = [(value', [location])]
+    findValueVariables _ (Full.List values) = values >>= findNodeVariables
+    findValueVariables _ (Full.Object fields) = fields
+        >>= findNodeVariables . getField @"value"
+    findValueVariables _ _ = []
    makeError operationName (variableName, locations') = Error
        { message = errorMessage operationName variableName
        , locations = locations'
@ -702,8 +725,7 @@ uniqueInputFieldNamesRule =
  where
    go (Full.Node (Full.Object fields) _) = filterFieldDuplicates fields
    go _ = mempty
-    filterFieldDuplicates fields =
-        filterDuplicates getFieldName "input field" fields
+    filterFieldDuplicates = filterDuplicates getFieldName "input field"
    getFieldName (Full.ObjectField fieldName _ location') = (fieldName, location')
    constGo (Full.Node (Full.ConstObject fields) _) = filterFieldDuplicates fields
    constGo _ = mempty
@ -821,7 +843,7 @@ knownArgumentNamesRule = ArgumentsRule fieldRule directiveRule
            . Schema.directives . schema
        Full.Argument argumentName _ location' <- lift $ Seq.fromList arguments
        case available of
-            Just (Schema.Directive _ _ definitions)
+            Just (Schema.Directive _ definitions _ _)
                | not $ HashMap.member argumentName definitions ->
                    pure $ makeError argumentName directiveName location'
            _ -> lift mempty
@ -837,23 +859,23 @@ knownArgumentNamesRule = ArgumentsRule fieldRule directiveRule
        , "\"."
        ]

-- | GraphQL servers define what directives they support. For each usage of a
-- directive, the directive must be available on that server.
+-- | GraphQL services define what directives they support. For each usage of a
+-- directive, the directive must be available on that service.
 knownDirectiveNamesRule :: Rule m
 knownDirectiveNamesRule = DirectivesRule $ const $ \directives' -> do
    definitions' <- asks $ Schema.directives . schema
-    let directiveSet = HashSet.fromList $ fmap directiveName directives'
-    let definitionSet = HashSet.fromList $ HashMap.keys definitions'
-    let difference = HashSet.difference directiveSet definitionSet
-    let undefined' = filter (definitionFilter difference) directives'
+    let directiveSet = HashSet.fromList $ fmap (getField @"name") directives'
+        definitionSet = HashSet.fromList $ HashMap.keys definitions'
+        difference = HashSet.difference directiveSet definitionSet
+        undefined' = filter (definitionFilter difference) directives'
    lift $ Seq.fromList $ makeError <$> undefined'
  where
+    definitionFilter :: HashSet Full.Name -> Full.Directive -> Bool
    definitionFilter difference = flip HashSet.member difference
-        . directiveName
-    directiveName (Full.Directive directiveName' _ _) = directiveName'
-    makeError (Full.Directive directiveName' _ location') = Error
-        { message = errorMessage directiveName'
-        , locations = [location']
+        . getField @"name"
+    makeError Full.Directive{..} = Error
+        { message = errorMessage name
+        , locations = [location]
        }
    errorMessage directiveName' = concat
        [ "Unknown directive \"@"
@ -890,9 +912,9 @@ knownInputFieldNamesRule = ValueRule go constGo
        , "\"."
        ]

-- | GraphQL servers define what directives they support and where they support
+-- | GraphQL services define what directives they support and where they support
 -- them. For each usage of a directive, the directive must be used in a location
-- that the server has declared support for.
+-- that the service has declared support for.
 directivesInValidLocationsRule :: Rule m
 directivesInValidLocationsRule = DirectivesRule directivesRule
  where
@ -901,7 +923,7 @@ directivesInValidLocationsRule = DirectivesRule directivesRule
        maybeDefinition <- asks
            $ HashMap.lookup directiveName . Schema.directives . schema
        case maybeDefinition of
-            Just (Schema.Directive _ allowedLocations _)
+            Just (Schema.Directive _ _ _ allowedLocations)
                | directiveLocation `notElem` allowedLocations -> pure $ Error
                    { message = errorMessage directiveName directiveLocation
                    , locations = [location]
@ -931,7 +953,7 @@ providedRequiredArgumentsRule = ArgumentsRule fieldRule directiveRule
        available <- asks
            $ HashMap.lookup directiveName . Schema.directives . schema
        case available of
-            Just (Schema.Directive _ _ definitions) ->
+            Just (Schema.Directive _ definitions _ _) ->
                let forEach = go (directiveMessage directiveName) arguments location'
                 in lift $ HashMap.foldrWithKey forEach Seq.empty definitions
            _ -> lift mempty
@ -1045,18 +1067,12 @@ overlappingFieldsCanBeMergedRule = OperationDefinitionRule $ \case
    go selectionSet selectionType = do
        fieldTuples <- evalStateT (collectFields selectionType selectionSet) HashSet.empty
        fieldsInSetCanMerge fieldTuples
-    fieldsInSetCanMerge :: forall m
-        . HashMap Full.Name (NonEmpty (Full.Field, Type.CompositeType m))
-        -> ReaderT (Validation m) Seq Error
    fieldsInSetCanMerge fieldTuples = do
        validation <- ask
        let (lonely, paired) = flattenPairs fieldTuples
        let reader = flip runReaderT validation
        lift $ foldMap (reader . visitLonelyFields) lonely
            <> foldMap (reader . forEachFieldTuple) paired
-    forEachFieldTuple :: forall m
-        . (FieldInfo m, FieldInfo m)
-        -> ReaderT (Validation m) Seq Error
    forEachFieldTuple (fieldA, fieldB) =
        case (parent fieldA, parent fieldB) of
            (parentA@Type.CompositeObjectType{}, parentB@Type.CompositeObjectType{})
@ -1083,10 +1099,6 @@ overlappingFieldsCanBeMergedRule = OperationDefinitionRule $ \case
        let Full.Field _ _ _ _ subSelections _ = node
            compositeFieldType = Type.outToComposite type'
         in maybe (lift Seq.empty) (go subSelections) compositeFieldType
-    sameResponseShape :: forall m
-        . FieldInfo m
-        -> FieldInfo m
-        -> ReaderT (Validation m) Seq Error
    sameResponseShape fieldA fieldB =
        let Full.Field _ _ _ _ selectionsA _ = node fieldA
            Full.Field _ _ _ _ selectionsB _ = node fieldB
@ -1331,8 +1343,8 @@ variablesInAllowedPositionRule = OperationDefinitionRule $ \case
        -> Type.CompositeType m
        -> t Full.Selection
        -> ValidationState m (Seq Error)
-    visitSelectionSet variables selectionType selections =
-        foldM (evaluateSelection variables selectionType) mempty selections
+    visitSelectionSet variables selectionType =
+        foldM (evaluateSelection variables selectionType) mempty
    evaluateFieldSelection variables selections accumulator = \case
        Just newParentType -> do
            let folder = evaluateSelection variables newParentType
@ -1399,7 +1411,7 @@ variablesInAllowedPositionRule = OperationDefinitionRule $ \case
        let Full.Directive directiveName arguments _ = directive
        directiveDefinitions <- lift $ asks $ Schema.directives . schema
        case HashMap.lookup directiveName directiveDefinitions of
-            Just (Schema.Directive _ _ directiveArguments) ->
+            Just (Schema.Directive _ directiveArguments _ _) ->
                mapArguments variables directiveArguments arguments
            Nothing -> pure mempty
    mapArguments variables argumentTypes = fmap fold
@ -1502,15 +1514,6 @@ variablesInAllowedPositionRule = OperationDefinitionRule $ \case
    hasNonNullVariableDefaultValue (Just (Full.Node Full.ConstNull _)) = False
    hasNonNullVariableDefaultValue Nothing = False
    hasNonNullVariableDefaultValue _ = True
-    unwrapInType (In.NonNullScalarType nonNullType) =
-        Just $ In.NamedScalarType nonNullType
-    unwrapInType (In.NonNullEnumType nonNullType) =
-        Just $ In.NamedEnumType nonNullType
-    unwrapInType (In.NonNullInputObjectType nonNullType) =
-        Just $ In.NamedInputObjectType nonNullType
-    unwrapInType (In.NonNullListType nonNullType) =
-        Just $ In.ListType nonNullType
-    unwrapInType _ = Nothing
    makeError variableDefinition expectedType =
        let Full.VariableDefinition variableName variableType _ location' =
                variableDefinition
@ -1527,6 +1530,17 @@ variablesInAllowedPositionRule = OperationDefinitionRule $ \case
            , locations = [location']
            }

