Replace the old executor

2021-09-03 22:47:49 +02:00 · 2021-09-03 22:47:49 +02:00 · b96d75f447
parent 7b4c7e2b8c
commit b96d75f447
11 changed files with 480 additions and 2007 deletions
--- a/graphql.cabal
+++ b/graphql.cabal
@ -47,11 +47,8 @@ library
      Language.GraphQL.Validate.Validation
      Test.Hspec.GraphQL
  other-modules:
-      Language.GraphQL.Execute.Execution
      Language.GraphQL.Execute.Internal
-      Language.GraphQL.Execute.Subscribe
      Language.GraphQL.Execute.Transform
-      Language.GraphQL.Executor
      Language.GraphQL.Type.Definition
      Language.GraphQL.Type.Internal
      Language.GraphQL.Validate.Rules
--- a/src/Language/GraphQL/AST.hs
+++ b/src/Language/GraphQL/AST.hs
@ -1,6 +1,4 @@
-{- 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 Safe #-}

 -- | Target AST for parser.
 module Language.GraphQL.AST
--- a/src/Language/GraphQL/Error.hs
+++ b/src/Language/GraphQL/Error.hs
@ -1,3 +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 DuplicateRecordFields #-}
 {-# LANGUAGE ExistentialQuantification #-}
 {-# LANGUAGE RecordWildCards #-}
--- a/src/Language/GraphQL/Execute.hs
+++ b/src/Language/GraphQL/Execute.hs
@ -4,17 +4,13 @@

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

 module Language.GraphQL.Execute
-   ( Error(..)
-   , Operation(..)
-   , Path(..)
-   , Response(..)
+   ( module Language.GraphQL.Execute.Coerce
   , execute
   ) where

@ -29,32 +25,27 @@ import Control.Monad.Catch
     , catches
     )
 import Control.Monad.Trans.Class (MonadTrans(..))
-import Control.Monad.Trans.Reader (ReaderT(..), ask, local, runReaderT)
-import Control.Monad.Trans.Writer (WriterT(..), runWriterT, tell)
-import qualified Control.Monad.Trans.Reader as Reader
+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.Bifunctor (first)
 import Data.Foldable (find)
-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.List (intercalate)
 import Data.List.NonEmpty (NonEmpty(..))
 import qualified Data.List.NonEmpty as NonEmpty
-import Data.Maybe (fromMaybe, isJust)
-import Data.Sequence (Seq, (><))
+import Data.Maybe (fromMaybe)
+import Data.Sequence (Seq)
 import qualified Data.Sequence as Seq
 import Data.Text (Text)
 import qualified Data.Text as Text
 import Data.Typeable (cast)
 import GHC.Records (HasField(..))
-import qualified Language.GraphQL.Execute.Coerce as Coerce
+import Language.GraphQL.Execute.Coerce
 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.Type.In as In
 import qualified Language.GraphQL.Type.Out as Out
 import qualified Language.GraphQL.Type as Type
@ -64,41 +55,11 @@ import qualified Language.GraphQL.Type.Schema as Schema
 import Language.GraphQL.Error
    ( Error(..)
    , Response(..)
-   , Path(..)
-   , ResponseEventStream
-   )
-import Numeric (showFloat)
-
-data Replacement m = Replacement
-    { variableValues :: Type.Subs
-    , fragmentDefinitions :: HashMap Full.Name Full.FragmentDefinition
-    , visitedFragments :: HashSet Full.Name
-    , types :: HashMap Full.Name (Type m)
-    }
-
-newtype TransformT m a = TransformT
-    { runTransformT :: ReaderT (Replacement m) m a
-    }
-
-instance Functor m => Functor (TransformT m) where
-    fmap f = TransformT . fmap f . runTransformT
-
-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
+    , Path(..)
+    , ResolverException(..)
+    , ResponseEventStream
+    )
+import Prelude hiding (null)

 newtype ExecutorT m a = ExecutorT
    { runExecutorT :: ReaderT (HashMap Full.Name (Type m)) (WriterT (Seq Error) m) a
@ -139,29 +100,31 @@ graphQLExceptionFromException e = do
    GraphQLException graphqlException <- fromException e
    cast graphqlException

-data ResolverException = forall e. Exception e => ResolverException e
+data ResultException = forall e. Exception e => ResultException e

-instance Show ResolverException where
-    show (ResolverException e) = show e
+instance Show ResultException where
+    show (ResultException e) = show e

-instance Exception ResolverException where
+instance Exception ResultException where
    toException = graphQLExceptionToException
    fromException = graphQLExceptionFromException

-data FieldError
-    = ResultCoercionError
-    | NullResultError
+resultExceptionToException :: Exception e => e -> SomeException
+resultExceptionToException = toException . ResultException

-instance Show FieldError where
-    show ResultCoercionError = "Result coercion failed."
-    show NullResultError = "Non-Nullable field resolver returned Null."
+resultExceptionFromException :: Exception e => SomeException -> Maybe e
+resultExceptionFromException e = do
+    ResultException resultException <- fromException e
+    cast resultException

-newtype FieldException = FieldException FieldError
-    deriving Show
+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
+    toException = graphQLExceptionToException
+    fromException = graphQLExceptionFromException

 data ValueCompletionException = ValueCompletionException String Type.Value

@ -175,11 +138,11 @@ instance Show ValueCompletionException where
        ]

 instance Exception ValueCompletionException where
-    toException = graphQLExceptionToException
-    fromException = graphQLExceptionFromException
+    toException = resultExceptionToException
+    fromException = resultExceptionFromException

 data InputCoercionException =
-    InputCoercionException String In.Type (Maybe (Full.Node Input))
+    InputCoercionException String In.Type (Maybe (Full.Node Transform.Input))

 instance Show InputCoercionException where
    show (InputCoercionException argumentName argumentType Nothing) = concat
@ -203,14 +166,27 @@ instance Exception InputCoercionException where
    toException = graphQLExceptionToException
    fromException = graphQLExceptionFromException

-data QueryError
-   = OperationNameRequired
-   | OperationNotFound String
-   | CoercionError Full.VariableDefinition
-   | UnknownInputType Full.VariableDefinition
+newtype ResultCoercionException = ResultCoercionException String

-asks :: Monad m => forall a. (Replacement m -> a) -> TransformT m a
-asks = TransformT . Reader.asks
+instance Show ResultCoercionException where
+    show (ResultCoercionException typeRepresentation) = concat
+        [ "Unable to coerce result to "
+        , typeRepresentation
+        , "."
+        ]
+
+instance Exception ResultCoercionException where
+    toException = resultExceptionToException
+    fromException = resultExceptionFromException
+
+data QueryError
+    = OperationNameRequired
+    | OperationNotFound String
+    | CoercionError Full.VariableDefinition
+    | UnknownInputType Full.VariableDefinition
+
+tell :: Monad m => Seq Error -> ExecutorT m ()
+tell = ExecutorT . lift . Writer.tell

 queryError :: QueryError -> Error
 queryError OperationNameRequired =
@ -241,232 +217,7 @@ queryError (UnknownInputType variableDefinition) =
            ]
     in Error{ message = queryErrorMessage, locations = [location], path = [] }

-data Operation m
-    = Operation Full.OperationType (Seq (Selection m)) Full.Location
-
-data Selection m
-    = FieldSelection (Field m)
-    | FragmentSelection (Fragment m)
-
-data Field m = Field
-    (Maybe Full.Name)
-    Full.Name
-    (HashMap Full.Name (Full.Node Input))
-    (Seq (Selection m))
-    Full.Location
-
-data Fragment m = Fragment
-    (Type.Internal.CompositeType m) (Seq (Selection m)) Full.Location
-
-data Input
-    = Variable Type.Value
-    | Int Int32
-    | Float Double
-    | String Text
-    | Boolean Bool
-    | Null
-    | Enum Full.Name
-    | List [Input]
-    | Object (HashMap Full.Name Input)
-    deriving Eq
-
-instance Show Input where
-    showList = mappend . showList'
-      where
-        showList' list = "[" ++ intercalate ", " (show <$> list) ++ "]"
-    show (Int integer) = show integer
-    show (Float float') = showFloat float' mempty
-    show (String text) = "\"" <> Text.foldr (mappend . Full.escape) "\"" text
-    show (Boolean boolean') = show boolean'
-    show Null = "null"
-    show (Enum name) = Text.unpack name
-    show (List list) = show list
-    show (Object fields) = unwords
-        [ "{"
-        , intercalate ", " (HashMap.foldrWithKey showObject [] fields)
-        , "}"
-        ]
-      where
-        showObject key value accumulator =
-            concat [Text.unpack key, ": ", show value] : accumulator
-    show variableValue = show variableValue
-
-document :: Full.Document
-    -> ([Full.OperationDefinition], HashMap Full.Name Full.FragmentDefinition)
-document = foldr filterOperation ([], HashMap.empty)
-  where
-    filterOperation (Full.ExecutableDefinition executableDefinition) accumulator
-        | Full.DefinitionOperation operationDefinition' <- executableDefinition =
-            first (operationDefinition' :) accumulator
-        | Full.DefinitionFragment fragmentDefinition <- executableDefinition
-        , Full.FragmentDefinition fragmentName _ _ _ _ <- fragmentDefinition =
-            HashMap.insert fragmentName fragmentDefinition <$> accumulator
-    filterOperation _ accumulator = accumulator -- Type system definitions.
-
-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 [Type.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.Internal.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.Internal.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 Type.Directive
-directive (Full.Directive name' arguments' _)
-    = Type.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
-    -> HashMap Full.Name a
-insertIfGiven name (Just v) = HashMap.insert name v
-insertIfGiven _ _ = id
-
-node :: Monad m => Full.Node Full.Value -> TransformT m (Maybe (Full.Node Input))
-node Full.Node{node = node', ..} =
-    traverse Full.Node <$> input node' <*> pure location
-
-execute :: (MonadCatch m, Coerce.VariableValue a, Coerce.Serialize b)
+execute :: (MonadCatch m, VariableValue a, Serialize b)
    => Schema m -- ^ Resolvers.
    -> Maybe Text -- ^ Operation name.
    -> HashMap Full.Name a -- ^ Variable substitution function.
@ -475,7 +226,7 @@ execute :: (MonadCatch m, Coerce.VariableValue a, Coerce.Serialize b)
 execute schema' operationName subs document' =
    executeRequest schema' document' (Text.unpack <$> operationName) subs

