diff --git a/CHANGELOG.md b/CHANGELOG.md
index acc418d..374dab1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -17,6 +17,7 @@ and this project adheres to
   interfaces they implement.
 - Show instances for GraphQL type definitions in the `Type` modules.
 - Custom Show instances for type and value representations in the AST.
+- `AST.Document.escape` escapes a single character in a `StringValue`.
 
 ## [0.11.0.0] - 2020-11-07
 ### Changed
diff --git a/graphql.cabal b/graphql.cabal
index f51e6d1..c84156d 100644
--- a/graphql.cabal
+++ b/graphql.cabal
@@ -4,7 +4,7 @@ cabal-version: 2.2
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 55ede035c74b423b607abac158fb26436e4ea06a43fecdcc2ef42423e63597fc
+-- hash: f04885cb80064cc5262a6059af1977c0da58df82e3373eb619e7238f3066c466
 
 name:           graphql
 version:        0.11.0.0
@@ -86,7 +86,7 @@ test-suite graphql-test
       Language.GraphQL.Execute.CoerceSpec
       Language.GraphQL.ExecuteSpec
       Language.GraphQL.Type.OutSpec
-      Language.GraphQL.ValidateSpec
+      Language.GraphQL.Validate.RulesSpec
       Test.DirectiveSpec
       Test.FragmentSpec
       Test.RootOperationSpec
diff --git a/tests/Language/GraphQL/Validate/RulesSpec.hs b/tests/Language/GraphQL/Validate/RulesSpec.hs
new file mode 100644
index 0000000..6f90436
--- /dev/null
+++ b/tests/Language/GraphQL/Validate/RulesSpec.hs
@@ -0,0 +1,914 @@
+{- This Source Code Form is subject to the terms of the Mozilla Public License,
+   v. 2.0. If a copy of the MPL was not distributed with this file, You can
+   obtain one at https://mozilla.org/MPL/2.0/. -}
+
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE QuasiQuotes #-}
+
+module Language.GraphQL.Validate.RulesSpec
+    ( spec
+    ) where
+
+import Data.Foldable (toList)
+import qualified Data.HashMap.Strict as HashMap
+import Data.Text (Text)
+import qualified Language.GraphQL.AST as AST
+import Language.GraphQL.Type
+import qualified Language.GraphQL.Type.In as In
+import qualified Language.GraphQL.Type.Out as Out
+import Language.GraphQL.Validate
+import Test.Hspec (Spec, context, describe, it, shouldBe, shouldContain)
+import Text.Megaparsec (parse, errorBundlePretty)
+import Text.RawString.QQ (r)
+
+petSchema :: Schema IO
+petSchema = schema queryType Nothing (Just subscriptionType) mempty
+
+queryType :: ObjectType IO
+queryType = ObjectType "Query" Nothing [] $ HashMap.fromList
+    [ ("dog", dogResolver)
+    , ("cat", catResolver)
+    , ("findDog", findDogResolver)
+    ]
+  where
+    dogField = Field Nothing (Out.NamedObjectType dogType) mempty
+    dogResolver = ValueResolver dogField $ pure Null
+    findDogArguments = HashMap.singleton "complex"
+        $ In.Argument Nothing (In.NonNullInputObjectType dogDataType) Nothing
+    findDogField = Field Nothing (Out.NamedObjectType dogType) findDogArguments
+    findDogResolver = ValueResolver findDogField $ pure Null
+    catField = Field Nothing (Out.NamedObjectType catType) mempty
+    catResolver = ValueResolver catField $ pure Null
+
+catCommandType :: EnumType
+catCommandType = EnumType "CatCommand" Nothing $ HashMap.fromList
+    [ ("JUMP", EnumValue Nothing)
+    ]
+
+catType :: ObjectType IO
+catType = ObjectType "Cat" Nothing [petType] $ HashMap.fromList
+    [ ("name", nameResolver)
+    , ("nickname", nicknameResolver)
+    , ("doesKnowCommand", doesKnowCommandResolver)
+    , ("meowVolume", meowVolumeResolver)
+    ]
+  where
+    meowVolumeField = Field Nothing (Out.NamedScalarType int) mempty