+unwrapInType :: In.Type -> Maybe In.Type
+unwrapInType (In.NonNullScalarType nonNullType) =
+    Just $ In.NamedScalarType nonNullType
+unwrapInType (In.NonNullEnumType nonNullType) =
+    Just $ In.NamedEnumType nonNullType
+unwrapInType (In.NonNullInputObjectType nonNullType) =
+    Just $ In.NamedInputObjectType nonNullType
+unwrapInType (In.NonNullListType nonNullType) =
+    Just $ In.ListType nonNullType
+unwrapInType _ = Nothing
+
 -- | Literal values must be compatible with the type expected in the position
 -- they are found as per the coercion rules.
 --
@ -1540,7 +1554,7 @@ valuesOfCorrectTypeRule = ValueRule go constGo
    go (Just inputType) value
        | Just constValue <- toConstNode value =
            lift $ check inputType constValue
-    go _ _ = lift mempty
+    go _ _ = lift mempty -- This rule checks only literals.
    toConstNode Full.Node{..} = flip Full.Node location <$> toConst node
    toConst (Full.Variable _) = Nothing
    toConst (Full.Int integer) = Just $ Full.ConstInt integer
@ -1550,9 +1564,9 @@ valuesOfCorrectTypeRule = ValueRule go constGo
    toConst Full.Null = Just Full.ConstNull
    toConst (Full.Enum enum) = Just $ Full.ConstEnum enum
    toConst (Full.List values) =
-        Just $ Full.ConstList $ catMaybes $ toConst <$> values
+        Just $ Full.ConstList $ mapMaybe toConstNode values
    toConst (Full.Object fields) = Just $ Full.ConstObject
-        $ catMaybes $ constObjectField <$> fields
+        $ mapMaybe constObjectField fields
    constObjectField Full.ObjectField{..}
        | Just constValue <- toConstNode value =
            Just $ Full.ObjectField name constValue location
@ -1582,24 +1596,36 @@ valuesOfCorrectTypeRule = ValueRule go constGo
        , Full.ConstEnum memberValue <- node
        , HashMap.member memberValue members = mempty
    check (In.InputObjectBaseType objectType) Full.Node{ node }
-        | In.InputObjectType _ _ typeFields <- objectType
-        , Full.ConstObject valueFields <- node =
-            foldMap (checkObjectField typeFields) valueFields
+        -- Skip, objects are checked recursively by the validation traverser.
+        | In.InputObjectType{}  <- objectType
+        , Full.ConstObject{} <- node = mempty
    check (In.ListBaseType listType) constValue@Full.Node{ .. }
-        | Full.ConstList listValues <- node =
-            foldMap (check listType) $ flip Full.Node location <$> listValues
+        | Full.ConstList values <- node =
+            foldMap (checkNull listType) values
        | otherwise = check listType constValue
    check inputType Full.Node{ .. } = pure $ Error
        { message = concat
            [ "Value "
-            , show node, " cannot be coerced to type \""
+            , show node
+            , " cannot be coerced to type \""
            , show inputType
            , "\"."
            ]
        , locations = [location]
        }
-    checkObjectField typeFields Full.ObjectField{..}
-        | Just typeField <- HashMap.lookup name typeFields
-        , In.InputField _ fieldType _ <- typeField =
-            check fieldType value
-    checkObjectField _ _ = mempty
+    checkNull inputType constValue =
+        let checkResult = check inputType constValue
+         in case null checkResult of
+            True
+                | Just unwrappedType <- unwrapInType inputType
+                , Full.Node{ node = Full.ConstNull, .. } <- constValue ->
+                    pure $ Error
+                        { message = concat
+                            [ "List of non-null values of type \""
+                            , show unwrappedType
+                            , "\" cannot contain null values."
+                            ]
+                        , locations = [location]
+                        }
+                | otherwise -> mempty
+            _ -> checkResult
--- a/src/Test/Hspec/GraphQL.hs
+++ b/src/Test/Hspec/GraphQL.hs
@ -1,41 +0,0 @@
-{- This Source Code Form is subject to the terms of the Mozilla Public License,
-   v. 2.0. If a copy of the MPL was not distributed with this file, You can
-   obtain one at https://mozilla.org/MPL/2.0/. -}
-
-{-# LANGUAGE ExplicitForAll #-}
-{-# LANGUAGE OverloadedStrings #-}
-
-- | Test helpers.
-module Test.Hspec.GraphQL
-    ( shouldResolve
-    , shouldResolveTo
-    ) where
-
-import Control.Monad.Catch (MonadCatch)
-import qualified Data.Aeson as Aeson
-import qualified Data.HashMap.Strict as HashMap
-import Data.Text (Text)
-import Language.GraphQL.Error
-import Test.Hspec.Expectations (Expectation, expectationFailure, shouldBe, shouldNotSatisfy)
-
-- | Asserts that a query resolves to some value.
-shouldResolveTo :: MonadCatch m
-    => Either (ResponseEventStream m Aeson.Value) Aeson.Object
-    -> Aeson.Object
-    -> Expectation
-shouldResolveTo (Right actual) expected = actual `shouldBe` expected
-shouldResolveTo _ _ = expectationFailure
-    "the query is expected to resolve to a value, but it resolved to an event stream"
-
-- | Asserts that the response doesn't contain any errors.
-shouldResolve :: MonadCatch m
-    => (Text -> IO (Either (ResponseEventStream m Aeson.Value) Aeson.Object))
-    -> Text
-    -> Expectation
-shouldResolve executor query = do
-    actual <- executor query
-    case actual of
-        Right response ->
-            response `shouldNotSatisfy` HashMap.member "errors"
-        _ -> expectationFailure
-            "the query is expected to resolve to a value, but it resolved to an event stream"
--- a/stack.yaml
+++ b/stack.yaml
@ -1,9 +0,0 @@
-resolver: lts-17.2
-
-packages:
- .
-
-extra-deps: []
-flags: {}
-
-pvp-bounds: lower
--- a/tests/Language/GraphQL/AST/Arbitrary.hs
+++ b/tests/Language/GraphQL/AST/Arbitrary.hs
@ -0,0 +1,130 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+module Language.GraphQL.AST.Arbitrary
+    ( AnyArgument(..)
+    , AnyLocation(..)
+    , AnyName(..)
+    , AnyNode(..)
+    , AnyObjectField(..)
+    , AnyValue(..)
+    , printArgument
+    ) where
+
+import qualified Language.GraphQL.AST.Document as Doc
+import Test.QuickCheck.Arbitrary (Arbitrary (arbitrary))
+import Test.QuickCheck (oneof, elements, listOf, resize, NonEmptyList (..))
+import Test.QuickCheck.Gen (Gen (..))
+import Data.Text (Text)
+import qualified Data.Text as Text
+import Data.Functor ((<&>))
+
+newtype AnyPrintableChar = AnyPrintableChar
+    { getAnyPrintableChar :: Char
+    } deriving (Eq, Show)
+
+alpha :: String
+alpha = ['a'..'z'] <> ['A'..'Z']
+
+num :: String
+num = ['0'..'9']
+
+instance Arbitrary AnyPrintableChar where
+    arbitrary = AnyPrintableChar <$> elements chars
+        where
+           chars = alpha <> num <> ['_']
+
+newtype AnyPrintableText = AnyPrintableText
+    { getAnyPrintableText :: Text
+    } deriving (Eq, Show)
+
+instance Arbitrary AnyPrintableText where
+    arbitrary = do
+        nonEmptyStr <- getNonEmpty <$> (arbitrary :: Gen (NonEmptyList AnyPrintableChar))
+        pure $ AnyPrintableText
+            $ Text.pack
+            $ map getAnyPrintableChar nonEmptyStr
+
+-- https://spec.graphql.org/June2018/#Name
+newtype AnyName = AnyName
+    { getAnyName :: Text
+    } deriving (Eq, Show)
+
+instance Arbitrary AnyName where
+    arbitrary = do
+        firstChar <- elements $ alpha <> ['_']
+        rest <- (arbitrary :: Gen [AnyPrintableChar])
+        pure $ AnyName
+            $ Text.pack
+            $ firstChar : map getAnyPrintableChar rest
+
+newtype AnyLocation = AnyLocation
+    { getAnyLocation :: Doc.Location
+    } deriving (Eq, Show)
+
+instance Arbitrary AnyLocation where
+    arbitrary = AnyLocation <$> (Doc.Location <$> arbitrary <*> arbitrary)
+
+newtype AnyNode a = AnyNode
+    { getAnyNode :: Doc.Node a
+    } deriving (Eq, Show)
+
+instance Arbitrary a => Arbitrary (AnyNode a) where
+    arbitrary = do
+        (AnyLocation location') <- arbitrary
+        node' <- flip Doc.Node location' <$> arbitrary
+        pure $ AnyNode node'
+
+newtype AnyObjectField a = AnyObjectField
+    { getAnyObjectField :: Doc.ObjectField a
+    } deriving (Eq, Show)
+
+instance Arbitrary a => Arbitrary (AnyObjectField a) where
+    arbitrary = do
+        name' <- getAnyName <$> arbitrary
+        value' <- getAnyNode <$> arbitrary
+        location' <- getAnyLocation <$> arbitrary
+        pure $ AnyObjectField $ Doc.ObjectField name' value' location'
+
+newtype AnyValue = AnyValue
+    { getAnyValue :: Doc.Value
+    } deriving (Eq, Show)
+
+instance Arbitrary AnyValue
+  where
+    arbitrary =
+        let variableGen :: Gen Doc.Value
+            variableGen = Doc.Variable . getAnyName <$> arbitrary
+            listGen :: Gen [Doc.Node Doc.Value]
+            listGen = (resize 5 . listOf) nodeGen
+            nodeGen :: Gen (Doc.Node Doc.Value)
+            nodeGen = fmap getAnyNode arbitrary <&> fmap getAnyValue
+            objectGen :: Gen [Doc.ObjectField Doc.Value]
+            objectGen = resize 1
+                $ fmap getNonEmpty arbitrary
+                <&> map (fmap getAnyValue . getAnyObjectField)
+         in AnyValue <$> oneof
+            [ variableGen
+            , Doc.Int <$> arbitrary
+            , Doc.Float <$> arbitrary
+            , Doc.String . getAnyPrintableText <$> arbitrary
+            , Doc.Boolean <$> arbitrary
+            , MkGen $ \_ _ -> Doc.Null
+            , Doc.Enum . getAnyName <$> arbitrary
+            , Doc.List <$> listGen
+            , Doc.Object <$> objectGen
+            ]
+
+newtype AnyArgument a = AnyArgument
+    { getAnyArgument :: Doc.Argument
+    } deriving (Eq, Show)
+
+instance Arbitrary a => Arbitrary (AnyArgument a) where
+    arbitrary = do
+        name' <- getAnyName <$> arbitrary
+        (AnyValue value') <- arbitrary
+        (AnyLocation location') <- arbitrary
+        pure $ AnyArgument $ Doc.Argument name' (Doc.Node value' location') location'
+
+printArgument :: AnyArgument AnyValue -> Text
+printArgument (AnyArgument (Doc.Argument name' (Doc.Node value' _) _)) =
+    name' <> ": " <> (Text.pack . show) value'
--- a/tests/Language/GraphQL/AST/DocumentSpec.hs
+++ b/tests/Language/GraphQL/AST/DocumentSpec.hs
@ -18,3 +18,9 @@ spec = do
                    ]
                expected = "{ field1: 1.2, field2: null }"
             in show object `shouldBe` expected
+
+    describe "Description" $
+        it "keeps content when merging with no description" $
+            let expected = Description $ Just "Left description"
+                actual = expected <> Description Nothing
+             in actual `shouldBe` expected
--- a/tests/Language/GraphQL/AST/EncoderSpec.hs
+++ b/tests/Language/GraphQL/AST/EncoderSpec.hs
@ -1,25 +1,22 @@
 {-# LANGUAGE OverloadedStrings #-}
-{-# LANGUAGE QuasiQuotes #-}
 module Language.GraphQL.AST.EncoderSpec
    ( spec
    ) where

+import Data.List.NonEmpty (NonEmpty(..))
 import qualified Language.GraphQL.AST.Document as Full
 import Language.GraphQL.AST.Encoder
 import Test.Hspec (Spec, context, describe, it, shouldBe, shouldStartWith, shouldEndWith, shouldNotContain)
 import Test.QuickCheck (choose, oneof, forAll)
-import Text.RawString.QQ (r)
-import Data.Text.Lazy (cons, toStrict, unpack)
+import qualified Data.Text.Lazy as Text.Lazy
+import qualified Language.GraphQL.AST.DirectiveLocation as DirectiveLocation

 spec :: Spec
 spec = do
    describe "value" $ do
-        context "null value" $ do
-            let testNull formatter = value formatter Full.Null `shouldBe` "null"
-            it "minified" $ testNull minified
-            it "pretty" $ testNull pretty
-
        context "minified" $ do
+            it "encodes null" $
+                value minified Full.Null `shouldBe` "null"
            it "escapes \\" $
                value minified (Full.String "\\") `shouldBe` "\"\\\\\""
            it "escapes double quotes" $
@ -45,79 +42,96 @@ spec = do
                it "~" $ value minified (Full.String "\x007E") `shouldBe` "\"~\""

        context "pretty" $ do
+            it "encodes null" $
+                value pretty Full.Null `shouldBe` "null"
+
            it "uses strings for short string values" $
                value pretty (Full.String "Short text") `shouldBe` "\"Short text\""
            it "uses block strings for text with new lines, with newline symbol" $
-                value pretty (Full.String "Line 1\nLine 2")
-                    `shouldBe` [r|"""
-  Line 1
-  Line 2
-"""|]
+                let expected = "\"\"\"\n\
+                        \  Line 1\n\
+                        \  Line 2\n\
+                        \\"\"\""
+                    actual = value pretty $ Full.String "Line 1\nLine 2"
+                 in actual `shouldBe` expected
            it "uses block strings for text with new lines, with CR symbol" $
-                value pretty (Full.String "Line 1\rLine 2")
-                    `shouldBe` [r|"""
-  Line 1
-  Line 2
-"""|]
+                let expected = "\"\"\"\n\
+                        \  Line 1\n\
+                        \  Line 2\n\
+                        \\"\"\""
+                    actual = value pretty $ Full.String "Line 1\rLine 2"
+                 in actual `shouldBe` expected
            it "uses block strings for text with new lines, with CR symbol followed by newline" $
-                value pretty (Full.String "Line 1\r\nLine 2")
-                    `shouldBe` [r|"""
-  Line 1
-  Line 2
-"""|]
+                let expected = "\"\"\"\n\
+                        \  Line 1\n\
+                        \  Line 2\n\
+                        \\"\"\""
+                    actual = value pretty $ Full.String "Line 1\r\nLine 2"
+                 in actual `shouldBe` expected
            it "encodes as one line string if has escaped symbols" $ do
-                let
-                  genNotAllowedSymbol = oneof
-                    [ choose ('\x0000', '\x0008')
-                    , choose ('\x000B', '\x000C')
-                    , choose ('\x000E', '\x001F')
-                    , pure '\x007F'
-                    ]
-
+                let genNotAllowedSymbol = oneof
+                        [ choose ('\x0000', '\x0008')
+                        , choose ('\x000B', '\x000C')
+                        , choose ('\x000E', '\x001F')
+                        , pure '\x007F'
+                        ]
                forAll genNotAllowedSymbol $ \x -> do
-                    let
-                      rawValue = "Short \n" <> cons x "text"
-                      encoded = value pretty (Full.String $ toStrict rawValue)
-                    shouldStartWith (unpack encoded) "\""
-                    shouldEndWith (unpack encoded) "\""
-                    shouldNotContain (unpack encoded) "\"\"\""
+                    let rawValue = "Short \n" <> Text.Lazy.cons x "text"
+                        encoded = Text.Lazy.unpack
+                            $ value pretty
+                            $ Full.String
+                            $ Text.Lazy.toStrict rawValue
+                    shouldStartWith encoded "\""
+                    shouldEndWith encoded "\""
+                    shouldNotContain encoded "\"\"\""