-executeRequest :: (MonadCatch m, Coerce.Serialize a, Coerce.VariableValue b)
+executeRequest :: (MonadCatch m, Serialize a, VariableValue b)
    => Schema m
    -> Full.Document
    -> Maybe String
@ -486,35 +237,36 @@ executeRequest schema sourceDocument operationName variableValues = do
    case operationAndVariables of
        Left queryError' -> pure
            $ Right
-            $ Response Coerce.null $ pure $ queryError queryError'
+            $ Response null $ pure $ queryError queryError'
        Right operation
-            | Operation Full.Query topSelections _operationLocation <- operation ->
+            | Transform.Operation Full.Query topSelections _operationLocation <- operation ->
                 Right <$> executeQuery topSelections schema
-            | Operation Full.Mutation topSelections operationLocation <- operation ->
+            | Transform.Operation Full.Mutation topSelections operationLocation <- operation ->
                Right <$> executeMutation topSelections schema operationLocation
-            | Operation Full.Subscription topSelections operationLocation <- operation ->
+            | Transform.Operation Full.Subscription topSelections operationLocation <- operation ->
                either rightErrorResponse Left <$> subscribe topSelections schema operationLocation
  where
    schemaTypes = Schema.types schema
-    (operationDefinitions, fragmentDefinitions') = document sourceDocument
+    (operationDefinitions, fragmentDefinitions') =
+        Transform.document sourceDocument
    buildOperation = do
        operationDefinition <- getOperation operationDefinitions operationName
        coercedVariableValues <- coerceVariableValues
            schemaTypes
            operationDefinition
            variableValues
-        let replacement = Replacement
+        let replacement = Transform.Replacement
                { variableValues = coercedVariableValues
                , fragmentDefinitions = fragmentDefinitions'
                , visitedFragments = mempty
                , types = schemaTypes
                }
        pure $ flip runReaderT replacement
-            $ runTransformT
-            $ transform operationDefinition
+            $ Transform.runTransformT
+            $ Transform.transform operationDefinition

-rightErrorResponse :: Coerce.Serialize b => forall a. Error -> Either a (Response b)
-rightErrorResponse = Right . Response Coerce.null . pure
+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
@ -527,8 +279,8 @@ getOperation operations (Just givenOperationName)
    findOperationByName _ = False
 getOperation _ _ = Left OperationNameRequired

-executeQuery :: (MonadCatch m, Coerce.Serialize a)
-    => Seq (Selection m)
+executeQuery :: (MonadCatch m, Serialize a)
+    => Seq (Transform.Selection m)
    -> Schema m
    -> m (Response a)
 executeQuery topSelections schema = do
@ -536,11 +288,26 @@ executeQuery topSelections schema = do
    (data', errors) <- runWriterT
        $ flip runReaderT (Schema.types schema)
        $ runExecutorT
-        $ executeSelectionSet topSelections queryType Type.Null []
+        $ catch (executeSelectionSet topSelections queryType Type.Null [])
+        handleException
    pure $ Response data' errors

-executeMutation :: (MonadCatch m, Coerce.Serialize a)
-    => Seq (Selection m)
+handleException :: (MonadCatch m, Serialize a)
+    => FieldException
+    -> ExecutorT m a
+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)
@ -549,15 +316,16 @@ executeMutation topSelections schema operationLocation
        (data', errors) <- runWriterT
            $ flip runReaderT (Schema.types schema)
            $ runExecutorT
-            $ executeSelectionSet topSelections mutationType Type.Null []
+            $ catch (executeSelectionSet topSelections mutationType Type.Null [])
+            handleException
        pure $ Response data' errors
    | otherwise = pure
-        $ Response Coerce.null
+        $ Response null
        $ Seq.singleton
        $ Error "Schema doesn't support mutations." [operationLocation] []

-executeSelectionSet :: (MonadCatch m, Coerce.Serialize a)
-    => Seq (Selection m)
+executeSelectionSet :: (MonadCatch m, Serialize a)
+    => Seq (Transform.Selection m)
    -> Out.ObjectType m
    -> Type.Value
    -> [Path]
@ -565,62 +333,80 @@ executeSelectionSet :: (MonadCatch m, Coerce.Serialize a)
 executeSelectionSet selections objectType objectValue errorPath = do
    let groupedFieldSet = collectFields objectType selections
    resolvedValues <- OrderedMap.traverseMaybe go groupedFieldSet
-    coerceResult (Out.NonNullObjectType objectType) $ Coerce.Object resolvedValues
+    coerceResult (Out.NonNullObjectType objectType) $ Object resolvedValues
  where
    executeField' fields resolver =
        executeField objectValue fields resolver errorPath
    Out.ObjectType _ _ _ resolvers = objectType
-    go fields@(Field _ fieldName _ _ _ :| _) =
+    go fields@(Transform.Field _ fieldName _ _ _ :| _) =
        traverse (executeField' fields) $ HashMap.lookup fieldName resolvers

-fieldsSegment :: forall m. NonEmpty (Field m) -> Path
-fieldsSegment (Field alias fieldName _ _ _ :| _) =
+fieldsSegment :: forall m. NonEmpty (Transform.Field m) -> Path
+fieldsSegment (Transform.Field alias fieldName _ _ _ :| _) =
    Segment (fromMaybe fieldName alias)

-executeField :: (MonadCatch m, Coerce.Serialize a)
+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 (Field m)
+    -> NonEmpty (Transform.Field m)
    -> Out.Resolver m
    -> [Path]
    -> ExecutorT m a
-executeField objectValue fields resolver errorPath =
-    let Field _ fieldName inputArguments _ fieldLocation :| _ = fields
+executeField objectValue fields (viewResolver -> resolverPair) errorPath =
+    let Transform.Field _ fieldName inputArguments _ fieldLocation :| _ = fields
     in catches (go fieldName inputArguments)
-        [ Handler (inputCoercionHandler fieldLocation)
-        , Handler (graphqlExceptionHandler fieldLocation)
+        [ Handler nullResultHandler
+        , Handler (inputCoercionHandler fieldLocation)
+        , Handler (resultHandler fieldLocation)
+        , Handler (resolverHandler fieldLocation)
        ]
  where
-    inputCoercionHandler :: (MonadCatch m, Coerce.Serialize a)
+    inputCoercionHandler :: (MonadCatch m, Serialize a)
        => Full.Location
        -> InputCoercionException
        -> ExecutorT m a
    inputCoercionHandler _ e@(InputCoercionException _ _ (Just valueNode)) =
        let argumentLocation = getField @"location" valueNode
-         in exceptionHandler argumentLocation $ displayException e
-    inputCoercionHandler fieldLocation e =
-        exceptionHandler fieldLocation $ displayException e
-    graphqlExceptionHandler :: (MonadCatch m, Coerce.Serialize a)
+         in exceptionHandler argumentLocation e
+    inputCoercionHandler fieldLocation e = exceptionHandler fieldLocation e
+    resultHandler :: (MonadCatch m, Serialize a)
        => Full.Location
-        -> GraphQLException
+        -> ResultException
        -> ExecutorT m a
-    graphqlExceptionHandler fieldLocation e =
-        exceptionHandler fieldLocation $ displayException e
-    exceptionHandler errorLocation exceptionText =
-        let newError = Error (Text.pack exceptionText) [errorLocation]
-                $ reverse
-                $ fieldsSegment fields : errorPath
-         in ExecutorT (lift $ tell $ Seq.singleton newError) >> pure Coerce.null
+    resultHandler = exceptionHandler
+    resolverHandler :: (MonadCatch m, Serialize a)
+        => Full.Location
+        -> ResolverException
+        -> ExecutorT m a
+    resolverHandler = exceptionHandler
+    nullResultHandler :: (MonadCatch m, Serialize a)
+        => FieldException
+        -> ExecutorT m a
+    nullResultHandler e@(FieldException fieldLocation errorPath' next) =
+        let newError = constructError next fieldLocation errorPath'
+         in if Out.isNonNullType fieldType
+             then throwM e
+             else returnError newError
+    exceptionHandler errorLocation e =
+        let newPath = fieldsSegment fields : errorPath
+            newError = constructError e errorLocation newPath
+         in if Out.isNonNullType fieldType
+             then throwM $ FieldException errorLocation newPath e
+             else returnError newError
+    returnError newError = tell (Seq.singleton newError) >> pure null
    go fieldName inputArguments = do
-        let (Out.Field _ fieldType argumentTypes, resolveFunction) =
-                resolverField resolver
        argumentValues <- coerceArgumentValues argumentTypes inputArguments
        resolvedValue <-
-            resolveFieldValue resolveFunction objectValue fieldName argumentValues
+           resolveFieldValue resolveFunction objectValue fieldName argumentValues
        completeValue fieldType fields errorPath resolvedValue
-    resolverField (Out.ValueResolver resolverField' resolveFunction) =
-        (resolverField', resolveFunction)
-    resolverField (Out.EventStreamResolver resolverField' resolveFunction _) =
-        (resolverField', resolveFunction)
+    (resolverField, resolveFunction) = resolverPair
+    Out.Field _ fieldType argumentTypes = resolverField