+    meowVolumeResolver = ValueResolver meowVolumeField $ pure $ Int 3
+    doesKnowCommandField = Field Nothing (Out.NonNullScalarType boolean)
+        $ HashMap.singleton "catCommand"
+        $ In.Argument Nothing (In.NonNullEnumType catCommandType) Nothing
+    doesKnowCommandResolver = ValueResolver doesKnowCommandField
+        $ pure $ Boolean True
+
+nameResolver :: Resolver IO
+nameResolver = ValueResolver nameField $ pure "Name"
+  where
+    nameField = Field Nothing (Out.NonNullScalarType string) mempty
+
+nicknameResolver :: Resolver IO
+nicknameResolver = ValueResolver nicknameField $ pure "Nickname"
+  where
+    nicknameField = Field Nothing (Out.NamedScalarType string) mempty
+
+dogCommandType :: EnumType
+dogCommandType = EnumType "DogCommand" Nothing $ HashMap.fromList
+    [ ("SIT", EnumValue Nothing)
+    , ("DOWN", EnumValue Nothing)
+    , ("HEEL", EnumValue Nothing)
+    ]
+
+dogType :: ObjectType IO
+dogType = ObjectType "Dog" Nothing [petType] $ HashMap.fromList
+    [ ("name", nameResolver)
+    , ("nickname", nicknameResolver)
+    , ("barkVolume", barkVolumeResolver)
+    , ("doesKnowCommand", doesKnowCommandResolver)
+    , ("isHousetrained", isHousetrainedResolver)
+    , ("owner", ownerResolver)
+    ]
+  where
+    barkVolumeField = Field Nothing (Out.NamedScalarType int) mempty
+    barkVolumeResolver = ValueResolver barkVolumeField $ pure $ Int 3
+    doesKnowCommandField = Field Nothing (Out.NonNullScalarType boolean)
+        $ HashMap.singleton "dogCommand"
+        $ In.Argument Nothing (In.NonNullEnumType dogCommandType) Nothing
+    doesKnowCommandResolver = ValueResolver doesKnowCommandField
+        $ pure $ Boolean True
+    isHousetrainedField = Field Nothing (Out.NonNullScalarType boolean)
+        $ HashMap.singleton "atOtherHomes"
+        $ In.Argument Nothing (In.NamedScalarType boolean) Nothing
+    isHousetrainedResolver = ValueResolver isHousetrainedField
+        $ pure $ Boolean True
+    ownerField = Field Nothing (Out.NamedObjectType humanType) mempty
+    ownerResolver = ValueResolver ownerField $ pure Null
+
+dogDataType :: InputObjectType
+dogDataType = InputObjectType "DogData" Nothing
+    $ HashMap.singleton "name" nameInputField
+  where
+    nameInputField = InputField Nothing (In.NonNullScalarType string) Nothing
+
+sentientType :: InterfaceType IO
+sentientType = InterfaceType "Sentient" Nothing []
+    $ HashMap.singleton "name"
+    $ Field Nothing (Out.NonNullScalarType string) mempty
+
+petType :: InterfaceType IO
+petType = InterfaceType "Pet" Nothing []
+    $ HashMap.singleton "name"
+    $ Field Nothing (Out.NonNullScalarType string) mempty
+
+subscriptionType :: ObjectType IO
+subscriptionType = ObjectType "Subscription" Nothing [] $ HashMap.fromList
+    [ ("newMessage", newMessageResolver)
+    , ("disallowedSecondRootField", newMessageResolver)
+    ]
+  where
+    newMessageField = Field Nothing (Out.NonNullObjectType messageType) mempty
+    newMessageResolver = ValueResolver newMessageField
+        $ pure $ Object HashMap.empty
+
+messageType :: ObjectType IO
+messageType = ObjectType "Message" Nothing [] $ HashMap.fromList
+    [ ("sender", senderResolver)
+    , ("body", bodyResolver)
+    ]
+  where
+    senderField = Field Nothing (Out.NonNullScalarType string) mempty
+    senderResolver = ValueResolver senderField $ pure "Sender"
+    bodyField = Field Nothing (Out.NonNullScalarType string) mempty
+    bodyResolver = ValueResolver bodyField $ pure "Message body."