-            it "Hello world" $ value pretty (Full.String "Hello,\n  World!\n\nYours,\n  GraphQL.")
-              `shouldBe` [r|"""
-  Hello,
-    World!
+            it "Hello world" $
+                let actual = value pretty
+                        $ Full.String "Hello,\n  World!\n\nYours,\n  GraphQL."
+                    expected = "\"\"\"\n\
+                        \  Hello,\n\
+                        \    World!\n\
+                        \\n\
+                        \  Yours,\n\
+                        \    GraphQL.\n\
+                        \\"\"\""
+                  in actual `shouldBe` expected

-  Yours,
-    GraphQL.
-"""|]
-
-            it "has only newlines" $ value pretty (Full.String "\n") `shouldBe` [r|"""
-
-
-"""|]
+            it "has only newlines" $
+                let actual = value pretty $ Full.String "\n"
+                    expected = "\"\"\"\n\n\n\"\"\""
+                 in actual `shouldBe` expected
            it "has newlines and one symbol at the begining" $
-              value pretty (Full.String "a\n\n") `shouldBe` [r|"""
-  a
-
-
-"""|]
+                let actual = value pretty $ Full.String "a\n\n"
+                    expected = "\"\"\"\n\
+                        \  a\n\
+                        \\n\
+                        \\n\
+                        \\"\"\""
+                 in actual `shouldBe` expected
            it "has newlines and one symbol at the end" $
-              value pretty (Full.String "\n\na") `shouldBe` [r|"""
-
-
-  a
-"""|]
+                let actual = value pretty $ Full.String "\n\na"
+                    expected = "\"\"\"\n\
+                        \\n\
+                        \\n\
+                        \  a\n\
+                        \\"\"\""
+                 in actual `shouldBe` expected
            it "has newlines and one symbol in the middle" $
-              value pretty (Full.String "\na\n") `shouldBe` [r|"""
-
-  a
-
-"""|]
-            it "skip trailing whitespaces" $ value pretty (Full.String "  Short\ntext    ")
-              `shouldBe` [r|"""
-  Short
-  text
-"""|]
+                let actual = value pretty $ Full.String "\na\n"
+                    expected = "\"\"\"\n\
+                        \\n\
+                        \  a\n\
+                        \\n\
+                        \\"\"\""
+                 in actual `shouldBe` expected
+            it "skip trailing whitespaces" $
+                let actual = value pretty $ Full.String "  Short\ntext    "
+                    expected = "\"\"\"\n\
+                        \  Short\n\
+                        \  text\n\
+                        \\"\"\""
+                 in actual `shouldBe` expected

    describe "definition" $
        it "indents block strings in arguments" $
@ -128,10 +142,113 @@ spec = do
                fieldSelection = pure $ Full.FieldSelection field
                operation = Full.DefinitionOperation
                    $ Full.SelectionSet fieldSelection location
-             in definition pretty operation `shouldBe` [r|{
-  field(message: """
-    line1
-    line2
-  """)
-}
-|]
+                expected = "{\n\
+                    \  field(message: \"\"\"\n\
+                    \    line1\n\
+                    \    line2\n\
+                    \  \"\"\")\n\
+                    \}\n"
+                actual = definition pretty operation
+             in actual `shouldBe` expected
+
+    describe "operationType" $
+        it "produces lowercase mutation operation type" $
+            let actual = operationType pretty Full.Mutation
+             in actual `shouldBe` "mutation"
+
+    describe "typeSystemDefinition" $ do
+        it "produces a schema with an indented operation type definition" $
+            let queryType = Full.OperationTypeDefinition Full.Query "QueryRootType"
+                mutationType = Full.OperationTypeDefinition Full.Mutation "MutationType"
+                operations = queryType :| pure mutationType
+                definition' = Full.SchemaDefinition [] operations
+                expected = "schema {\n\
+                    \  query: QueryRootType\n\
+                    \  mutation: MutationType\n\
+                    \}\n"
+                actual = typeSystemDefinition pretty definition'
+             in actual `shouldBe` expected
+
+        it "encodes a scalar type definition" $
+            let uuidType = Full.ScalarTypeDefinition mempty "UUID" mempty
+                definition' = Full.TypeDefinition uuidType
+                expected = "scalar UUID"
+                actual = typeSystemDefinition pretty definition'
+             in actual `shouldBe` expected
+
+        it "encodes an interface definition" $
+            let someType = Full.TypeNamed "String"
+                argument = Full.InputValueDefinition mempty "arg" someType Nothing mempty
+                arguments = Full.ArgumentsDefinition [argument]
+                definition' = Full.TypeDefinition
+                    $ Full.InterfaceTypeDefinition mempty "UUID" (Full.ImplementsInterfaces []) mempty
+                    $ pure
+                    $ Full.FieldDefinition mempty "value" arguments someType mempty
+                expected = "interface UUID {\n\
+                    \  value(arg: String): String\n\
+                    \}"
+                actual = typeSystemDefinition pretty definition'
+             in actual `shouldBe` expected
+
+        it "encodes an union definition" $
+            let definition' = Full.TypeDefinition
+                    $ Full.UnionTypeDefinition mempty "SearchResult" mempty
+                    $ Full.UnionMemberTypes ["Photo", "Person"]
+                expected = "union SearchResult =\n\
+                    \  | Photo\n\
+                    \  | Person"
+                actual = typeSystemDefinition pretty definition'
+             in actual `shouldBe` expected
+
+        it "encodes an enum definition" $
+            let values =
+                    [ Full.EnumValueDefinition mempty "NORTH" mempty
+                    , Full.EnumValueDefinition mempty "EAST" mempty
+                    , Full.EnumValueDefinition mempty "SOUTH" mempty
+                    , Full.EnumValueDefinition mempty "WEST" mempty
+                    ]
+                definition' = Full.TypeDefinition
+                    $ Full.EnumTypeDefinition mempty "Direction" mempty values
+                expected = "enum Direction {\n\
+                    \  NORTH\n\
+                    \  EAST\n\
+                    \  SOUTH\n\
+                    \  WEST\n\
+                    \}"
+                actual = typeSystemDefinition pretty definition'
+             in actual `shouldBe` expected
+
+        it "encodes an input type" $
+            let intType = Full.TypeNonNull $ Full.NonNullTypeNamed "Int"
+                stringType = Full.TypeNamed "String"
+                fields =
+                    [ Full.InputValueDefinition mempty "a" stringType Nothing mempty
+                    , Full.InputValueDefinition mempty "b" intType Nothing mempty
+                    ]
+                definition' = Full.TypeDefinition
+                    $ Full.InputObjectTypeDefinition mempty "ExampleInputObject" mempty fields
+                expected = "input ExampleInputObject {\n\
+                    \  a: String\n\
+                    \  b: Int!\n\
+                    \}"
+                actual = typeSystemDefinition pretty definition'
+             in actual `shouldBe` expected
+
+        context "directive definition" $ do
+            it "encodes a directive definition" $ do
+                let definition' = Full.DirectiveDefinition mempty "example" mempty False
+                        $ pure
+                        $ DirectiveLocation.ExecutableDirectiveLocation DirectiveLocation.Field
+                    expected = "@example() on\n\
+                        \  | FIELD"
+                    actual = typeSystemDefinition pretty definition'
+                 in actual `shouldBe` expected
+
+            it "encodes a repeatable directive definition" $ do
+                let definition' = Full.DirectiveDefinition mempty "example" mempty True
+                        $ pure
+                        $ DirectiveLocation.ExecutableDirectiveLocation DirectiveLocation.Field
+                    expected = "@example() repeatable on\n\
+                        \  | FIELD"
+                    actual = typeSystemDefinition pretty definition'
+                 in actual `shouldBe` expected
--- a/tests/Language/GraphQL/AST/LexerSpec.hs
+++ b/tests/Language/GraphQL/AST/LexerSpec.hs
@ -1,5 +1,4 @@
 {-# LANGUAGE OverloadedStrings #-}
-{-# LANGUAGE QuasiQuotes #-}
 module Language.GraphQL.AST.LexerSpec
    ( spec
    ) where
@ -10,7 +9,6 @@ import Language.GraphQL.AST.Lexer
 import Test.Hspec (Spec, context, describe, it)
 import Test.Hspec.Megaparsec (shouldParse, shouldFailOn, shouldSucceedOn)
 import Text.Megaparsec (ParseErrorBundle, parse)
-import Text.RawString.QQ (r)

 spec :: Spec
 spec = describe "Lexer" $ do
@ -19,38 +17,39 @@ spec = describe "Lexer" $ do
            parse unicodeBOM "" `shouldSucceedOn` "\xfeff"

        it "lexes strings" $ do
-            parse string "" [r|"simple"|] `shouldParse` "simple"
-            parse string "" [r|" white space "|] `shouldParse` " white space "
-            parse string "" [r|"quote \""|] `shouldParse` [r|quote "|]
-            parse string "" [r|"escaped \n"|] `shouldParse` "escaped \n"
-            parse string "" [r|"slashes \\ \/"|] `shouldParse` [r|slashes \ /|]
-            parse string "" [r|"unicode \u1234\u5678\u90AB\uCDEF"|]
+            parse string "" "\"simple\"" `shouldParse` "simple"
+            parse string "" "\" white space \"" `shouldParse` " white space "
+            parse string "" "\"quote \\\"\"" `shouldParse` "quote \""
+            parse string "" "\"escaped \\n\"" `shouldParse` "escaped \n"
+            parse string "" "\"slashes \\\\ \\/\"" `shouldParse` "slashes \\ /"
+            parse string "" "\"unicode \\u1234\\u5678\\u90AB\\uCDEF\""
                `shouldParse` "unicode ሴ噸邫췯"

        it "lexes block string" $ do
-            parse blockString "" [r|"""simple"""|] `shouldParse` "simple"
-            parse blockString "" [r|""" white space """|]
+            parse blockString "" "\"\"\"simple\"\"\"" `shouldParse` "simple"
+            parse blockString "" "\"\"\" white space \"\"\""
                `shouldParse` " white space "
-            parse blockString "" [r|"""contains " quote"""|]
-                `shouldParse` [r|contains " quote|]
-            parse blockString "" [r|"""contains \""" triplequote"""|]
-                `shouldParse` [r|contains """ triplequote|]
+            parse blockString "" "\"\"\"contains \" quote\"\"\""
+                `shouldParse` "contains \" quote"
+            parse blockString "" "\"\"\"contains \\\"\"\" triplequote\"\"\""
+                `shouldParse` "contains \"\"\" triplequote"
            parse blockString "" "\"\"\"multi\nline\"\"\"" `shouldParse` "multi\nline"
            parse blockString "" "\"\"\"multi\rline\r\nnormalized\"\"\""
                `shouldParse` "multi\nline\nnormalized"
            parse blockString "" "\"\"\"multi\rline\r\nnormalized\"\"\""
                `shouldParse` "multi\nline\nnormalized"
-            parse blockString "" [r|"""unescaped \n\r\b\t\f\u1234"""|]
-                `shouldParse` [r|unescaped \n\r\b\t\f\u1234|]
-            parse blockString "" [r|"""slashes \\ \/"""|]
-                `shouldParse` [r|slashes \\ \/|]
-            parse blockString "" [r|"""
-
-                spans
-                  multiple
-                    lines
-
-                """|] `shouldParse` "spans\n  multiple\n    lines"
+            parse blockString "" "\"\"\"unescaped \\n\\r\\b\\t\\f\\u1234\"\"\""
+                `shouldParse` "unescaped \\n\\r\\b\\t\\f\\u1234"
+            parse blockString "" "\"\"\"slashes \\\\ \\/\"\"\""
+                `shouldParse` "slashes \\\\ \\/"
+            parse blockString "" "\"\"\"\n\
+                \\n\
+                \  spans\n\
+                \    multiple\n\
+                \      lines\n\
+                \\n\
+                \\"\"\""
+                `shouldParse` "spans\n  multiple\n    lines"