 resolveFieldValue :: MonadCatch m
    => Out.Resolve m
@ -651,34 +437,33 @@ resolveAbstractType abstractType values'
            _ -> pure Nothing
    | otherwise = pure Nothing

-completeValue :: (MonadCatch m, Coerce.Serialize a)
+completeValue :: (MonadCatch m, Serialize a)
    => Out.Type m
-    -> NonEmpty (Field m)
+    -> NonEmpty (Transform.Field m)
    -> [Path]
    -> Type.Value
    -> ExecutorT m a
-completeValue outputType _ _ Type.Null
-    | Out.isNonNullType outputType = throwFieldError NullResultError
-    | otherwise = pure Coerce.null
+completeValue (Out.isNonNullType -> False) _ _ Type.Null =
+    pure null
 completeValue outputType@(Out.ListBaseType listType) fields errorPath (Type.List list)
-    = foldM go (0, []) list >>= coerceResult outputType . Coerce.List . snd
+    = foldM go (0, []) list >>= coerceResult outputType . List . snd
  where
    go (index, accumulator) listItem = do
        let updatedPath = Index index : errorPath
        completedValue <- completeValue listType fields updatedPath listItem
        pure (index + 1, completedValue : accumulator)
 completeValue outputType@(Out.ScalarBaseType _) _ _ (Type.Int int) =
-    coerceResult outputType $ Coerce.Int int
+    coerceResult outputType $ Int int
 completeValue outputType@(Out.ScalarBaseType _) _ _ (Type.Boolean boolean) =
-    coerceResult outputType $ Coerce.Boolean boolean
+    coerceResult outputType $ Boolean boolean
 completeValue outputType@(Out.ScalarBaseType _) _ _ (Type.Float float) =
-    coerceResult outputType $ Coerce.Float float
+    coerceResult outputType $ Float float
 completeValue outputType@(Out.ScalarBaseType _) _ _ (Type.String string) =
-    coerceResult outputType $ Coerce.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 $ Coerce.Enum enum
+        then coerceResult outputType $ Enum enum
        else throwM
            $ ValueCompletionException (show outputType)
            $ Type.Enum enum
@ -708,28 +493,25 @@ completeValue outputType@(Out.UnionBaseType unionType) fields errorPath result
 completeValue outputType _ _ result =
    throwM $ ValueCompletionException (show outputType) result

-coerceResult :: (MonadCatch m, Coerce.Serialize a)
+coerceResult :: (MonadCatch m, Serialize a)
    => Out.Type m
-    -> Coerce.Output a
+    -> Output a
    -> ExecutorT m a
 coerceResult outputType result
-    | Just serialized <- Coerce.serialize outputType result = pure serialized
-    | otherwise = throwFieldError ResultCoercionError
+    | Just serialized <- serialize outputType result = pure serialized
+    | otherwise = throwM $ ResultCoercionException $ show outputType

 mergeSelectionSets :: MonadCatch m
-    => NonEmpty (Field m)
-    -> Seq (Selection m)
+    => NonEmpty (Transform.Field m)
+    -> Seq (Transform.Selection m)
 mergeSelectionSets = foldr forEach mempty
  where
-    forEach (Field _ _ _ fieldSelectionSet _) selectionSet' =
+    forEach (Transform.Field _ _ _ fieldSelectionSet _) selectionSet' =
        selectionSet' <> fieldSelectionSet

-throwFieldError :: MonadCatch m => FieldError -> m a
-throwFieldError = throwM . FieldException
-
 coerceArgumentValues :: MonadCatch m
    => HashMap Full.Name In.Argument
-    -> HashMap Full.Name (Full.Node Input)
+    -> HashMap Full.Name (Full.Node Transform.Input)
    -> m Type.Subs
 coerceArgumentValues argumentDefinitions argumentValues =
    HashMap.foldrWithKey c pure argumentDefinitions mempty
@ -754,53 +536,53 @@ coerceArgumentValues argumentDefinitions argumentValues =
                    $ Just inputValue
                | otherwise -> throwM
                    $ InputCoercionException (Text.unpack argumentName) variableType Nothing
-    matchFieldValues' = Coerce.matchFieldValues coerceArgumentValue
+    matchFieldValues' = matchFieldValues coerceArgumentValue
        $ Full.node <$> argumentValues
-    coerceArgumentValue inputType (Int integer) =
-        Coerce.coerceInputLiteral inputType (Type.Int integer)
-    coerceArgumentValue inputType (Boolean boolean) =
-        Coerce.coerceInputLiteral inputType (Type.Boolean boolean)
-    coerceArgumentValue inputType (String string) =
-        Coerce.coerceInputLiteral inputType (Type.String string)
-    coerceArgumentValue inputType (Float float) =
-        Coerce.coerceInputLiteral inputType (Type.Float float)
-    coerceArgumentValue inputType (Enum enum) =
-        Coerce.coerceInputLiteral inputType (Type.Enum enum)
-    coerceArgumentValue inputType Null
+    coerceArgumentValue inputType (Transform.Int integer) =
+        coerceInputLiteral inputType (Type.Int integer)
+    coerceArgumentValue inputType (Transform.Boolean boolean) =
+        coerceInputLiteral inputType (Type.Boolean boolean)
+    coerceArgumentValue inputType (Transform.String string) =
+        coerceInputLiteral inputType (Type.String string)
+    coerceArgumentValue inputType (Transform.Float float) =
+        coerceInputLiteral inputType (Type.Float float)
+    coerceArgumentValue inputType (Transform.Enum enum) =
+        coerceInputLiteral inputType (Type.Enum enum)
+    coerceArgumentValue inputType Transform.Null
        | In.isNonNullType inputType = Nothing
-        | otherwise = Coerce.coerceInputLiteral inputType Type.Null
-    coerceArgumentValue (In.ListBaseType inputType) (List list) =
+        | otherwise = coerceInputLiteral inputType Type.Null
+    coerceArgumentValue (In.ListBaseType inputType) (Transform.List list) =
        let coerceItem = coerceArgumentValue inputType
         in Type.List <$> traverse coerceItem list
-    coerceArgumentValue (In.InputObjectBaseType inputType) (Object object)
+    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 _ (Variable variable) = pure variable
+    coerceArgumentValue _ (Transform.Variable variable) = pure variable
    coerceArgumentValue _ _ = Nothing
    forEachField object variableName (In.InputField _ variableType defaultValue) =
-        Coerce.matchFieldValues coerceArgumentValue object variableName variableType defaultValue
+        matchFieldValues coerceArgumentValue object variableName variableType defaultValue