+
+humanType :: ObjectType IO
+humanType = ObjectType "Human" Nothing [sentientType] $ HashMap.fromList
+    [ ("name", nameResolver)
+    , ("pets", petsResolver)
+    ]
+  where
+    petsField =
+        Field Nothing (Out.ListType $ Out.NonNullInterfaceType petType) mempty
+    petsResolver = ValueResolver petsField $ pure $ List []
+
+validate :: Text -> [Error]
+validate queryString =
+    case parse AST.document "" queryString of
+        Left parseErrors -> error $ errorBundlePretty parseErrors
+        Right ast -> toList $ document petSchema specifiedRules ast
+
+spec :: Spec
+spec =
+    describe "document" $ do
+        context "executableDefinitionsRule" $
+            it "rejects type definitions" $
+                let queryString = [r|
+                  query getDogName {
+                    dog {
+                      name
+                      color
+                    }
+                  }
+
+                  extend type Dog {
+                    color: String
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Definition must be OperationDefinition or \
+                            \FragmentDefinition."
+                        , locations = [AST.Location 9 19]
+                        }
+                 in validate queryString `shouldContain` [expected]
+
+        context "singleFieldSubscriptionsRule" $ do
+            it "rejects multiple subscription root fields" $
+                let queryString = [r|
+                  subscription sub {
+                    newMessage {
+                      body
+                      sender
+                    }
+                    disallowedSecondRootField
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Subscription \"sub\" must select only one top \
+                            \level field."
+                        , locations = [AST.Location 2 19]
+                        }
+                 in validate queryString `shouldContain` [expected]
+
+            it "rejects multiple subscription root fields coming from a fragment" $
+                let queryString = [r|
+                  subscription sub {
+                    ...multipleSubscriptions
+                  }
+
+                  fragment multipleSubscriptions on Subscription {
+                    newMessage {
+                      body
+                      sender
+                    }
+                    disallowedSecondRootField
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Subscription \"sub\" must select only one top \
+                            \level field."
+                        , locations = [AST.Location 2 19]
+                        }
+                 in validate queryString `shouldContain` [expected]
+
+            it "finds corresponding subscription fragment" $
+                let queryString = [r|
+                  subscription sub {
+                    ...anotherSubscription
+                    ...multipleSubscriptions
+                  }
+                  fragment multipleSubscriptions on Subscription {
+                    newMessage {
+                      body
+                    }
+                    disallowedSecondRootField {
+                      sender
+                    }
+                  }
+                  fragment anotherSubscription on Subscription {
+                    newMessage {
+                      body
+                      sender
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Subscription \"sub\" must select only one top \
+                            \level field."
+                        , locations = [AST.Location 2 19]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "loneAnonymousOperationRule" $
+            it "rejects multiple anonymous operations" $
+                let queryString = [r|
+                  {
+                    dog {
+                      name
+                    }
+                  }
+
+                  query getName {
+                    dog {
+                      owner {
+                        name
+                      }
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "This anonymous operation must be the only defined \
+                            \operation."
+                        , locations = [AST.Location 2 19]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "uniqueOperationNamesRule" $
+            it "rejects operations with the same name" $
+                let queryString = [r|
+                  query dogOperation {
+                    dog {
+                      name
+                    }
+                  }
+
+                  mutation dogOperation {
+                    mutateDog {
+                      id
+                    }
+                }
+                |]
+                    expected = Error
+                        { message =
+                            "There can be only one operation named \
+                            \\"dogOperation\"."
+                        , locations = [AST.Location 2 19, AST.Location 8 19]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "uniqueFragmentNamesRule" $
+            it "rejects fragments with the same name" $
+                let queryString = [r|
+                  {
+                    dog {
+                      ...fragmentOne
+                    }
+                  }
+
+                  fragment fragmentOne on Dog {
+                    name
+                  }
+
+                  fragment fragmentOne on Dog {
+                    owner {
+                      name
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "There can be only one fragment named \
+                            \\"fragmentOne\"."
+                        , locations = [AST.Location 8 19, AST.Location 12 19]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "fragmentSpreadTargetDefinedRule" $
+            it "rejects the fragment spread without a target" $
+                let queryString = [r|
+                  {
+                    dog {
+                      ...undefinedFragment
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Fragment target \"undefinedFragment\" is \
+                            \undefined."
+                        , locations = [AST.Location 4 23]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "fragmentSpreadTypeExistenceRule" $ do
+            it "rejects fragment spreads without an unknown target type" $
+                let queryString = [r|
+                  {
+                    dog {
+                      ...notOnExistingType
+                    }
+                  }
+                  fragment notOnExistingType on NotInSchema {
+                    name
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Fragment \"notOnExistingType\" is specified on \
+                            \type \"NotInSchema\" which doesn't exist in the \
+                            \schema."