        it "lexes numbers" $ do
            parse integer "" "4" `shouldParse` (4 :: Int)
@ -84,7 +83,7 @@ spec = describe "Lexer" $ do

    context "Implementation tests" $ do
        it "lexes empty block strings" $
-            parse blockString "" [r|""""""|] `shouldParse` ""
+            parse blockString "" "\"\"\"\"\"\"" `shouldParse` ""
        it "lexes ampersand" $
            parse amp "" "&" `shouldParse` "&"
        it "lexes schema extensions" $
--- a/tests/Language/GraphQL/AST/ParserSpec.hs
+++ b/tests/Language/GraphQL/AST/ParserSpec.hs
@ -1,131 +1,184 @@
 {-# LANGUAGE OverloadedStrings #-}
-{-# LANGUAGE QuasiQuotes #-}
 module Language.GraphQL.AST.ParserSpec
    ( spec
    ) where

 import Data.List.NonEmpty (NonEmpty(..))
+import qualified Data.Text as Text
 import Language.GraphQL.AST.Document
+import qualified Language.GraphQL.AST.DirectiveLocation as DirLoc
 import Language.GraphQL.AST.Parser
-import Test.Hspec (Spec, describe, it)
-import Test.Hspec.Megaparsec (shouldParse, shouldFailOn, shouldSucceedOn)
+import Test.Hspec (Spec, describe, it, context)
+import Test.Hspec.Megaparsec
+    ( shouldParse
+    , shouldFailOn
+    , parseSatisfies
+    , shouldSucceedOn
+    )
 import Text.Megaparsec (parse)
-import Text.RawString.QQ (r)
+import Test.QuickCheck (property, NonEmptyList (..), mapSize)
+import Language.GraphQL.AST.Arbitrary

 spec :: Spec
 spec = describe "Parser" $ do
    it "accepts BOM header" $
        parse document "" `shouldSucceedOn` "\xfeff{foo}"

-    it "accepts block strings as argument" $
-        parse document "" `shouldSucceedOn` [r|{
-              hello(text: """Argument""")
-            }|]
+    context "Arguments" $ do
+        it "accepts block strings as argument" $
+            parse document "" `shouldSucceedOn`
+                "{ hello(text: \"\"\"Argument\"\"\") }"

-    it "accepts strings as argument" $
-        parse document "" `shouldSucceedOn` [r|{
-              hello(text: "Argument")
-            }|]
+        it "accepts strings as argument" $
+            parse document "" `shouldSucceedOn` "{ hello(text: \"Argument\") }"

-    it "accepts two required arguments" $
-        parse document "" `shouldSucceedOn` [r|
-            mutation auth($username: String!, $password: String!){
-              test
-            }|]
+        it "accepts int as argument" $
+            parse document "" `shouldSucceedOn` "{ user(id: 4) }"

-    it "accepts two string arguments" $
-        parse document "" `shouldSucceedOn` [r|
-            mutation auth{
-              test(username: "username", password: "password")
-            }|]
+        it "accepts boolean as argument" $
+            parse document "" `shouldSucceedOn`
+                "{ hello(flag: true) { field1 } }"

-    it "accepts two block string arguments" $
-        parse document "" `shouldSucceedOn` [r|
-            mutation auth{
-              test(username: """username""", password: """password""")
-            }|]
+        it "accepts float as argument" $
+            parse document "" `shouldSucceedOn`
+                "{ body(height: 172.5) { height } }"
+
+        it "accepts empty list as argument" $
+            parse document "" `shouldSucceedOn` "{ query(list: []) { field1 } }"
+
+        it "accepts two required arguments" $
+            parse document "" `shouldSucceedOn`
+                "mutation auth($username: String!, $password: String!) { test }"
+
+        it "accepts two string arguments" $
+            parse document "" `shouldSucceedOn`
+                "mutation auth { test(username: \"username\", password: \"password\") }"
+
+        it "accepts two block string arguments" $
+            let given = "mutation auth {\n\
+                    \  test(username: \"\"\"username\"\"\", password: \"\"\"password\"\"\")\n\
+                    \}"
+             in parse document "" `shouldSucceedOn` given
+
+        it "fails to parse an empty argument list in parens" $
+            parse document "" `shouldFailOn` "{ test() }"
+
+        it "accepts any arguments" $ mapSize (const 10) $ property $ \xs ->
+            let arguments' = map printArgument
+                    $ getNonEmpty (xs :: NonEmptyList (AnyArgument AnyValue))
+                query' = "query(" <> Text.intercalate ", " arguments' <> ")"
+             in parse document "" `shouldSucceedOn` ("{ " <> query' <> " }")

    it "parses minimal schema definition" $
-        parse document "" `shouldSucceedOn` [r|schema { query: Query }|]
+        parse document "" `shouldSucceedOn` "schema { query: Query }"

    it "parses minimal scalar definition" $
-        parse document "" `shouldSucceedOn` [r|scalar Time|]
+        parse document "" `shouldSucceedOn` "scalar Time"

    it "parses ImplementsInterfaces" $
-        parse document "" `shouldSucceedOn` [r|
-            type Person implements NamedEntity & ValuedEntity {
-              name: String
-            }
-        |]
+        parse document "" `shouldSucceedOn`
+            "type Person implements NamedEntity & ValuedEntity {\n\
+            \  name: String\n\
+            \}"

    it "parses a  type without ImplementsInterfaces" $
-        parse document "" `shouldSucceedOn` [r|
-            type Person {
-              name: String
-            }
-        |]
+        parse document "" `shouldSucceedOn`
+            "type Person {\n\
+            \  name: String\n\
+            \}"

    it "parses ArgumentsDefinition in an ObjectDefinition" $
-        parse document "" `shouldSucceedOn` [r|
-            type Person {
-              name(first: String, last: String): String
-            }
-        |]
+        parse document "" `shouldSucceedOn`
+            "type Person {\n\
+            \  name(first: String, last: String): String\n\
+            \}"

    it "parses minimal union type definition" $
-        parse document "" `shouldSucceedOn` [r|
-            union SearchResult = Photo | Person
-        |]
+        parse document "" `shouldSucceedOn`
+            "union SearchResult = Photo | Person"

    it "parses minimal interface type definition" $
-        parse document "" `shouldSucceedOn` [r|
-            interface NamedEntity {
-              name: String
-            }
-        |]
+        parse document "" `shouldSucceedOn`
+            "interface NamedEntity {\n\
+            \  name: String\n\
+            \}"
+
+    it "parses ImplementsInterfaces on interfaces" $
+        parse document "" `shouldSucceedOn`
+            "interface Person implements NamedEntity & ValuedEntity {\n\
+            \  name: String\n\
+            \}"

    it "parses minimal enum type definition" $
-        parse document "" `shouldSucceedOn` [r|
-            enum Direction {
-              NORTH
-              EAST
-              SOUTH
-              WEST
-            }
-        |]
-
-    it "parses minimal enum type definition" $
-        parse document "" `shouldSucceedOn` [r|
-            enum Direction {
-              NORTH
-              EAST
-              SOUTH
-              WEST
-            }
-        |]
+        parse document "" `shouldSucceedOn`
+            "enum Direction {\n\
+            \  NORTH\n\
+            \  EAST\n\
+            \  SOUTH\n\
+            \  WEST\n\
+            \}"

    it "parses minimal input object type definition" $
-        parse document "" `shouldSucceedOn` [r|
-            input Point2D {
-              x: Float
-              y: Float
-            }
-        |]
+        parse document "" `shouldSucceedOn`
+            "input Point2D {\n\
+            \  x: Float\n\
+            \  y: Float\n\
+            \}"

    it "parses minimal input enum definition with an optional pipe" $
-        parse document "" `shouldSucceedOn` [r|
-            directive @example on
-              | FIELD
-              | FRAGMENT_SPREAD
-        |]
+        parse document "" `shouldSucceedOn`
+            "directive @example on\n\
+            \  | FIELD\n\
+            \  | FRAGMENT_SPREAD"
+
+    it "parses two minimal directive definitions" $
+        let directive name' loc = TypeSystemDefinition
+                $ DirectiveDefinition
+                    (Description Nothing)
+                    name'
+                    (ArgumentsDefinition [])
+                    False
+                    (loc :| [])
+            example1 = directive "example1"
+                (DirLoc.TypeSystemDirectiveLocation DirLoc.FieldDefinition)
+                (Location {line = 1, column = 1})
+            example2 = directive "example2"
+                (DirLoc.ExecutableDirectiveLocation DirLoc.Field)
+                (Location {line = 2, column = 1})
+            testSchemaExtension = example1 :| [example2]
+            query = Text.unlines
+                [ "directive @example1 on FIELD_DEFINITION"
+                , "directive @example2 on FIELD"
+                ]
+         in parse document "" query `shouldParse` testSchemaExtension
+
+    it "parses a directive definition with a default empty list argument" $
+        let argumentValue = Just
+                $ Node (ConstList [])
+                $ Location{ line = 1, column = 33 }
+            loc = DirLoc.TypeSystemDirectiveLocation DirLoc.FieldDefinition
+            argumentValueDefinition = InputValueDefinition
+                (Description Nothing)
+                "foo"
+                (TypeList (TypeNamed "String"))
+                argumentValue
+                []
+            definition = DirectiveDefinition
+                (Description Nothing)
+                "test"
+                (ArgumentsDefinition [argumentValueDefinition])
+                False
+                (loc :| [])
+            directive = TypeSystemDefinition definition
+                $ Location{ line = 1, column = 1 }
+            query = "directive @test(foo: [String] = []) on FIELD_DEFINITION"
+         in parse document "" query `shouldParse` (directive :| [])

    it "parses schema extension with a new directive" $
-        parse document "" `shouldSucceedOn`[r|
-            extend schema @newDirective
-        |]
+        parse document "" `shouldSucceedOn` "extend schema @newDirective"

    it "parses schema extension with an operation type definition" $
-        parse document "" `shouldSucceedOn` [r|extend schema { query: Query }|]
+        parse document "" `shouldSucceedOn` "extend schema { query: Query }"

    it "parses schema extension with an operation type and directive" $
        let newDirective = Directive "newDirective" [] $ Location 1 15
@ -134,38 +187,42 @@ spec = describe "Parser" $ do
                $ OperationTypeDefinition Query "Query" :| []
            testSchemaExtension = TypeSystemExtension schemaExtension
                $ Location 1 1
-            query = [r|extend schema @newDirective { query: Query }|]
+            query = "extend schema @newDirective { query: Query }"
         in parse document "" query `shouldParse` (testSchemaExtension :| [])

+    it "parses a repeatable directive definition" $
+        let given = "directive @test repeatable on FIELD_DEFINITION"
+            isRepeatable (TypeSystemDefinition definition' _ :| [])
+                | DirectiveDefinition _ _ _ repeatable _ <- definition' = repeatable
+            isRepeatable _ = False
+         in parse document "" given `parseSatisfies` isRepeatable
+
    it "parses an object extension" $
-        parse document "" `shouldSucceedOn` [r|
-            extend type Story {
-              isHiddenLocally: Boolean
-            }
-        |]
+        parse document "" `shouldSucceedOn`
+            "extend type Story { isHiddenLocally: Boolean }"

    it "rejects variables in DefaultValue" $
-        parse document "" `shouldFailOn` [r|
-            query ($book: String = "Zarathustra", $author: String = $book) {
-              title
-            }
-        |]
+        parse document "" `shouldFailOn`
+            "query ($book: String = \"Zarathustra\", $author: String = $book) {\n\
+            \  title\n\
+            \}"
+
+    it "rejects empty selection set" $
+       parse document "" `shouldFailOn` "query { innerField {} }"

    it "parses documents beginning with a comment" $
-        parse document "" `shouldSucceedOn` [r|
-            """
-            Query
-            """
-            type Query {
-                queryField: String
-            }
-        |]
+        parse document "" `shouldSucceedOn`
+            "\"\"\"\n\
+            \Query\n\
+            \\"\"\"\n\
+            \type Query {\n\
+            \  queryField: String\n\
+            \}"

    it "parses subscriptions" $
-        parse document "" `shouldSucceedOn` [r|
-            subscription NewMessages {
-              newMessage(roomId: 123) {
-                sender
-              }
-            }
-        |]
+        parse document "" `shouldSucceedOn`
+            "subscription NewMessages {\n\
+            \  newMessage(roomId: 123) {\n\
+            \    sender\n\
+            \  }\n\
+            \}"
--- a/tests/Language/GraphQL/ErrorSpec.hs
+++ b/tests/Language/GraphQL/ErrorSpec.hs
@ -7,18 +7,30 @@ module Language.GraphQL.ErrorSpec
    ( spec
    ) where