 collectFields :: Monad m
    => Out.ObjectType m
-    -> Seq (Selection m)
-    -> OrderedMap (NonEmpty (Field m))
+    -> Seq (Transform.Selection m)
+    -> OrderedMap (NonEmpty (Transform.Field m))
 collectFields objectType = foldl forEach OrderedMap.empty
  where
-    forEach groupedFields (FieldSelection fieldSelection) =
-        let Field maybeAlias fieldName _ _ _ = fieldSelection
+    forEach groupedFields (Transform.FieldSelection fieldSelection) =
+        let Transform.Field maybeAlias fieldName _ _ _ = fieldSelection
            responseKey = fromMaybe fieldName maybeAlias
         in OrderedMap.insert responseKey (fieldSelection :| []) groupedFields
-    forEach groupedFields (FragmentSelection selectionFragment)
-        | Fragment fragmentType fragmentSelectionSet _ <- selectionFragment
+    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, Coerce.VariableValue b)
+coerceVariableValues :: (Monad m, VariableValue b)
    => HashMap Full.Name (Schema.Type m)
    -> Full.OperationDefinition
    -> HashMap Full.Name b
@ -818,7 +600,7 @@ coerceVariableValues types operationDefinition' variableValues
         in case Type.Internal.lookupInputType variableTypeName types of
            Just variableType ->
                maybe (Left $ CoercionError variableDefinition) Right
-                    $ Coerce.matchFieldValues
+                    $ matchFieldValues
                        coerceVariableValue'
                        variableValues
                        variableName
@ -828,8 +610,8 @@ coerceVariableValues types operationDefinition' variableValues
            Nothing -> Left $ UnknownInputType variableDefinition
    forEach _ coercedValuesOrError = coercedValuesOrError
    coerceVariableValue' variableType value'
-        = Coerce.coerceVariableValue variableType value'
-        >>= Coerce.coerceInputLiteral variableType
+        = coerceVariableValue variableType value'
+        >>= coerceInputLiteral variableType

 constValue :: Full.ConstValue -> Type.Value
 constValue (Full.ConstInt i) = Type.Int i
@ -845,8 +627,8 @@ constValue (Full.ConstObject o) =
    constObjectField Full.ObjectField{value = value', ..} =
        (name, constValue $ Full.node value')

-subscribe :: (MonadCatch m, Coerce.Serialize a)
-    => Seq (Selection m)
+subscribe :: (MonadCatch m, Serialize a)
+    => Seq (Transform.Selection m)
    -> Schema m
    -> Full.Location
    -> m (Either Error (ResponseEventStream m a))
@ -861,10 +643,10 @@ subscribe fields schema objectLocation
    | otherwise = pure $ Left
        $ Error "Schema doesn't support subscriptions." [] []

-mapSourceToResponseEvent :: (MonadCatch m, Coerce.Serialize a)
+mapSourceToResponseEvent :: (MonadCatch m, Serialize a)
    => HashMap Full.Name (Type m)
    -> Out.ObjectType m
-    -> Seq (Selection m)
+    -> Seq (Transform.Selection m)
    -> Out.SourceEventStream m
    -> m (ResponseEventStream m a)
 mapSourceToResponseEvent types' subscriptionType fields sourceStream
@ -876,11 +658,12 @@ createSourceEventStream :: MonadCatch m
    => HashMap Full.Name (Type m)
    -> Out.ObjectType m
    -> Full.Location
-    -> Seq (Selection m)
+    -> Seq (Transform.Selection m)
    -> m (Either Error (Out.SourceEventStream m))
 createSourceEventStream _types subscriptionType objectLocation fields
    | [fieldGroup] <- OrderedMap.elems groupedFieldSet
-    , Field _ fieldName arguments' _ errorLocation <- NonEmpty.head fieldGroup
+    , Transform.Field _ fieldName arguments' _ errorLocation <-
+        NonEmpty.head fieldGroup
    , Out.ObjectType _ _ _ fieldTypes <- subscriptionType
    , resolverT <- fieldTypes HashMap.! fieldName
    , Out.EventStreamResolver fieldDefinition _ resolver <- resolverT
@ -889,16 +672,15 @@ createSourceEventStream _types subscriptionType objectLocation fields
            Left _ -> pure
                $ Left
                $ Error "Argument coercion failed." [errorLocation] []
-            Right  argumentValues -> left (singleError' [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 []
+    singleError :: [Full.Location] -> String -> Error
+    singleError errorLocations message = Error (Text.pack message) errorLocations []

 resolveFieldEventStream :: MonadCatch m
    => Type.Value
@ -917,15 +699,16 @@ resolveFieldEventStream result args resolver =
        , Type.values = result
        }