+                        , locations = [AST.Location 4 23]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+            it "rejects inline fragments without a target" $
+                let queryString = [r|
+                  {
+                    ... on NotInSchema {
+                      name
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Inline fragment is specified on type \
+                            \\"NotInSchema\" which doesn't exist in the schema."
+                        , locations = [AST.Location 3 21]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "fragmentsOnCompositeTypesRule" $ do
+            it "rejects fragments on scalar types" $
+                let queryString = [r|
+                  {
+                    dog {
+                      ...fragOnScalar
+                    }
+                  }
+                  fragment fragOnScalar on Int {
+                    name
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Fragment cannot condition on non composite type \
+                            \\"Int\"."
+                        , locations = [AST.Location 7 19]
+                        }
+                 in validate queryString `shouldContain` [expected]
+
+            it "rejects inline fragments on scalar types" $
+                let queryString = [r|
+                  {
+                    ... on Boolean {
+                      name
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Fragment cannot condition on non composite type \
+                            \\"Boolean\"."
+                        , locations = [AST.Location 3 21]
+                        }
+                in validate queryString `shouldContain` [expected]
+
+        context "noUnusedFragmentsRule" $
+            it "rejects unused fragments" $
+                let queryString = [r|
+                  fragment nameFragment on Dog { # unused
+                    name
+                  }
+
+                  {
+                    dog {
+                      name
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Fragment \"nameFragment\" is never used."
+                        , locations = [AST.Location 2 19]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "noFragmentCyclesRule" $
+            it "rejects spreads that form cycles" $
+                let queryString = [r|
+                  {
+                    dog {
+                      ...nameFragment
+                    }
+                  }
+                  fragment nameFragment on Dog {
+                    name
+                    ...barkVolumeFragment
+                  }
+                  fragment barkVolumeFragment on Dog {
+                    barkVolume
+                    ...nameFragment
+                  }
+                |]
+                    error1 = Error
+                        { message =
+                            "Cannot spread fragment \"barkVolumeFragment\" \
+                            \within itself (via barkVolumeFragment -> \
+                            \nameFragment -> barkVolumeFragment)."
+                        , locations = [AST.Location 11 19]
+                        }
+                    error2 = Error
+                        { message =
+                            "Cannot spread fragment \"nameFragment\" within \
+                            \itself (via nameFragment -> barkVolumeFragment -> \
+                            \nameFragment)."
+                        , locations = [AST.Location 7 19]
+                        }
+                 in validate queryString `shouldBe` [error1, error2]
+
+        context "uniqueArgumentNamesRule" $
+            it "rejects duplicate field arguments" $
+                let queryString = [r|
+                  {
+                    dog {
+                      isHousetrained(atOtherHomes: true, atOtherHomes: true)
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "There can be only one argument named \
+                            \\"atOtherHomes\"."
+                        , locations = [AST.Location 4 38, AST.Location 4 58]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "uniqueDirectiveNamesRule" $
+            it "rejects more than one directive per location" $
+                let queryString = [r|
+                  query ($foo: Boolean = true, $bar: Boolean = false) {
+                    dog @skip(if: $foo) @skip(if: $bar) {
+                      name
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "There can be only one directive named \"skip\"."
+                        , locations = [AST.Location 3 25, AST.Location 3 41]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "uniqueVariableNamesRule" $
+            it "rejects duplicate variables" $
+                let queryString = [r|
+                  query houseTrainedQuery($atOtherHomes: Boolean, $atOtherHomes: Boolean) {
+                    dog {
+                      isHousetrained(atOtherHomes: $atOtherHomes)
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "There can be only one variable named \
+                            \\"atOtherHomes\"."
+                        , locations = [AST.Location 2 43, AST.Location 2 67]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "variablesAreInputTypesRule" $
+            it "rejects non-input types as variables" $
+                let queryString = [r|
+                  query takesDogBang($dog: Dog!) {
+                    dog {
+                      isHousetrained(atOtherHomes: $dog)
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Variable \"$dog\" cannot be non-input type \
+                            \\"Dog\"."