-import qualified Data.Aeson as Aeson
-import qualified Data.Sequence as Seq
+import Data.List.NonEmpty (NonEmpty (..))
 import Language.GraphQL.Error
-import Test.Hspec ( Spec
-                  , describe
-                  , it
-                  , shouldBe
-                  )
+import qualified Language.GraphQL.Type as Type
+import Test.Hspec
+    ( Spec
+    , describe
+    , it
+    , shouldBe
+    )
+import Text.Megaparsec (PosState(..))
+import Text.Megaparsec.Error (ParseError(..), ParseErrorBundle(..))
+import Text.Megaparsec.Pos (SourcePos(..), mkPos)

 spec :: Spec
-spec = describe "singleError" $
-    it "constructs an error with the given message" $
-        let errors'' = Seq.singleton $ Error "Message." [] []
-            expected = Response Aeson.Null errors''
-         in singleError "Message." `shouldBe` expected
+spec = describe "parseError" $
+    it "generates response with a single error" $ do
+        let parseErrors = TrivialError 0 Nothing mempty :| []
+            posState = PosState
+                { pstateInput = ""
+                , pstateOffset = 0
+                , pstateSourcePos = SourcePos "" (mkPos 1) (mkPos 1)
+                , pstateTabWidth = mkPos 1
+                , pstateLinePrefix = ""
+                }
+        Response Type.Null actual <-
+            parseError (ParseErrorBundle parseErrors posState)
+        length actual `shouldBe` 1
--- a/tests/Language/GraphQL/Execute/CoerceSpec.hs
+++ b/tests/Language/GraphQL/Execute/CoerceSpec.hs
@ -7,12 +7,8 @@ module Language.GraphQL.Execute.CoerceSpec
    ( spec
    ) where

-import Data.Aeson as Aeson ((.=))
-import qualified Data.Aeson as Aeson
-import qualified Data.Aeson.Types as Aeson
 import qualified Data.HashMap.Strict as HashMap
 import Data.Maybe (isNothing)
-import Data.Scientific (scientific)
 import qualified Language.GraphQL.Execute.Coerce as Coerce
 import Language.GraphQL.Type
 import qualified Language.GraphQL.Type.In as In
@ -27,81 +23,11 @@ direction = EnumType "Direction" Nothing  $ HashMap.fromList
    , ("WEST", EnumValue Nothing)
    ]

-singletonInputObject :: In.Type
-singletonInputObject = In.NamedInputObjectType type'
-  where
-    type' = In.InputObjectType "ObjectName" Nothing inputFields
-    inputFields = HashMap.singleton "field" field
-    field = In.InputField Nothing (In.NamedScalarType string) Nothing
-
 namedIdType :: In.Type
 namedIdType = In.NamedScalarType id

 spec :: Spec
-spec = do
-    describe "VariableValue Aeson" $ do
-        it "coerces strings" $
-            let expected = Just (String "asdf")
-                actual = Coerce.coerceVariableValue
-                    (In.NamedScalarType string) (Aeson.String "asdf")
-             in actual `shouldBe` expected
-        it "coerces non-null strings" $
-            let expected = Just (String "asdf")
-                actual = Coerce.coerceVariableValue
-                    (In.NonNullScalarType string) (Aeson.String "asdf")
-             in actual `shouldBe` expected
-        it "coerces booleans" $
-            let expected = Just (Boolean True)
-                actual = Coerce.coerceVariableValue
-                    (In.NamedScalarType boolean) (Aeson.Bool True)
-             in actual `shouldBe` expected
-        it "coerces zero to an integer" $
-            let expected = Just (Int 0)
-                actual = Coerce.coerceVariableValue
-                    (In.NamedScalarType int) (Aeson.Number 0)
-             in actual `shouldBe` expected
-        it "rejects fractional if an integer is expected" $
-            let actual = Coerce.coerceVariableValue
-                    (In.NamedScalarType int) (Aeson.Number $ scientific 14 (-1))
-             in actual `shouldSatisfy` isNothing
-        it "coerces float numbers" $
-            let expected = Just (Float 1.4)
-                actual = Coerce.coerceVariableValue
-                    (In.NamedScalarType float) (Aeson.Number $ scientific 14 (-1))
-             in actual `shouldBe` expected
-        it "coerces IDs" $
-            let expected = Just (String "1234")
-                json = Aeson.String "1234"
-                actual = Coerce.coerceVariableValue namedIdType json
-             in actual `shouldBe` expected
-        it "coerces input objects" $
-            let actual = Coerce.coerceVariableValue singletonInputObject
-                    $ Aeson.object ["field" .= ("asdf" :: Aeson.Value)]
-                expected = Just $ Object $ HashMap.singleton "field" "asdf"
-             in actual `shouldBe` expected
-        it "skips the field if it is missing in the variables" $
-            let actual = Coerce.coerceVariableValue
-                    singletonInputObject Aeson.emptyObject
-                expected = Just $ Object HashMap.empty
-             in actual `shouldBe` expected
-        it "fails if input object value contains extra fields" $
-            let actual = Coerce.coerceVariableValue singletonInputObject
-                    $ Aeson.object variableFields
-                variableFields =
-                    [ "field" .= ("asdf" :: Aeson.Value)
-                    , "extra" .= ("qwer" :: Aeson.Value)
-                    ]
-             in actual `shouldSatisfy` isNothing
-        it "preserves null" $
-            let actual = Coerce.coerceVariableValue namedIdType Aeson.Null
-             in actual `shouldBe` Just Null
-        it "preserves list order" $
-            let list = Aeson.toJSONList ["asdf" :: Aeson.Value, "qwer"]
-                listType = (In.ListType $ In.NamedScalarType string)
-                actual = Coerce.coerceVariableValue listType list
-                expected = Just $ List [String "asdf", String "qwer"]
-             in actual `shouldBe` expected
-
+spec =
    describe "coerceInputLiteral" $ do
        it "coerces enums" $
            let expected = Just (Enum "NORTH")
--- a/tests/Language/GraphQL/Execute/OrderedMapSpec.hs
+++ b/tests/Language/GraphQL/Execute/OrderedMapSpec.hs
@ -0,0 +1,72 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
+{-# LANGUAGE OverloadedStrings #-}
+
+module Language.GraphQL.Execute.OrderedMapSpec
+    ( spec
+    ) where
+
+import Language.GraphQL.Execute.OrderedMap (OrderedMap)
+import qualified Language.GraphQL.Execute.OrderedMap as OrderedMap
+import Test.Hspec (Spec, describe, it, shouldBe, shouldSatisfy)
+
+spec :: Spec
+spec =
+    describe "OrderedMap" $ do
+        it "creates an empty map" $
+            (mempty :: OrderedMap String) `shouldSatisfy` null
+
+        it "creates a singleton" $
+            let value :: String
+                value = "value"
+             in OrderedMap.size (OrderedMap.singleton "key" value) `shouldBe` 1
+
+        it "combines inserted vales" $
+            let key = "key"
+                map1 = OrderedMap.singleton key ("1" :: String)
+                map2 = OrderedMap.singleton key ("2" :: String)
+             in OrderedMap.lookup key (map1 <> map2) `shouldBe` Just "12"
+
+        it "shows the map" $
+            let actual = show
+                    $ OrderedMap.insert "key1" "1"
+                    $ OrderedMap.singleton "key2" ("2" :: String)
+                expected = "fromList [(\"key2\",\"2\"),(\"key1\",\"1\")]"
+             in actual `shouldBe` expected
+
+        it "traverses a map of just values" $
+            let actual = sequence
+                    $ OrderedMap.insert "key1" (Just "2")
+                    $ OrderedMap.singleton "key2" $ Just ("1" :: String)
+                expected = Just
+                    $ OrderedMap.insert "key1" "2"
+                    $ OrderedMap.singleton "key2" ("1" :: String)
+             in actual `shouldBe` expected
+
+        it "traverses a map with a Nothing" $
+            let actual = sequence
+                    $ OrderedMap.insert "key1" Nothing
+                    $ OrderedMap.singleton "key2" $ Just ("1" :: String)
+                expected = Nothing
+             in actual `shouldBe` expected
+
+        it "combines two maps preserving the order of the second one" $
+            let map1 :: OrderedMap String
+                map1 = OrderedMap.insert "key2" "2"
+                    $ OrderedMap.singleton "key1" "1"
+                map2 :: OrderedMap String
+                map2 = OrderedMap.insert "key4" "4"
+                    $ OrderedMap.singleton "key3" "3"
+                expected = OrderedMap.insert "key4" "4"
+                    $ OrderedMap.insert "key3" "3"
+                    $ OrderedMap.insert "key2" "2"
+                    $ OrderedMap.singleton "key1" "1"
+             in (map1 <> map2) `shouldBe` expected
+
+        it "replaces existing values" $
+            let key = "key"
+                actual = OrderedMap.replace key ("2" :: String)
+                        $ OrderedMap.singleton key ("1" :: String)
+             in OrderedMap.lookup key actual `shouldBe` Just "2"
--- a/tests/Language/GraphQL/ExecuteSpec.hs
+++ b/tests/Language/GraphQL/ExecuteSpec.hs
@ -2,66 +2,207 @@
   v. 2.0. If a copy of the MPL was not distributed with this file, You can
   obtain one at https://mozilla.org/MPL/2.0/. -}

+{-# LANGUAGE DuplicateRecordFields #-}
+{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE NamedFieldPuns #-}
 {-# LANGUAGE OverloadedStrings #-}
-{-# LANGUAGE QuasiQuotes #-}
+
 module Language.GraphQL.ExecuteSpec
    ( spec
    ) where

-import Control.Exception (SomeException)
-import Data.Aeson ((.=))
-import qualified Data.Aeson as Aeson
-import Data.Aeson.Types (emptyObject)
+import Control.Exception (Exception(..))
+import Control.Monad.Catch (throwM)
 import Data.Conduit
 import Data.HashMap.Strict (HashMap)
 import qualified Data.HashMap.Strict as HashMap
-import Language.GraphQL.AST (Document, Name)
+import Data.Typeable (cast)
+import Language.GraphQL.AST (Document, Location(..), Name)
 import Language.GraphQL.AST.Parser (document)
 import Language.GraphQL.Error
-import Language.GraphQL.Execute
-import Language.GraphQL.Type as Type
-import Language.GraphQL.Type.Out as Out
+import Language.GraphQL.Execute (execute)
+import qualified Language.GraphQL.Type.Schema as Schema
+import qualified Language.GraphQL.Type as Type
+import Language.GraphQL.Type
+import qualified Language.GraphQL.Type.In as In
+import qualified Language.GraphQL.Type.Out as Out
+import Prelude hiding (id)
 import Test.Hspec (Spec, context, describe, it, shouldBe)
-import Text.Megaparsec (parse)
-import Text.RawString.QQ (r)
+import Text.Megaparsec (parse, errorBundlePretty)
+import Schemas.HeroSchema (heroSchema)
+import Data.Maybe (fromJust)
+import qualified Data.Sequence as Seq
+import Data.Text (Text)
+import qualified Data.Text as Text
+import Test.Hspec.Expectations
+    ( Expectation
+    , expectationFailure
+    )
+import Data.Either (fromRight)

-philosopherSchema :: Schema (Either SomeException)
-philosopherSchema = schema queryType Nothing (Just subscriptionType) mempty
+data PhilosopherException = PhilosopherException
+    deriving Show