-executeSubscriptionEvent :: (MonadCatch m, Coerce.Serialize a)
+executeSubscriptionEvent :: (MonadCatch m, Serialize a)
    => HashMap Full.Name (Type m)
    -> Out.ObjectType m
-    -> Seq (Selection m)
+    -> Seq (Transform.Selection m)
    -> Type.Value
    -> m (Response a)
 executeSubscriptionEvent types' objectType fields initialValue = do
    (data', errors) <- runWriterT
        $ flip runReaderT types'
        $ runExecutorT
-        $ executeSelectionSet fields objectType initialValue []
+        $ catch (executeSelectionSet fields objectType initialValue [])
+        handleException
    pure $ Response data' errors
--- a/src/Language/GraphQL/Execute/Execution.hs
+++ b/src/Language/GraphQL/Execute/Execution.hs
@ -1,253 +0,0 @@
-{-# LANGUAGE ExplicitForAll #-}
-{-# LANGUAGE LambdaCase #-}
-{-# 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 qualified Data.List.NonEmpty as NonEmpty
-import Data.HashMap.Strict (HashMap)
-import qualified Data.HashMap.Strict as HashMap
-import Data.Maybe (fromMaybe)
-import Data.Sequence (Seq(..))
-import qualified Data.Text as Text
-import qualified Language.GraphQL.AST as Full
-import Language.GraphQL.Error
-import Language.GraphQL.Execute.Coerce
-import Language.GraphQL.Execute.Internal
-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.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
-    -> Full.Location
-    -> CollectErrsT m Type.Value
-resolveFieldValue result args resolver location' =
-    catch (lift $ runReaderT resolver context) handleFieldError
-  where
-    handleFieldError :: MonadCatch m
-        => ResolverException
-        -> CollectErrsT m Type.Value
-    handleFieldError e
-        = addError Type.Null
-        $ Error (Text.pack $ displayException e) [location'] []
-    context = Type.Context
-        { Type.arguments = Type.Arguments args
-        , Type.values = result
-        }
-
-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.SelectionField field) =
-        let responseKey = aliasOrName field
-         in OrderedMap.insert responseKey (field :| []) groupedFields
-    forEach groupedFields (Transform.SelectionFragment selectionFragment)
-        | Transform.Fragment fragmentType fragmentSelectionSet <- selectionFragment
-        , Internal.doesFragmentTypeApply fragmentType objectType =
-            let fragmentGroupedFieldSet = collectFields objectType fragmentSelectionSet
-             in groupedFields <> fragmentGroupedFieldSet
-        | otherwise = groupedFields
-
-aliasOrName :: forall m. Transform.Field m -> Full.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' _ location' = NonEmpty.head fields
-        case coerceArgumentValues argumentDefinitions arguments' of
-            Left [] ->
-                let errorMessage = "Not all required arguments are specified."
-                 in addError null $ Error errorMessage [location'] []
-            Left errorLocations -> addError null
-                $ Error "Argument coercing failed." errorLocations []
-            Right argumentValues -> do
-                answer <- resolveFieldValue prev argumentValues resolver location'
-                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 (firstFieldLocation fields) . List
-completeValue outputType@(Out.ScalarBaseType _) fields (Type.Int int) =
-    coerceResult outputType (firstFieldLocation fields) $ Int int
-completeValue outputType@(Out.ScalarBaseType _) fields (Type.Boolean boolean) =
-    coerceResult outputType (firstFieldLocation fields) $ Boolean boolean
-completeValue outputType@(Out.ScalarBaseType _) fields (Type.Float float) =
-    coerceResult outputType (firstFieldLocation fields) $ Float float
-completeValue outputType@(Out.ScalarBaseType _) fields (Type.String string) =
-    coerceResult outputType (firstFieldLocation fields) $ String string
-completeValue outputType@(Out.EnumBaseType enumType) fields (Type.Enum enum) =
-    let Type.EnumType _ _ enumMembers = enumType
-        location = firstFieldLocation fields
-     in if HashMap.member enum enumMembers
-        then coerceResult outputType location $ Enum enum
-        else addError null $ Error "Enum value completion failed." [location] []
-completeValue (Out.ObjectBaseType objectType) fields result
-    = executeSelectionSet result objectType (firstFieldLocation fields)
-    $ mergeSelectionSets fields
-completeValue (Out.InterfaceBaseType interfaceType) fields result
-    | Type.Object objectMap <- result = do
-        let abstractType = Internal.AbstractInterfaceType interfaceType
-        let location = firstFieldLocation fields
-        concreteType <- resolveAbstractType abstractType objectMap
-        case concreteType of
-            Just objectType -> executeSelectionSet result objectType location
-                $ mergeSelectionSets fields
-            Nothing -> addError null
-                $ Error "Interface value completion failed." [location] []
-completeValue (Out.UnionBaseType unionType) fields result
-    | Type.Object objectMap <- result = do
-        let abstractType = Internal.AbstractUnionType unionType
-        let  location = firstFieldLocation fields
-        concreteType <- resolveAbstractType abstractType objectMap
-        case concreteType of
-            Just objectType -> executeSelectionSet result objectType
-                location $ mergeSelectionSets fields
-            Nothing -> addError null
-                $ Error "Union value completion failed." [location] []
-completeValue _ (Transform.Field _ _ _ _ location :| _) _ =
-    addError null $ Error "Value completion failed." [location] []
-
-mergeSelectionSets :: MonadCatch m
-    => NonEmpty (Transform.Field m)
-    -> Seq (Transform.Selection m)
-mergeSelectionSets = foldr forEach mempty
-  where
-    forEach (Transform.Field _ _ _ fieldSelectionSet _) selectionSet =
-        selectionSet <> fieldSelectionSet
-
-firstFieldLocation :: MonadCatch m => NonEmpty (Transform.Field m) -> Full.Location
-firstFieldLocation (Transform.Field _ _ _ _ fieldLocation :| _) = fieldLocation
-
-coerceResult :: (MonadCatch m, Serialize a)
-    => Out.Type m
-    -> Full.Location
-    -> Output a
-    -> CollectErrsT m a
-coerceResult outputType parentLocation result
-    | Just serialized <- serialize outputType result = pure serialized
-    | otherwise = addError null
-        $ Error "Result coercion failed." [parentLocation] []
-
-- | 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
-    -> Full.Location
-    -> Seq (Transform.Selection m)
-    -> CollectErrsT m a
-executeSelectionSet result objectType@(Out.ObjectType _ _ _ resolvers) objectLocation selectionSet = do
-    let fields = collectFields objectType selectionSet
-    resolvedValues <- OrderedMap.traverseMaybe forEach fields
-    coerceResult (Out.NonNullObjectType objectType) objectLocation
-        $ 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 Full.Name In.Argument
-    -> HashMap Full.Name (Full.Node Transform.Input)
-    -> Either [Full.Location] Type.Subs
-coerceArgumentValues argumentDefinitions argumentNodes =
-    HashMap.foldrWithKey forEach (pure mempty) argumentDefinitions
-  where
-    forEach argumentName (In.Argument _ variableType defaultValue) = \case
-        Right resultMap
-            | Just matchedValues
-                <- matchFieldValues' argumentName variableType defaultValue $ Just resultMap
-                -> Right matchedValues
-            | otherwise -> Left $ generateError argumentName []
-        Left errorLocations
-            | Just _
-                <- matchFieldValues' argumentName variableType defaultValue $ pure mempty
-                -> Left errorLocations
-            | otherwise -> Left $ generateError argumentName errorLocations
-    generateError argumentName errorLocations =
-        case HashMap.lookup argumentName argumentNodes of
-            Just (Full.Node _ errorLocation) -> [errorLocation]
-            Nothing -> errorLocations
-    matchFieldValues' = matchFieldValues coerceArgumentValue (Full.node <$> argumentNodes)
-    coerceArgumentValue inputType (Transform.Int integer) =
-        coerceInputLiteral inputType (Type.Int integer)
-    coerceArgumentValue inputType (Transform.Boolean boolean) =
-        coerceInputLiteral inputType (Type.Boolean boolean)
-    coerceArgumentValue inputType (Transform.String string) =
-        coerceInputLiteral inputType (Type.String string)
-    coerceArgumentValue inputType (Transform.Float float) =
-        coerceInputLiteral inputType (Type.Float float)
-    coerceArgumentValue inputType (Transform.Enum enum) =
-        coerceInputLiteral inputType (Type.Enum enum)
-    coerceArgumentValue inputType Transform.Null
-        | In.isNonNullType inputType = Nothing
-        | otherwise = coerceInputLiteral inputType Type.Null
-    coerceArgumentValue (In.ListBaseType inputType) (Transform.List list) =
-        let coerceItem = coerceInputLiteral inputType
-         in Type.List <$> 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/Subscribe.hs
+++ b/src/Language/GraphQL/Execute/Subscribe.hs
@ -1,113 +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.Arrow (left)
-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.List.NonEmpty as NonEmpty
-import Data.Sequence (Seq(..))
-import qualified Language.GraphQL.AST as Full
-import Language.GraphQL.Execute.Coerce
-import Language.GraphQL.Execute.Execution
-import Language.GraphQL.Execute.Internal
-import qualified Language.GraphQL.Execute.OrderedMap as OrderedMap
-import qualified Language.GraphQL.Execute.Transform as Transform
-import Language.GraphQL.Error
-    ( Error(..)
-    , ResolverException
-    , Response
-    , ResponseEventStream
-    , runCollectErrs
-    )
-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
-
-subscribe :: (MonadCatch m, Serialize a)
-    => HashMap Full.Name (Type m)
-    -> Out.ObjectType m
-    -> Full.Location
-    -> Seq (Transform.Selection m)
-    -> m (Either Error (ResponseEventStream m a))
-subscribe types' objectType objectLocation fields = do
-    sourceStream <-
-        createSourceEventStream types' objectType objectLocation fields
-    let traverser =
-            mapSourceToResponseEvent types' objectType objectLocation fields
-    traverse traverser sourceStream
-
-mapSourceToResponseEvent :: (MonadCatch m, Serialize a)
-    => HashMap Full.Name (Type m)
-    -> Out.ObjectType m
-    -> Full.Location
-    -> Seq (Transform.Selection m)
-    -> Out.SourceEventStream m
-    -> m (ResponseEventStream m a)
-mapSourceToResponseEvent types' subscriptionType objectLocation fields sourceStream
-    = pure
-    $ sourceStream
-    .| mapMC (executeSubscriptionEvent types' subscriptionType objectLocation 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
-
-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
-    -> Full.Location
-    -> Seq (Transform.Selection m)
-    -> Definition.Value
-    -> m (Response a)
-executeSubscriptionEvent types' objectType objectLocation fields initialValue
-    = runCollectErrs types'
-    $ executeSelectionSet initialValue objectType objectLocation fields
--- a/src/Language/GraphQL/Execute/Transform.hs
+++ b/src/Language/GraphQL/Execute/Transform.hs
@ -6,7 +6,7 @@
 {-# 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:
@ -21,65 +21,84 @@
 -- 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
+    , 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.
 data Operation m
-    = Query (Maybe Text) (Seq (Selection m)) Full.Location
-    | Mutation (Maybe Text) (Seq (Selection m)) Full.Location
-    | Subscription (Maybe Text) (Seq (Selection m)) Full.Location
+    = Operation Full.OperationType (Seq (Selection m)) Full.Location
+
+data Selection m
+    = FieldSelection (Field m)
+    | FragmentSelection (Fragment m)

-- | Single GraphQL field.
 data Field m = Field
    (Maybe Full.Name)
    Full.Name
@ -87,339 +106,214 @@ data Field m = Field
    (Seq (Selection m))
    Full.Location

-- | 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.Location
-
-- | Query error types.
-data QueryError
-    = OperationNotFound Text
-    | OperationNameRequired
-    | CoercionError
-    | EmptyDocument
-    | UnsupportedRootOperation
-
-instance Show QueryError where
-    show (OperationNotFound operationName) = unwords
-        ["Operation", Text.unpack operationName, "couldn't be found in the document."]
-    show OperationNameRequired = "Missing operation name."
-    show CoercionError = "Coercion error."
-    show EmptyDocument =
-        "The document doesn't contain any executable operations."
-    show UnsupportedRootOperation =
-        "Root operation type couldn't be found in the schema."
+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

-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
+instance Show Input where
+    showList = mappend . showList'
+      where
+        showList' list = "[" ++ intercalate ", " (show <$> list) ++ "]"
+    show (Int integer) = show integer
+    show (Float float') = showFloat float' mempty
+    show (String text) = "\"" <> Text.foldr (mappend . Full.escape) "\"" text
+    show (Boolean boolean') = show boolean'
+    show Null = "null"
+    show (Enum name) = Text.unpack name
+    show (List list) = show list
+    show (Object fields) = unwords
+        [ "{"
+        , intercalate ", " (HashMap.foldrWithKey showObject [] fields)
+        , "}"
+        ]
+      where
+        showObject key value accumulator =
+            concat [Text.unpack key, ": ", show value] : accumulator
+    show variableValue = show variableValue
+
+document :: Full.Document
+    -> ([Full.OperationDefinition], HashMap Full.Name Full.FragmentDefinition)
+document = foldr filterOperation ([], HashMap.empty)
  where
-    matchingName (OperationDefinition _ name _ _ _ _) =
-        name == Just operationName
+    filterOperation (Full.ExecutableDefinition executableDefinition) accumulator
+        | Full.DefinitionOperation operationDefinition' <- executableDefinition =
+            first (operationDefinition' :) accumulator
+        | Full.DefinitionFragment fragmentDefinition <- executableDefinition
+        , Full.FragmentDefinition fragmentName _ _ _ _ <- fragmentDefinition =
+            HashMap.insert fragmentName fragmentDefinition <$> accumulator
+    filterOperation _ accumulator = accumulator -- Type system definitions.