-queryType :: Out.ObjectType (Either SomeException)
+instance Exception PhilosopherException where
+    toException = toException. ResolverException
+    fromException e = do
+        ResolverException resolverException <- fromException e
+        cast resolverException
+
+philosopherSchema :: Schema IO
+philosopherSchema =
+    schemaWithTypes Nothing queryType Nothing subscriptionRoot extraTypes mempty
+  where
+    subscriptionRoot = Just subscriptionType
+    extraTypes =
+        [ Schema.ObjectType bookType
+        , Schema.ObjectType bookCollectionType
+        ]
+
+queryType :: Out.ObjectType IO
 queryType = Out.ObjectType "Query" Nothing []
-    $ HashMap.singleton "philosopher" 
-    $ ValueResolver philosopherField
-    $ pure $ Type.Object mempty
+    $ HashMap.fromList
+    [ ("philosopher", ValueResolver philosopherField philosopherResolver)
+    , ("throwing", ValueResolver throwingField throwingResolver)
+    , ("count", ValueResolver countField countResolver)
+    , ("sequence", ValueResolver sequenceField sequenceResolver)
+    , ("withInputObject", ValueResolver withInputObjectField withInputObjectResolver)
+    ]
  where
    philosopherField =
-        Out.Field Nothing (Out.NonNullObjectType philosopherType) HashMap.empty
+        Out.Field Nothing (Out.NamedObjectType philosopherType)
+        $ HashMap.singleton "id"
+        $ In.Argument Nothing (In.NamedScalarType id) Nothing
+    philosopherResolver = pure $ Object mempty
+    throwingField =
+        let fieldType = Out.ListType $ Out.NonNullScalarType string
+         in Out.Field Nothing fieldType HashMap.empty
+    throwingResolver :: Resolve IO
+    throwingResolver = throwM PhilosopherException
+    countField =
+        let fieldType = Out.NonNullScalarType int
+         in Out.Field Nothing fieldType HashMap.empty
+    countResolver = pure ""
+    sequenceField =
+        let fieldType = Out.ListType $ Out.NonNullScalarType int
+         in Out.Field Nothing fieldType HashMap.empty
+    sequenceResolver = pure intSequence
+    withInputObjectResolver = pure $ Type.Int 0
+    withInputObjectField =
+        Out.Field Nothing (Out.NonNullScalarType int) $ HashMap.fromList
+            [("values", In.Argument Nothing withInputObjectArgumentType Nothing)]
+    withInputObjectArgumentType = In.NonNullListType
+        $ In.NonNullInputObjectType inputObjectType

-philosopherType :: Out.ObjectType (Either SomeException)
+inputObjectType :: In.InputObjectType
+inputObjectType = In.InputObjectType "InputObject" Nothing $
+    HashMap.singleton "name" $
+        In.InputField Nothing (In.NonNullScalarType int) Nothing
+
+intSequence :: Value
+intSequence = Type.List [Type.Int 1, Type.Int 2, Type.Int 3]
+
+musicType :: Out.ObjectType IO
+musicType = Out.ObjectType "Music" Nothing []
+    $ HashMap.fromList resolvers
+  where
+    resolvers =
+        [ ("instrument", ValueResolver instrumentField instrumentResolver)
+        ]
+    instrumentResolver = pure $ String "piano"
+    instrumentField = Out.Field Nothing (Out.NonNullScalarType string) HashMap.empty
+
+poetryType :: Out.ObjectType IO
+poetryType = Out.ObjectType "Poetry" Nothing []
+    $ HashMap.fromList resolvers
+  where
+    resolvers =
+        [ ("genre", ValueResolver genreField genreResolver)
+        ]
+    genreResolver = pure $ String "Futurism"
+    genreField = Out.Field Nothing (Out.NonNullScalarType string) HashMap.empty
+
+interestType :: Out.UnionType IO
+interestType = Out.UnionType "Interest" Nothing [musicType, poetryType]
+
+philosopherType :: Out.ObjectType IO
 philosopherType = Out.ObjectType "Philosopher" Nothing []
    $ HashMap.fromList resolvers
  where
    resolvers =
        [ ("firstName", ValueResolver firstNameField firstNameResolver)
        , ("lastName", ValueResolver lastNameField lastNameResolver)
+        , ("school", ValueResolver schoolField schoolResolver)
+        , ("interest", ValueResolver interestField interestResolver)
+        , ("majorWork", ValueResolver majorWorkField majorWorkResolver)
+        , ("century", ValueResolver centuryField centuryResolver)
+        , ("firstLanguage", ValueResolver firstLanguageField firstLanguageResolver)
        ]
    firstNameField =
        Out.Field Nothing (Out.NonNullScalarType string) HashMap.empty
-    firstNameResolver = pure $ Type.String "Friedrich"
+    firstNameResolver = pure $ String "Friedrich"
    lastNameField
        = Out.Field Nothing (Out.NonNullScalarType string) HashMap.empty
-    lastNameResolver = pure $ Type.String "Nietzsche"
+    lastNameResolver = pure $ String "Nietzsche"
+    schoolField
+       = Out.Field Nothing (Out.NonNullEnumType schoolType) HashMap.empty
+    schoolResolver = pure $ Enum "EXISTENTIALISM"
+    interestField
+        = Out.Field Nothing (Out.NonNullUnionType interestType) HashMap.empty
+    interestResolver = pure
+        $ Object
+        $ HashMap.fromList [("instrument", "piano")]
+    majorWorkField
+        = Out.Field Nothing (Out.NonNullInterfaceType workType) HashMap.empty
+    majorWorkResolver = pure
+        $ Object
+        $ HashMap.fromList
+            [ ("title", "Also sprach Zarathustra: Ein Buch für Alle und Keinen")
+            ]
+    centuryField =
+        Out.Field Nothing (Out.NonNullScalarType int) HashMap.empty
+    centuryResolver = pure $ Float 18.5
+    firstLanguageField
+        = Out.Field Nothing (Out.NonNullScalarType string) HashMap.empty
+    firstLanguageResolver = pure Null

-subscriptionType :: Out.ObjectType (Either SomeException)
+workType :: Out.InterfaceType IO
+workType = Out.InterfaceType "Work" Nothing []
+    $ HashMap.fromList fields
+  where
+    fields = [("title", titleField)]
+    titleField = Out.Field Nothing (Out.NonNullScalarType string) HashMap.empty
+
+bookType :: Out.ObjectType IO
+bookType = Out.ObjectType "Book" Nothing [workType]
+    $ HashMap.fromList resolvers
+  where
+    resolvers =
+        [ ("title", ValueResolver titleField titleResolver)
+        ]
+    titleField = Out.Field Nothing (Out.NonNullScalarType string) HashMap.empty
+    titleResolver = pure "Also sprach Zarathustra: Ein Buch für Alle und Keinen"
+
+bookCollectionType :: Out.ObjectType IO
+bookCollectionType = Out.ObjectType "Book" Nothing [workType]
+    $ HashMap.fromList resolvers
+  where
+    resolvers =
+        [ ("title", ValueResolver titleField titleResolver)
+        ]
+    titleField = Out.Field Nothing (Out.NonNullScalarType string) HashMap.empty
+    titleResolver = pure "The Three Critiques"
+
+subscriptionType :: Out.ObjectType IO
 subscriptionType = Out.ObjectType "Subscription" Nothing []
    $ HashMap.singleton "newQuote"
-    $ EventStreamResolver quoteField (pure $ Type.Object mempty)
-    $ pure $ yield $ Type.Object mempty
+    $ EventStreamResolver quoteField (pure $ Object mempty)
+    $ pure $ yield $ Object mempty
  where
    quoteField =
        Out.Field Nothing (Out.NonNullObjectType quoteType) HashMap.empty

-quoteType :: Out.ObjectType (Either SomeException)
+quoteType :: Out.ObjectType IO
 quoteType = Out.ObjectType "Quote" Nothing []
    $ HashMap.singleton "quote"
    $ ValueResolver quoteField
@ -70,63 +211,260 @@ quoteType = Out.ObjectType "Quote" Nothing []
    quoteField =
        Out.Field Nothing (Out.NonNullScalarType string) HashMap.empty

-type EitherStreamOrValue = Either
-    (ResponseEventStream (Either SomeException) Aeson.Value)
-    (Response Aeson.Value)
+schoolType :: Type.EnumType
+schoolType = EnumType "School" Nothing $ HashMap.fromList
+    [ ("NOMINALISM", EnumValue Nothing)
+    , ("REALISM", EnumValue Nothing)
+    , ("IDEALISM", EnumValue Nothing)
+    ]

-execute' :: Document -> Either SomeException EitherStreamOrValue
-execute' =
-    execute philosopherSchema Nothing (mempty :: HashMap Name Aeson.Value)
+type EitherStreamOrValue = Either
+    (ResponseEventStream IO Type.Value)
+    (Response Type.Value)
+
+-- Asserts that a query resolves to a value.
+shouldResolveTo :: Text.Text -> Response Type.Value -> Expectation
+shouldResolveTo querySource expected =
+    case parse document "" querySource of
+        (Right parsedDocument) ->
+            execute philosopherSchema Nothing (mempty :: HashMap Name Type.Value) parsedDocument >>= go
+        (Left errorBundle) -> expectationFailure $ errorBundlePretty errorBundle
+  where
+    go = \case
+        Right result -> shouldBe result expected
+        Left _ -> expectationFailure
+            "the query is expected to resolve to a value, but it resolved to an event stream"
+
+-- Asserts that the executor produces an error that starts with a string.
+shouldContainError :: Either (ResponseEventStream IO Type.Value) (Response Type.Value)
+    -> Text
+    -> Expectation
+shouldContainError streamOrValue expected =
+    case streamOrValue of
+        Right response -> respond response
+        Left _ -> expectationFailure
+            "the query is expected to resolve to a value, but it resolved to an event stream"
+  where
+    startsWith :: Text.Text -> Text.Text -> Bool
+    startsWith xs ys = Text.take (Text.length ys) xs == ys
+    respond :: Response Type.Value -> Expectation
+    respond Response{ errors }
+        | any ((`startsWith` expected) . message) errors = pure ()
+        | otherwise = expectationFailure
+            "the query is expected to execute with errors, but the response doesn't contain any errors"
+
+parseAndExecute :: Schema IO
+    -> Maybe Text
+    -> HashMap Name Type.Value
+    -> Text
+    -> IO (Either (ResponseEventStream IO Type.Value) (Response Type.Value))
+parseAndExecute schema' operation variables
+    = either (pure . parseError) (execute schema' operation variables)
+    . parse document ""

 spec :: Spec
 spec =
    describe "execute" $ do
        it "rejects recursive fragments" $
-            let sourceQuery = [r|
-              {
-                ...cyclicFragment
-              }
-
-              fragment cyclicFragment on Query {
-                ...cyclicFragment
-              }
-            |]
-                expected = Response emptyObject mempty
-                Right (Right actual) = either (pure . parseError) execute'
-                    $ parse document "" sourceQuery
-             in actual `shouldBe` expected
+            let sourceQuery = "\
+              \{\n\
+              \  ...cyclicFragment\n\
+              \}\n\
+              \\n\
+              \fragment cyclicFragment on Query {\n\
+              \  ...cyclicFragment\n\
+              \}\
+              \"
+                expected = Response (Object mempty) mempty
+             in sourceQuery `shouldResolveTo` expected

        context "Query" $ do
            it "skips unknown fields" $
-                let data'' = Aeson.object
-                        [ "philosopher" .= Aeson.object
-                            [ "firstName" .= ("Friedrich" :: String)
-                            ]
-                        ]
+                let data'' = Object
+                        $ HashMap.singleton "philosopher"
+                        $ Object
+                        $ HashMap.singleton "firstName"
+                        $ String "Friedrich"
                    expected = Response data'' mempty
-                    Right (Right actual) = either (pure . parseError) execute'
-                        $ parse document "" "{ philosopher { firstName surname } }"
-                in actual `shouldBe` expected
+                    sourceQuery = "{ philosopher { firstName surname } }"
+                in sourceQuery `shouldResolveTo` expected
            it "merges selections" $
-                let data'' = Aeson.object
-                        [ "philosopher" .= Aeson.object
-                            [ "firstName" .= ("Friedrich" :: String)
-                            , "lastName" .= ("Nietzsche" :: String)
+                let data'' = Object
+                        $ HashMap.singleton "philosopher"
+                        $ Object
+                        $ HashMap.fromList
+                            [ ("firstName", String "Friedrich")
+                            , ("lastName", String "Nietzsche")
                            ]
-                        ]
                    expected = Response data'' mempty
-                    Right (Right actual) = either (pure . parseError) execute'
-                        $ parse document "" "{ philosopher { firstName } philosopher { lastName } }"
-                in actual `shouldBe` expected
+                    sourceQuery = "{ philosopher { firstName } philosopher { lastName } }"
+                in sourceQuery `shouldResolveTo` expected
+
+            it "errors on invalid output enum values" $
+                let data'' = Object $ HashMap.singleton "philosopher" Null
+                    executionErrors = pure $ Error
+                        { message =
+                            "Value completion error. Expected type School!, found: EXISTENTIALISM."
+                        , locations = [Location 1 17]
+                        , path = [Segment "philosopher", Segment "school"]
+                        }
+                    expected = Response data'' executionErrors
+                    sourceQuery = "{ philosopher { school } }"
+                 in sourceQuery `shouldResolveTo` expected
+
+            it "gives location information for non-null unions" $
+                let data'' = Object $ HashMap.singleton "philosopher" Null
+                    executionErrors = pure $ Error
+                        { message =
+                            "Value completion error. Expected type Interest!, found: { instrument: \"piano\" }."
+                        , locations = [Location 1 17]
+                        , path = [Segment "philosopher", Segment "interest"]
+                        }
+                    expected = Response data'' executionErrors
+                    sourceQuery = "{ philosopher { interest } }"
+                 in sourceQuery `shouldResolveTo` expected
+
+            it "gives location information for invalid interfaces" $
+                let data'' = Object $ HashMap.singleton "philosopher" Null
+                    executionErrors = pure $ Error
+                        { message
+                            = "Value completion error. Expected type Work!, found:\
+                            \ { title: \"Also sprach Zarathustra: Ein Buch f\252r Alle und Keinen\" }."
+                        , locations = [Location 1 17]
+                        , path = [Segment "philosopher", Segment "majorWork"]
+                        }
+                    expected = Response data'' executionErrors
+                    sourceQuery = "{ philosopher { majorWork { title } } }"
+                 in sourceQuery `shouldResolveTo` expected
+
+            it "gives location information for failed result coercion" $
+                let data'' = Object $ HashMap.singleton "philosopher" Null
+                    executionErrors = pure $ Error
+                        { message = "Unable to coerce result to Int!."
+                        , locations = [Location 1 26]
+                        , path = [Segment "philosopher", Segment "century"]
+                        }
+                    expected = Response data'' executionErrors
+                    sourceQuery = "{ philosopher(id: \"1\") { century } }"
+                in sourceQuery `shouldResolveTo` expected
+
+            it "gives location information for failed result coercion" $
+                let data'' = Object $ HashMap.singleton "throwing" Null
+                    executionErrors = pure $ Error
+                        { message = "PhilosopherException"
+                        , locations = [Location 1 3]
+                        , path = [Segment "throwing"]
+                        }
+                    expected = Response data'' executionErrors
+                    sourceQuery = "{ throwing }"
+                in sourceQuery `shouldResolveTo` expected
+
+            it "sets data to null if a root field isn't nullable" $
+                let executionErrors = pure $ Error
+                        { message = "Unable to coerce result to Int!."
+                        , locations = [Location 1 3]
+                        , path = [Segment "count"]
+                        }
+                    expected = Response Null executionErrors
+                    sourceQuery = "{ count }"
+                in sourceQuery `shouldResolveTo` expected
+
+            it "detects nullability errors" $
+                let data'' = Object $ HashMap.singleton "philosopher" Null
+                    executionErrors = pure $ Error
+                        { message = "Value completion error. Expected type String!, found: null."
+                        , locations = [Location 1 26]
+                        , path = [Segment "philosopher", Segment "firstLanguage"]
+                        }
+                    expected = Response data'' executionErrors
+                    sourceQuery = "{ philosopher(id: \"1\") { firstLanguage } }"
+                in sourceQuery `shouldResolveTo` expected
+
+            it "returns list elements in the original order" $
+                let data'' = Object $ HashMap.singleton "sequence" intSequence
+                    expected = Response data'' mempty
+                    sourceQuery = "{ sequence }"
+                in sourceQuery `shouldResolveTo` expected
+
+            context "Arguments" $ do
+                it "gives location information for invalid scalar arguments" $
+                    let data'' = Object $ HashMap.singleton "philosopher" Null
+                        executionErrors = pure $ Error
+                            { message =
+                                "Argument \"id\" has invalid type. Expected type ID, found: True."
+                            , locations = [Location 1 15]
+                            , path = [Segment "philosopher"]
+                            }
+                        expected = Response data'' executionErrors
+                        sourceQuery = "{ philosopher(id: true) { lastName } }"
+                    in sourceQuery `shouldResolveTo` expected
+
+                it "puts an object in a list if needed" $
+                    let data'' = Object $ HashMap.singleton "withInputObject" $ Type.Int 0
+                        expected = Response data'' mempty
+                        sourceQuery = "{ withInputObject(values: { name: 0 }) }"
+                    in sourceQuery `shouldResolveTo` expected
+
+            context "queryError" $ do
+                let namedQuery name = "query " <> name <> " { philosopher(id: \"1\") { interest } }"
+                    twoQueries = namedQuery "A" <> " " <> namedQuery "B"
+
+                it "throws operation name is required error" $ do
+                    let expectedErrorMessage = "Operation name is required"
+                    actual <- parseAndExecute philosopherSchema Nothing mempty twoQueries
+                    actual `shouldContainError` expectedErrorMessage
+
+                it "throws operation not found error" $ do
+                    let expectedErrorMessage = "Operation \"C\" is not found"
+                    actual <- parseAndExecute philosopherSchema (Just "C") mempty twoQueries
+                    actual `shouldContainError` expectedErrorMessage
+
+                it "throws variable coercion error" $ do
+                    let data'' = Null
+                        executionErrors = pure $ Error
+                            { message = "Failed to coerce the variable $id: String."
+                            , locations =[Location 1 7]
+                            , path = []
+                            }
+                        expected = Response data'' executionErrors
+                        executeWithVars = execute philosopherSchema Nothing (HashMap.singleton "id" (Type.Int 1))
+                    Right actual <- either (pure . parseError) executeWithVars
+                        $ parse document "" "query($id: String) { philosopher(id: \"1\") { firstLanguage } }"
+                    actual `shouldBe` expected
+
+                it "throws variable unkown input type error" $
+                    let data'' = Null
+                        executionErrors = pure $ Error
+                            { message = "Variable $id has unknown type Cat."
+                            , locations =[Location 1 7]
+                            , path = []
+                            }
+                        expected = Response data'' executionErrors
+                        sourceQuery = "query($id: Cat) { philosopher(id: \"1\") { firstLanguage } }"
+                     in sourceQuery `shouldResolveTo` expected
+
+            context "Error path" $ do
+                let executeHero :: Document -> IO EitherStreamOrValue
+                    executeHero = execute heroSchema Nothing (HashMap.empty :: HashMap Name Type.Value)
+
+                it "at the beggining of the list" $ do
+                    Right actual <- either (pure . parseError) executeHero
+                        $ parse document "" "{ hero(id: \"1\") { friends { name } } }"
+                    let Response _ errors' = actual
+                        Error _ _ path' = fromJust $ Seq.lookup 0 errors'
+                        expected = [Segment "hero", Segment "friends", Index 0, Segment "name"]
+                     in path' `shouldBe` expected
+
        context "Subscription" $
-            it "subscribes" $
-                let data'' = Aeson.object
-                        [ "newQuote" .= Aeson.object
-                            [ "quote" .= ("Naturam expelles furca, tamen usque recurret." :: String)
-                            ]
-                        ]
+            it "subscribes" $ do
+                let data'' = Object
+                        $ HashMap.singleton "newQuote"
+                        $ Object
+                        $ HashMap.singleton "quote"
+                        $ String "Naturam expelles furca, tamen usque recurret."
                    expected = Response data'' mempty
-                    Right (Left stream) = either (pure . parseError) execute'
-                        $ parse document "" "subscription { newQuote { quote } }"
-                    Right (Just actual) = runConduit $ stream .| await
-                in actual `shouldBe` expected
+                Left stream <- execute philosopherSchema Nothing (mempty :: HashMap Name Type.Value)
+                    $ fromRight (error "Parse error")
+                    $ parse document "" "subscription { newQuote { quote } }"
+                Just actual <- runConduit $ stream .| await
+                actual `shouldBe` expected
--- a/tests/Language/GraphQL/Validate/RulesSpec.hs
+++ b/tests/Language/GraphQL/Validate/RulesSpec.hs
--- a/tests/Schemas/HeroSchema.hs
+++ b/tests/Schemas/HeroSchema.hs
@ -0,0 +1,70 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
+{-# LANGUAGE OverloadedStrings #-}
+
+module Schemas.HeroSchema (heroSchema) where
+
+import Control.Exception (Exception(..))
+import Control.Monad.Catch (throwM)
+import Language.GraphQL.Error (ResolverException (..))
+import qualified Language.GraphQL.Type.In as In
+import qualified Language.GraphQL.Type as Type
+import Language.GraphQL.Type.Schema (schemaWithTypes)
+import qualified Data.HashMap.Strict as HashMap
+import Data.Typeable (cast)
+import qualified Language.GraphQL.Type.Out as Out
+
+data HeroException = HeroException
+    deriving Show
+
+instance Exception HeroException where
+    toException = toException. ResolverException
+    fromException e = do
+        ResolverException resolverException <- fromException e
+        cast resolverException
+
+heroSchema :: Type.Schema IO
+heroSchema =
+    schemaWithTypes Nothing queryType Nothing Nothing [] mempty
+
+type ObjectType = Out.ObjectType IO
+
+queryType :: ObjectType
+queryType = Out.ObjectType "Query" Nothing []
+    $ HashMap.fromList
+        [ ("hero", Out.ValueResolver heroField heroResolver)
+        ]
+  where
+    heroField = Out.Field Nothing (Out.NamedObjectType heroType)
+        $ HashMap.singleton "id"
+        $ In.Argument Nothing (In.NamedScalarType Type.id) Nothing
+    heroResolver = pure $ Type.Object mempty