-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
+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
-    forEach variableDefinition coercedValues = do
-        let Full.VariableDefinition variableName variableTypeName defaultValue _ =
-                variableDefinition
-        let defaultValue' = constValue . Full.node <$> defaultValue
-        variableType <- Type.lookupInputType variableTypeName types
+    go accumulatedSelections currentSelection =
+        selection currentSelection <&> (accumulatedSelections ><)

-        Coerce.matchFieldValues
-            coerceVariableValue'
-            variableValues
-            variableName
-            variableType
-            defaultValue'
-            coercedValues
-    coerceVariableValue' variableType value'
-        = Coerce.coerceVariableValue variableType value'
-        >>= Coerce.coerceInputLiteral variableType
+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'

-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
+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
-    constObjectField Full.ObjectField{value = value', ..} =
-        (name, constValue $ Full.node value')
+    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 }

-- | 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
+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 location ->
-            OperationDefinition type' name variables directives' selections location
-        Full.SelectionSet selectionSet location ->
-            OperationDefinition Full.Query Nothing mempty mempty selectionSet location
-
-- * Operation
-
-operation :: OperationDefinition -> Replacement m -> Operation m
-operation operationDefinition replacement
-    = runIdentity
-    $ evalStateT (collectFragments >> transform operationDefinition) replacement
-  where
-    transform (OperationDefinition Full.Query name _ _ sels location) =
-        flip (Query name) location <$> appendSelection sels
-    transform (OperationDefinition Full.Mutation name _ _ sels location) =
-        flip (Mutation name) location <$> appendSelection sels
-    transform (OperationDefinition Full.Subscription name _ _ sels location) =
-        flip (Subscription name) location <$> appendSelection sels
-
-- * Selection
-
-selection
-    :: Full.Selection
-    ->  State (Replacement m) (Either (Seq (Selection m)) (Selection m))
-selection (Full.FieldSelection fieldSelection) =
-    maybe (Left mempty) (Right . SelectionField) <$> field fieldSelection
-selection (Full.FragmentSpreadSelection fragmentSelection)
-    = maybe (Left mempty) (Right . SelectionFragment)
-    <$> fragmentSpread fragmentSelection
-selection (Full.InlineFragmentSelection fragmentSelection) =
-    inlineFragment fragmentSelection
-
-field :: Full.Field ->  State (Replacement m) (Maybe (Field m))
-field (Full.Field alias name arguments' directives' selections location) = do
-    fieldArguments <- foldM go HashMap.empty arguments'
-    fieldSelections <- appendSelection selections
-    fieldDirectives <- Definition.selection <$> directives directives'
-    let field' = Field alias name fieldArguments fieldSelections location
-    pure $ field' <$ fieldDirectives
-  where
-    go arguments (Full.Argument name' (Full.Node value' _) location') = do
-        objectFieldValue <-  input value'
-        case objectFieldValue of
-            Just fieldValue ->
-                let argumentNode = Full.Node fieldValue location'
-                 in pure $ HashMap.insert name' argumentNode arguments
-            Nothing -> pure arguments
-
-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 <$> traverse (value . Full.node) list
-value (Full.Object object) =
-    Type.Object . HashMap.fromList <$> 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 . Full.node) 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/Executor.hs
+++ b/src/Language/GraphQL/Executor.hs
@ -1,847 +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 ExistentialQuantification #-}
-{-# LANGUAGE LambdaCase #-}
-{-# LANGUAGE NamedFieldPuns #-}
-{-# LANGUAGE OverloadedStrings #-}
-{-# LANGUAGE RecordWildCards #-}
-
-module Language.GraphQL.Executor
-   ( Error(..)
-   , Operation(..)
-   , Path(..)
-   , ResponseEventStream
-   , Response(..)
-   , execute
-   ) where
-
-import Conduit (ConduitT, mapMC, (.|))
-import Control.Arrow (left)
-import Control.Monad.Catch
-     ( Exception(..)
-     , MonadCatch(..)
-     , MonadThrow(..)
-     , SomeException(..)
-     )
-import Control.Monad.Trans.Class (MonadTrans(..))
-import Control.Monad.Trans.Reader (ReaderT(..), ask, local, runReaderT)
-import Control.Monad.Trans.Writer (WriterT(..), runWriterT, tell)
-import qualified Control.Monad.Trans.Reader as Reader
-import Control.Monad (foldM)
-import qualified Language.GraphQL.AST.Document as Full
-import Data.Bifunctor (first)
-import Data.Foldable (find)
-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.List.NonEmpty (NonEmpty(..))
-import qualified Data.List.NonEmpty as NonEmpty
-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 Data.Typeable (cast)
-import qualified Language.GraphQL.Execute.Coerce as Coerce
-import Language.GraphQL.Execute.OrderedMap (OrderedMap)
-import qualified Language.GraphQL.Execute.OrderedMap as OrderedMap
-import qualified Language.GraphQL.Type.In as In
-import qualified Language.GraphQL.Type.Out as Out
-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(..))
-
-data Replacement m = Replacement
-    { variableValues :: Type.Subs
-    , fragmentDefinitions :: HashMap Full.Name Full.FragmentDefinition
-    , visitedFragments :: HashSet Full.Name
-    , types :: HashMap Full.Name (Type m)
-    }
-
-newtype TransformT m a = TransformT
-    { runTransformT :: ReaderT (Replacement m) m a
-    }
-
-instance Functor m => Functor (TransformT m) where
-    fmap f = TransformT . fmap f . runTransformT
-
-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
-
-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 ResolverException = forall e. Exception e => ResolverException e
-
-instance Show ResolverException where
-    show (ResolverException e) = show e
-
-instance Exception ResolverException where
-    toException = graphQLExceptionToException
-    fromException = graphQLExceptionFromException
-
-data FieldError
-    = ArgumentTypeError
-    | MissingArgumentError
-    | EnumCompletionError
-    | InterfaceCompletionError
-    | UnionCompletionError
-    | ValueCompletionError
-    | ResultCoercionError
-    | NullResultError
-
-instance Show FieldError where
-    show ArgumentTypeError = "Invalid argument type."
-    show MissingArgumentError = "Required argument not specified."
-    show EnumCompletionError = "Enum value completion failed."
-    show InterfaceCompletionError = "Interface value completion failed."
-    show UnionCompletionError = "Union value completion failed."
-    show ValueCompletionError = "Value completion failed."
-    show ResultCoercionError = "Result coercion failed."
-    show NullResultError = "Non-Nullable field resolver returned Null."
-
-newtype FieldException = FieldException FieldError
-    deriving Show
-
-instance Exception FieldException where
-   toException = graphQLExceptionToException
-   fromException = graphQLExceptionFromException
-
-data QueryError
-   = OperationNameRequired
-   | OperationNotFound String
-   | CoercionError Full.VariableDefinition
-   | UnknownInputType Full.VariableDefinition
-
-asks :: Monad m => forall a. (Replacement m -> a) -> TransformT m a
-asks = TransformT . Reader.asks
-
-queryError :: QueryError -> Error
-queryError OperationNameRequired =
-    Error{ message = "Operation name is required.", locations = [], path = [] }
-queryError (OperationNotFound operationName) =
-    let queryErrorMessage = Text.concat
-            [ "Operation \""
-            , Text.pack operationName
-            , "\" not found."
-            ]
-     in Error{ message = queryErrorMessage, locations = [], path = [] }
-queryError (CoercionError variableDefinition) =
-    let Full.VariableDefinition variableName _ _ location = variableDefinition
-        queryErrorMessage = Text.concat
-            [ "Failed to coerce the variable \""
-            , variableName
-            , "\"."
-            ]
-     in Error{ message = queryErrorMessage, locations = [location], path = [] }
-queryError (UnknownInputType variableDefinition) =
-    let Full.VariableDefinition variableName variableTypeName _ location = variableDefinition
-        queryErrorMessage = Text.concat
-            [ "Variable \""
-            , variableName
-            , "\" has unknown type \""
-            , Text.pack $ show variableTypeName
-            , "\"."
-            ]
-     in Error{ message = queryErrorMessage, locations = [location], path = [] }
-
-data Operation m
-    = Operation Full.OperationType (Seq (Selection m)) Full.Location
-
-data Selection m
-    = FieldSelection (Field m)
-    | FragmentSelection (Fragment m)
-
-data Field m = Field
-    (Maybe Full.Name)
-    Full.Name
-    (HashMap Full.Name (Full.Node Input))
-    (Seq (Selection m))
-    Full.Location
-
-data Fragment m = Fragment
-    (Type.Internal.CompositeType m) (Seq (Selection m)) Full.Location
-
-data Input
-    = Variable Type.Value
-    | Int Int32
-    | Float Double
-    | String Text
-    | Boolean Bool
-    | Null
-    | Enum Full.Name
-    | List [Input]
-    | Object (HashMap Full.Name Input)
-
-document :: Full.Document
-    -> ([Full.OperationDefinition], HashMap Full.Name Full.FragmentDefinition)
-document = foldr filterOperation ([], HashMap.empty)
-  where
-    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.
-
-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 [Type.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.Internal.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.Internal.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 _) = do
-        argumentValue <- node valueNode
-        pure $ insertIfGiven name' argumentValue accumulator
-
-directive :: Monad m => Full.Directive -> TransformT m Type.Directive
-directive (Full.Directive name' arguments' _)
-    = Type.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
-    -> HashMap Full.Name a
-insertIfGiven name (Just v) = HashMap.insert name v
-insertIfGiven _ _ = id
-
-node :: Monad m => Full.Node Full.Value -> TransformT m (Maybe (Full.Node Input))
-node Full.Node{node = node', ..} =
-    traverse Full.Node <$> input node' <*> pure location
-
-execute :: (MonadCatch m, Coerce.VariableValue a, Coerce.Serialize b)
-    => Schema m -- ^ Resolvers.
-    -> Maybe Text -- ^ Operation name.
-    -> HashMap Full.Name a -- ^ Variable substitution function.
-    -> Full.Document -- @GraphQL@ document.
-    -> m (Either (ResponseEventStream m b) (Response b))
-execute schema' operationName subs document' =
-    executeRequest schema' document' (Text.unpack <$> operationName) subs
-
-executeRequest :: (MonadCatch m, Coerce.Serialize a, Coerce.VariableValue b)
-    => Schema m
-    -> Full.Document
-    -> Maybe String
-    -> HashMap Full.Name b
-    -> m (Either (ResponseEventStream m a) (Response a))
-executeRequest schema sourceDocument operationName variableValues = do
-    operationAndVariables <- sequence buildOperation
-    case operationAndVariables of
-        Left queryError' -> pure
-            $ Right
-            $ Response Coerce.null $ pure $ queryError queryError'
-        Right operation
-            | Operation Full.Query topSelections _operationLocation <- operation ->
-                 Right <$> executeQuery topSelections schema
-            | Operation Full.Mutation topSelections operationLocation <- operation ->
-                Right <$> executeMutation topSelections schema operationLocation
-            | Operation Full.Subscription topSelections operationLocation <- operation ->
-                either rightErrorResponse Left <$> subscribe topSelections schema operationLocation
-  where
-    schemaTypes = Schema.types schema
-    (operationDefinitions, fragmentDefinitions') = document sourceDocument
-    buildOperation = do
-        operationDefinition <- getOperation operationDefinitions operationName
-        coercedVariableValues <- coerceVariableValues
-            schemaTypes
-            operationDefinition
-            variableValues
-        let replacement = Replacement
-                { variableValues = coercedVariableValues
-                , fragmentDefinitions = fragmentDefinitions'
-                , visitedFragments = mempty
-                , types = schemaTypes
-                }
-        pure $ flip runReaderT replacement
-            $ runTransformT
-            $ transform operationDefinition
-
-rightErrorResponse :: Coerce.Serialize b => forall a. Error -> Either a (Response b)
-rightErrorResponse = Right . Response Coerce.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, Coerce.Serialize a)
-    => Seq (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
-        $ executeSelectionSet topSelections queryType Type.Null []
-    pure $ Response data' errors
-
-executeMutation :: (MonadCatch m, Coerce.Serialize a)
-    => Seq (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
-            $ executeSelectionSet topSelections mutationType Type.Null []
-        pure $ Response data' errors
-    | otherwise = pure
-        $ Response Coerce.null
-        $ Seq.singleton
-        $ Error "Schema doesn't support mutations." [operationLocation] []
-
-executeSelectionSet :: (MonadCatch m, Coerce.Serialize a)
-    => Seq (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) $ Coerce.Object resolvedValues
-  where
-    executeField' fields resolver =
-        executeField objectValue fields resolver errorPath
-    Out.ObjectType _ _ _ resolvers = objectType
-    go fields@(Field _ fieldName _ _ _ :| _) =
-        traverse (executeField' fields) $ HashMap.lookup fieldName resolvers
-
-fieldsSegment :: forall m. NonEmpty (Field m) -> Path
-fieldsSegment (Field alias fieldName _ _ _ :| _) =
-    Segment (fromMaybe fieldName alias)
-
-executeField :: (MonadCatch m, Coerce.Serialize a)
-    => Type.Value
-    -> NonEmpty (Field m)
-    -> Out.Resolver m
-    -> [Path]
-    -> ExecutorT m a
-executeField objectValue fields resolver errorPath =
-    let Field _ fieldName inputArguments _ fieldLocation :| _ = fields
-     in catch (go fieldName inputArguments) $ exceptionHandler fieldLocation
-  where
-    exceptionHandler :: (MonadCatch m, Coerce.Serialize a)
-        => Full.Location
-        -> GraphQLException
-        -> ExecutorT m a
-    exceptionHandler fieldLocation e =
-        let newError = Error (Text.pack $ displayException e) [fieldLocation] errorPath
-         in ExecutorT (lift $ tell $ Seq.singleton newError) >> pure Coerce.null
-    go fieldName inputArguments = do
-        let (Out.Field _ fieldType argumentTypes, resolveFunction) =
-                resolverField resolver
-        argumentValues <- coerceArgumentValues argumentTypes inputArguments
-        resolvedValue <-
-            resolveFieldValue resolveFunction objectValue fieldName argumentValues
-        completeValue fieldType fields errorPath resolvedValue
-    resolverField (Out.ValueResolver resolverField' resolveFunction) =
-        (resolverField', resolveFunction)
-    resolverField (Out.EventStreamResolver resolverField' resolveFunction _) =
-        (resolverField', resolveFunction)
-
-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
-
-completeValue :: (MonadCatch m, Coerce.Serialize a)
-    => Out.Type m
-    -> NonEmpty (Field m)
-    -> [Path]
-    -> Type.Value
-    -> ExecutorT m a
-completeValue outputType _ _ Type.Null
-    | Out.isNonNullType outputType = throwFieldError NullResultError
-    | otherwise = pure Coerce.null
-completeValue outputType@(Out.ListBaseType listType) fields errorPath (Type.List list)
-    = foldM go (0, []) list >>= coerceResult outputType . Coerce.List . snd
-  where
-    go (index, accumulator) listItem = do
-        let updatedPath = Index index : errorPath
-        completedValue <- completeValue listType fields updatedPath listItem
-        pure (index + 1, completedValue : accumulator)
-completeValue outputType@(Out.ScalarBaseType _) _ _ (Type.Int int) =
-    coerceResult outputType $ Coerce.Int int
-completeValue outputType@(Out.ScalarBaseType _) _ _ (Type.Boolean boolean) =
-    coerceResult outputType $ Coerce.Boolean boolean
-completeValue outputType@(Out.ScalarBaseType _) _ _ (Type.Float float) =
-    coerceResult outputType $ Coerce.Float float
-completeValue outputType@(Out.ScalarBaseType _) _ _ (Type.String string) =
-    coerceResult outputType $ Coerce.String string
-completeValue outputType@(Out.EnumBaseType enumType) _ _ (Type.Enum enum) =
-    let Type.EnumType _ _ enumMembers = enumType
-     in if HashMap.member enum enumMembers
-        then coerceResult outputType $ Coerce.Enum enum
-        else throwFieldError EnumCompletionError
-completeValue (Out.ObjectBaseType objectType) fields errorPath result
-    = executeSelectionSet (mergeSelectionSets fields) objectType result
-    $ fieldsSegment fields : errorPath
-completeValue (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 -> throwFieldError InterfaceCompletionError
-completeValue (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 -> throwFieldError UnionCompletionError
-completeValue _ _ _ _ = throwFieldError ValueCompletionError
-
-coerceResult :: (MonadCatch m, Coerce.Serialize a)
-    => Out.Type m
-    -> Coerce.Output a
-    -> ExecutorT m a
-coerceResult outputType result
-    | Just serialized <- Coerce.serialize outputType result = pure serialized
-    | otherwise = throwFieldError ResultCoercionError
-
-mergeSelectionSets :: MonadCatch m
-    => NonEmpty (Field m)
-    -> Seq (Selection m)
-mergeSelectionSets = foldr forEach mempty
-  where
-    forEach (Field _ _ _ fieldSelectionSet _) selectionSet' =
-        selectionSet' <> fieldSelectionSet
-
-throwFieldError :: MonadCatch m => FieldError -> m a
-throwFieldError = throwM . FieldException
-
-coerceArgumentValues :: MonadCatch m
-    => HashMap Full.Name In.Argument
-    -> HashMap Full.Name (Full.Node 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 _ <- HashMap.lookup argumentName argumentValues ->
-                    throwFieldError ArgumentTypeError
-                | otherwise -> throwFieldError MissingArgumentError
-    matchFieldValues' = Coerce.matchFieldValues coerceArgumentValue
-        $ Full.node <$> argumentValues
-    coerceArgumentValue inputType (Int integer) =
-        Coerce.coerceInputLiteral inputType (Type.Int integer)
-    coerceArgumentValue inputType (Boolean boolean) =
-        Coerce.coerceInputLiteral inputType (Type.Boolean boolean)
-    coerceArgumentValue inputType (String string) =
-        Coerce.coerceInputLiteral inputType (Type.String string)
-    coerceArgumentValue inputType (Float float) =
-        Coerce.coerceInputLiteral inputType (Type.Float float)
-    coerceArgumentValue inputType (Enum enum) =
-        Coerce.coerceInputLiteral inputType (Type.Enum enum)
-    coerceArgumentValue inputType Null
-        | In.isNonNullType inputType = Nothing
-        | otherwise = Coerce.coerceInputLiteral inputType Type.Null
-    coerceArgumentValue (In.ListBaseType inputType) (List list) =
-        let coerceItem = coerceArgumentValue inputType
-         in Type.List <$> traverse coerceItem list
-    coerceArgumentValue (In.InputObjectBaseType inputType) (Object object)
-        | In.InputObjectType _ _ inputFields <- inputType = 
-            let go = forEachField object
-                resultMap = HashMap.foldrWithKey go (pure mempty) inputFields
-             in Type.Object <$> resultMap
-    coerceArgumentValue _ (Variable variable) = pure variable
-    coerceArgumentValue _ _ = Nothing
-    forEachField object variableName (In.InputField _ variableType defaultValue) =
-        Coerce.matchFieldValues coerceArgumentValue object variableName variableType defaultValue
-
-collectFields :: Monad m
-    => Out.ObjectType m
-    -> Seq (Selection m)
-    -> OrderedMap (NonEmpty (Field m))
-collectFields objectType = foldl forEach OrderedMap.empty
-  where
-    forEach groupedFields (FieldSelection fieldSelection) =
-        let Field maybeAlias fieldName _ _ _ = fieldSelection
-            responseKey = fromMaybe fieldName maybeAlias
-         in OrderedMap.insert responseKey (fieldSelection :| []) groupedFields
-    forEach groupedFields (FragmentSelection selectionFragment)
-        | Fragment fragmentType fragmentSelectionSet _ <- selectionFragment
-        , Type.Internal.doesFragmentTypeApply fragmentType objectType =
-            let fragmentGroupedFieldSet =
-                    collectFields objectType fragmentSelectionSet
-             in groupedFields <> fragmentGroupedFieldSet
-        | otherwise = groupedFields
-
-coerceVariableValues :: (Monad m, Coerce.VariableValue b)
-    => HashMap Full.Name (Schema.Type m)
-    -> Full.OperationDefinition
-    -> HashMap Full.Name b
-    -> Either QueryError Type.Subs
-coerceVariableValues types operationDefinition' variableValues
-    | Full.OperationDefinition _ _ variableDefinitions _ _ _ <-
-        operationDefinition'
-    = foldr forEach (Right HashMap.empty) variableDefinitions
-    | otherwise = pure mempty
-  where
-    forEach variableDefinition (Right coercedValues) =
-        let Full.VariableDefinition variableName variableTypeName defaultValue _ =
-                variableDefinition
-            defaultValue' = constValue . Full.node <$> defaultValue
-         in case Type.Internal.lookupInputType variableTypeName types of
-            Just variableType ->
-                maybe (Left $ CoercionError variableDefinition) Right
-                    $ Coerce.matchFieldValues
-                        coerceVariableValue'
-                        variableValues
-                        variableName
-                        variableType
-                        defaultValue'
-                    $ Just coercedValues
-            Nothing -> Left $ UnknownInputType variableDefinition
-    forEach _ coercedValuesOrError = coercedValuesOrError
-    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 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')
-
-type ResponseEventStream m a = ConduitT () (Response a) m ()
-
-subscribe :: (MonadCatch m, Coerce.Serialize a)
-    => Seq (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, Coerce.Serialize a)
-    => HashMap Full.Name (Type m)
-    -> Out.ObjectType m
-    -> Seq (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 (Selection m)
-    -> m (Either Error (Out.SourceEventStream m))
-createSourceEventStream _types subscriptionType objectLocation fields
-    | [fieldGroup] <- OrderedMap.elems groupedFieldSet
-    , 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, Coerce.Serialize a)
-    => HashMap Full.Name (Type m)
-    -> Out.ObjectType m
-    -> Seq (Selection m)
-    -> Type.Value
-    -> m (Response a)
-executeSubscriptionEvent types' objectType fields initialValue = do
-    (data', errors) <- runWriterT
-        $ flip runReaderT types'
-        $ runExecutorT
-        $ executeSelectionSet fields objectType initialValue []
-    pure $ Response data' errors
--- a/src/Language/GraphQL/Type/Definition.hs
+++ b/src/Language/GraphQL/Type/Definition.hs
@ -1,4 +1,5 @@
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE Safe #-}