+
+stringField :: Out.Field IO
+stringField = Out.Field Nothing (Out.NonNullScalarType Type.string) HashMap.empty
+
+heroType :: ObjectType
+heroType = Out.ObjectType "Hero" Nothing [] $ HashMap.fromList resolvers
+  where
+    resolvers =
+        [ ("id", Out.ValueResolver stringField (pure $ Type.String "4111"))
+        , ("name", Out.ValueResolver stringField (pure $ Type.String "R2D2"))
+        , ("friends", Out.ValueResolver friendsField (pure $ Type.List [luke]))
+        ]
+    friendsField = Out.Field Nothing (Out.ListType $ Out.NonNullObjectType lukeType) HashMap.empty
+    -- This list values are ignored because of current realisation (types and resolvers are the same entity)
+    -- The values from lukeType will be used
+    luke = Type.Object $ HashMap.fromList
+        [ ("id", "dfdfdf")
+        , ("name", "dfdfdff")
+        ]
+
+lukeType :: ObjectType
+lukeType = Out.ObjectType "Luke" Nothing [] $ HashMap.fromList resolvers
+  where
+    resolvers =
+        [ ("id", Out.ValueResolver stringField (pure $ Type.String "1000"))
+        , ("name", Out.ValueResolver stringField (throwM HeroException))
+        ]
--- a/tests/Test/DirectiveSpec.hs
+++ b/tests/Test/DirectiveSpec.hs
@ -1,92 +0,0 @@
-{- This Source Code Form is subject to the terms of the Mozilla Public License,
-   v. 2.0. If a copy of the MPL was not distributed with this file, You can
-   obtain one at https://mozilla.org/MPL/2.0/. -}
-
-{-# LANGUAGE OverloadedStrings #-}
-{-# LANGUAGE QuasiQuotes #-}
-module Test.DirectiveSpec
-    ( spec
-    ) where
-
-import Data.Aeson (object, (.=))
-import qualified Data.Aeson as Aeson
-import qualified Data.HashMap.Strict as HashMap
-import Language.GraphQL
-import Language.GraphQL.Type
-import qualified Language.GraphQL.Type.Out as Out
-import Test.Hspec (Spec, describe, it)
-import Test.Hspec.GraphQL
-import Text.RawString.QQ (r)
-
-experimentalResolver :: Schema IO
-experimentalResolver = schema queryType Nothing Nothing mempty
-  where
-    queryType = Out.ObjectType "Query" Nothing []
-        $ HashMap.singleton "experimentalField"
-        $ Out.ValueResolver (Out.Field Nothing (Out.NamedScalarType int) mempty)
-        $ pure $ Int 5
-
-emptyObject :: Aeson.Object
-emptyObject = HashMap.singleton "data" $ object []
-
-spec :: Spec
-spec =
-    describe "Directive executor" $ do
-        it "should be able to @skip fields" $ do
-            let sourceQuery = [r|
-              {
-                experimentalField @skip(if: true)
-              }
-            |]
-
-            actual <- graphql experimentalResolver sourceQuery
-            actual `shouldResolveTo` emptyObject
-
-        it "should not skip fields if @skip is false" $ do
-            let sourceQuery = [r|
-              {
-                experimentalField @skip(if: false)
-              }
-            |]
-                expected = HashMap.singleton "data"
-                    $ object
-                        [ "experimentalField" .= (5 :: Int)
-                        ]
-            actual <- graphql experimentalResolver sourceQuery
-            actual `shouldResolveTo` expected
-
-        it "should skip fields if @include is false" $ do
-            let sourceQuery = [r|
-              {
-                experimentalField @include(if: false)
-              }
-            |]
-
-            actual <- graphql experimentalResolver sourceQuery
-            actual `shouldResolveTo` emptyObject
-
-        it "should be able to @skip a fragment spread" $ do
-            let sourceQuery = [r|
-              {
-                ...experimentalFragment @skip(if: true)
-              }
-
-              fragment experimentalFragment on Query {
-                experimentalField
-              }
-            |]
-
-            actual <- graphql experimentalResolver sourceQuery
-            actual `shouldResolveTo` emptyObject
-
-        it "should be able to @skip an inline fragment" $ do
-            let sourceQuery = [r|
-              {
-                ... on Query @skip(if: true) {
-                  experimentalField
-                }
-              }
-            |]
-
-            actual <- graphql experimentalResolver sourceQuery
-            actual `shouldResolveTo` emptyObject
--- a/tests/Test/FragmentSpec.hs
+++ b/tests/Test/FragmentSpec.hs
@ -1,198 +0,0 @@
-{- This Source Code Form is subject to the terms of the Mozilla Public License,
-   v. 2.0. If a copy of the MPL was not distributed with this file, You can
-   obtain one at https://mozilla.org/MPL/2.0/. -}
-
-{-# LANGUAGE OverloadedStrings #-}
-{-# LANGUAGE QuasiQuotes #-}
-module Test.FragmentSpec
-    ( spec
-    ) where
-
-import Data.Aeson ((.=))
-import qualified Data.Aeson as Aeson
-import qualified Data.HashMap.Strict as HashMap
-import Data.Text (Text)
-import Language.GraphQL
-import Language.GraphQL.Type
-import qualified Language.GraphQL.Type.Out as Out
-import Test.Hspec (Spec, describe, it)
-import Test.Hspec.GraphQL
-import Text.RawString.QQ (r)
-
-size :: (Text, Value)
-size = ("size", String "L")
-
-circumference :: (Text, Value)
-circumference = ("circumference", Int 60)
-
-garment :: Text -> (Text, Value)
-garment typeName =
-    ("garment",  Object $ HashMap.fromList
-        [ if typeName == "Hat" then circumference else size
-        , ("__typename", String typeName)
-        ]
-    )
-
-inlineQuery :: Text
-inlineQuery = [r|{
-  garment {
-    ... on Hat {
-      circumference
-    }
-    ... on Shirt {
-      size
-    }
-  }
-}|]
-
-shirtType :: Out.ObjectType IO
-shirtType = Out.ObjectType "Shirt" Nothing [] $ HashMap.fromList
-    [ ("size", sizeFieldType)
-    ]
-
-hatType :: Out.ObjectType IO
-hatType = Out.ObjectType "Hat" Nothing [] $ HashMap.fromList
-    [ ("size", sizeFieldType)
-    , ("circumference", circumferenceFieldType)
-    ]
-
-circumferenceFieldType :: Out.Resolver IO
-circumferenceFieldType
-    = Out.ValueResolver (Out.Field Nothing (Out.NamedScalarType int) mempty)
-    $ pure $ snd circumference
-
-sizeFieldType :: Out.Resolver IO
-sizeFieldType
-    = Out.ValueResolver (Out.Field Nothing (Out.NamedScalarType string) mempty)
-    $ pure $ snd size
-
-toSchema :: Text -> (Text, Value) -> Schema IO
-toSchema t (_, resolve) = schema queryType Nothing Nothing mempty
-  where
-    garmentType = Out.UnionType "Garment" Nothing [hatType, shirtType]
-    typeNameField = Out.Field Nothing (Out.NamedScalarType string) mempty
-    garmentField = Out.Field Nothing (Out.NamedUnionType garmentType) mempty
-    queryType =
-        case t of
-            "circumference" -> hatType
-            "size" -> shirtType
-            _ -> Out.ObjectType "Query" Nothing []
-                $ HashMap.fromList
-                    [ ("garment", ValueResolver garmentField (pure resolve))
-                    , ("__typename", ValueResolver typeNameField (pure $ String "Shirt"))
-                    ]
-
-spec :: Spec
-spec = do
-    describe "Inline fragment executor" $ do
-        it "chooses the first selection if the type matches" $ do
-            actual <- graphql (toSchema "Hat" $ garment "Hat") inlineQuery
-            let expected = HashMap.singleton "data"
-                    $ Aeson.object
-                        [ "garment" .= Aeson.object
-                            [ "circumference" .= (60 :: Int)
-                            ]
-                        ]
-             in actual `shouldResolveTo` expected
-
-        it "chooses the last selection if the type matches" $ do
-            actual <- graphql (toSchema "Shirt" $ garment "Shirt") inlineQuery
-            let expected = HashMap.singleton "data"
-                    $ Aeson.object
-                        [ "garment" .= Aeson.object
-                            [ "size" .= ("L" :: Text)
-                            ]
-                        ]
-             in actual `shouldResolveTo` expected
-
-        it "embeds inline fragments without type" $ do
-            let sourceQuery = [r|{
-              circumference
-              ... {
-                size
-              }
-            }|]
-            actual <- graphql (toSchema "circumference" circumference) sourceQuery
-            let expected = HashMap.singleton "data"
-                    $ Aeson.object
-                        [ "circumference" .= (60 :: Int)
-                        , "size" .= ("L" :: Text)
-                        ]
-             in actual `shouldResolveTo` expected
-
-        it "evaluates fragments on Query" $ do
-            let sourceQuery = [r|{
-              ... {
-                size
-              }
-            }|]
-             in graphql (toSchema "size" size) `shouldResolve` sourceQuery
-
-    describe "Fragment spread executor" $ do
-        it "evaluates fragment spreads" $ do
-            let sourceQuery = [r|
-              {
-                ...circumferenceFragment
-              }
-
-              fragment circumferenceFragment on Hat {
-                circumference
-              }
-            |]
-
-            actual <- graphql (toSchema "circumference" circumference) sourceQuery
-            let expected = HashMap.singleton "data"
-                    $ Aeson.object
-                        [ "circumference" .= (60 :: Int)
-                        ]
-             in actual `shouldResolveTo` expected
-
-        it "evaluates nested fragments" $ do
-            let sourceQuery = [r|
-              {
-                garment {
-                  ...circumferenceFragment
-                }
-              }
-
-              fragment circumferenceFragment on Hat {
-                ...hatFragment
-              }
-
-              fragment hatFragment on Hat {
-                circumference
-              }
-            |]
-
-            actual <- graphql (toSchema "Hat" $ garment "Hat") sourceQuery
-            let expected = HashMap.singleton "data"
-                    $ Aeson.object
-                        [ "garment" .= Aeson.object
-                            [ "circumference" .= (60 :: Int)
-                            ]
-                        ]
-             in actual `shouldResolveTo` expected
-
-        it "considers type condition" $ do
-            let sourceQuery = [r|
-              {
-                garment {
-                  ...circumferenceFragment
-                  ...sizeFragment
-                }
-              }
-              fragment circumferenceFragment on Hat {
-                circumference
-              }
-              fragment sizeFragment on Shirt {
-                size
-              }
-            |]
-                expected = HashMap.singleton "data"
-                    $ Aeson.object
-                        [ "garment" .= Aeson.object
-                            [ "circumference" .= (60 :: Int)
-                            ]
-                        ]
-            actual <- graphql (toSchema "Hat" $ garment "Hat") sourceQuery
-            actual `shouldResolveTo` expected
--- a/tests/Test/RootOperationSpec.hs
+++ b/tests/Test/RootOperationSpec.hs
@ -1,72 +0,0 @@
-{- This Source Code Form is subject to the terms of the Mozilla Public License,
-   v. 2.0. If a copy of the MPL was not distributed with this file, You can
-   obtain one at https://mozilla.org/MPL/2.0/. -}
-
-{-# LANGUAGE OverloadedStrings #-}
-{-# LANGUAGE QuasiQuotes #-}
-module Test.RootOperationSpec
-    ( spec
-    ) where
-
-import Data.Aeson ((.=), object)
-import qualified Data.HashMap.Strict as HashMap
-import Language.GraphQL
-import Test.Hspec (Spec, describe, it)
-import Text.RawString.QQ (r)
-import Language.GraphQL.Type
-import qualified Language.GraphQL.Type.Out as Out
-import Test.Hspec.GraphQL
-
-hatType :: Out.ObjectType IO
-hatType = Out.ObjectType "Hat" Nothing []
-    $ HashMap.singleton "circumference"
-    $ ValueResolver (Out.Field Nothing (Out.NamedScalarType int) mempty)
-    $ pure $ Int 60
-
-garmentSchema :: Schema IO
-garmentSchema = schema queryType (Just mutationType) Nothing mempty
-  where
-    queryType = Out.ObjectType "Query" Nothing [] hatFieldResolver
-    mutationType = Out.ObjectType "Mutation" Nothing [] incrementFieldResolver
-    garment = pure $ Object $ HashMap.fromList
-        [ ("circumference", Int 60)
-        ]
-    incrementFieldResolver = HashMap.singleton "incrementCircumference"
-        $ ValueResolver (Out.Field Nothing (Out.NamedScalarType int) mempty)
-        $ pure $ Int 61
-    hatField = Out.Field Nothing (Out.NamedObjectType hatType) mempty
-    hatFieldResolver =
-        HashMap.singleton "garment" $ ValueResolver hatField garment
-
-spec :: Spec
-spec =
-    describe "Root operation type" $ do
-        it "returns objects from the root resolvers" $ do
-            let querySource = [r|
-              {
-                garment {
-                  circumference
-                }
-              }
-            |]
-                expected = HashMap.singleton "data"
-                    $ object
-                        [ "garment" .= object
-                            [ "circumference" .= (60 :: Int)
-                            ]
-                        ]
-            actual <- graphql garmentSchema querySource
-            actual `shouldResolveTo` expected
-
-        it "chooses Mutation" $ do
-            let querySource = [r|
-              mutation {
-                incrementCircumference
-              }
-            |]
-                expected = HashMap.singleton "data"
-                    $ object
-                        [ "incrementCircumference" .= (61 :: Int)
-                        ]
-            actual <- graphql garmentSchema querySource
-            actual `shouldResolveTo` expected