 -- | Types that can be used as both input and output types.
 module Language.GraphQL.Type.Definition
--- a/src/Language/GraphQL/Type/In.hs
+++ b/src/Language/GraphQL/Type/In.hs
@ -3,6 +3,7 @@
   obtain one at https://mozilla.org/MPL/2.0/. -}

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

 -- | Input types and values.
--- a/tests/Language/GraphQL/ExecuteSpec.hs
+++ b/tests/Language/GraphQL/ExecuteSpec.hs
@ -54,18 +54,23 @@ queryType = Out.ObjectType "Query" Nothing []
    $ HashMap.fromList
    [ ("philosopher", ValueResolver philosopherField philosopherResolver)
    , ("genres", ValueResolver genresField genresResolver)
+    , ("count", ValueResolver countField countResolver)
    ]
  where
    philosopherField =
-        Out.Field Nothing (Out.NonNullObjectType philosopherType)
+        Out.Field Nothing (Out.NamedObjectType philosopherType)
        $ HashMap.singleton "id"
        $ In.Argument Nothing (In.NamedScalarType id) Nothing
    philosopherResolver = pure $ Object mempty
    genresField =
-      let fieldType = Out.ListType $ Out.NonNullScalarType string
-       in Out.Field Nothing fieldType HashMap.empty
+        let fieldType = Out.ListType $ Out.NonNullScalarType string
+         in Out.Field Nothing fieldType HashMap.empty
    genresResolver :: Resolve (Either SomeException)
    genresResolver = throwM PhilosopherException
+    countField =
+        let fieldType = Out.NonNullScalarType int
+         in Out.Field Nothing fieldType HashMap.empty
+    countResolver = pure ""

 musicType :: Out.ObjectType (Either SomeException)
 musicType = Out.ObjectType "Music" Nothing []
@ -230,9 +235,7 @@ spec =

            it "errors on invalid output enum values" $
                let data'' = Aeson.object
-                        [ "philosopher" .= Aeson.object
-                            [ "school" .= Aeson.Null
-                            ]
+                        [ "philosopher" .= Aeson.Null
                        ]
                    executionErrors = pure $ Error
                        { message =
@ -247,9 +250,7 @@ spec =

            it "gives location information for non-null unions" $
                let data'' = Aeson.object
-                        [ "philosopher" .= Aeson.object
-                            [ "interest" .= Aeson.Null
-                            ]
+                        [ "philosopher" .= Aeson.Null
                        ]
                    executionErrors = pure $ Error
                        { message =
@ -264,9 +265,7 @@ spec =

            it "gives location information for invalid interfaces" $
                let data'' = Aeson.object
-                        [ "philosopher" .= Aeson.object
-                            [ "majorWork" .= Aeson.Null
-                            ]
+                        [ "philosopher" .= Aeson.Null
                        ]
                    executionErrors = pure $ Error
                        { message
@ -297,14 +296,12 @@ spec =

            it "gives location information for failed result coercion" $
                let data'' = Aeson.object
-                        [ "philosopher" .= Aeson.object
-                            [ "century" .= Aeson.Null
-                            ]
+                        [ "philosopher" .= Aeson.Null
                        ]
                    executionErrors = pure $ Error
-                        { message = "Result coercion failed."
+                        { message = "Unable to coerce result to !Int."
                        , locations = [Location 1 26]
-                        , path = []
+                        , path = [Segment "philosopher", Segment "century"]
                        }
                    expected = Response data'' executionErrors
                    Right (Right actual) = either (pure . parseError) execute'
@ -318,13 +315,24 @@ spec =
                    executionErrors = pure $ Error
                        { message = "PhilosopherException"
                        , locations = [Location 1 3]
-                        , path = []
+                        , path = [Segment "genres"]
                        }
                    expected = Response data'' executionErrors
                    Right (Right actual) = either (pure . parseError) execute'
                        $ parse document "" "{ genres }"
                in actual `shouldBe` 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 Aeson.Null executionErrors
+                    Right (Right actual) = either (pure . parseError) execute'
+                        $ parse document "" "{ count }"
+                in actual `shouldBe` expected
+
        context "Subscription" $
            it "subscribes" $
                let data'' = Aeson.object