+                        , locations = [AST.Location 2 38]
+                        }
+                 in validate queryString `shouldContain` [expected]
+
+        context "noUndefinedVariablesRule" $
+            it "rejects undefined variables" $
+                let queryString = [r|
+                  query variableIsNotDefinedUsedInSingleFragment {
+                    dog {
+                      ...isHousetrainedFragment
+                    }
+                  }
+
+                  fragment isHousetrainedFragment on Dog {
+                    isHousetrained(atOtherHomes: $atOtherHomes)
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Variable \"$atOtherHomes\" is not defined by \
+                            \operation \
+                            \\"variableIsNotDefinedUsedInSingleFragment\"."
+                        , locations = [AST.Location 9 50]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "noUnusedVariablesRule" $
+            it "rejects unused variables" $
+                let queryString = [r|
+                  query variableUnused($atOtherHomes: Boolean) {
+                    dog {
+                      isHousetrained
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Variable \"$atOtherHomes\" is never used in \
+                            \operation \"variableUnused\"."
+                        , locations = [AST.Location 2 40]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "uniqueInputFieldNamesRule" $
+            it "rejects duplicate fields in input objects" $
+                let queryString = [r|
+                  {
+                    findDog(complex: { name: "Fido", name: "Jack" }) {
+                      name
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "There can be only one input field named \"name\"."
+                        , locations = [AST.Location 3 40, AST.Location 3 54]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "fieldsOnCorrectTypeRule" $
+            it "rejects undefined fields" $
+                let queryString = [r|
+                  {
+                    dog {
+                      meowVolume
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Cannot query field \"meowVolume\" on type \"Dog\"."
+                        , locations = [AST.Location 4 23]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "scalarLeafsRule" $
+            it "rejects scalar fields with not empty selection set" $
+                let queryString = [r|
+                  {
+                    dog {
+                      barkVolume {
+                        sinceWhen
+                      }
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Field \"barkVolume\" must not have a selection \
+                            \since type \"Int\" has no subfields."
+                        , locations = [AST.Location 4 23]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "knownArgumentNamesRule" $ do
+            it "rejects field arguments missing in the type" $
+                let queryString = [r|
+                  {
+                    dog {
+                      doesKnowCommand(command: CLEAN_UP_HOUSE, dogCommand: SIT)
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Unknown argument \"command\" on field \
+                            \\"Dog.doesKnowCommand\"."
+                        , locations = [AST.Location 4 39]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+            it "rejects directive arguments missing in the definition" $
+                let queryString = [r|
+                  {
+                    dog {
+                      isHousetrained(atOtherHomes: true) @include(unless: false, if: true)
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Unknown argument \"unless\" on directive \
+                            \\"@include\"."
+                        , locations = [AST.Location 4 67]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "knownDirectiveNamesRule" $
+            it "rejects undefined directives" $
+                let queryString = [r|
+                  {
+                    dog {
+                      isHousetrained(atOtherHomes: true) @ignore(if: true)
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message = "Unknown directive \"@ignore\"."
+                        , locations = [AST.Location 4 58]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "knownInputFieldNamesRule" $
+            it "rejects undefined input object fields" $
+                let queryString = [r|
+                  {
+                    findDog(complex: { favoriteCookieFlavor: "Bacon", name: "Jack" }) {
+                      name
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Field \"favoriteCookieFlavor\" is not defined \
+                            \by type \"DogData\"."
+                        , locations = [AST.Location 3 40]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "directivesInValidLocationsRule" $
+            it "rejects directives in invalid locations" $
+                let queryString = [r|
+                  query @skip(if: $foo) {
+                    dog {
+                      name
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Directive \"@skip\" may not be used on QUERY."
+                        , locations = [AST.Location 2 25]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "overlappingFieldsCanBeMergedRule" $ do
+            it "fails to merge fields of mismatching types" $
+                let queryString = [r|
+                  {
+                    dog {
+                      name: nickname
+                      name
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Fields \"name\" conflict because \"nickname\" and \
+                            \\"name\" are different fields. Use different \
+                            \aliases on the fields to fetch both if this was \
+                            \intentional."
+                        , locations = [AST.Location 4 23, AST.Location 5 23]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+            it "fails if the arguments of the same field don't match" $
+                let queryString = [r|
+                  {
+                    dog {
+                      doesKnowCommand(dogCommand: SIT)
+                      doesKnowCommand(dogCommand: HEEL)
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Fields \"doesKnowCommand\" conflict because they \
+                            \have different arguments. Use different aliases \
+                            \on the fields to fetch both if this was \
+                            \intentional."
+                        , locations = [AST.Location 4 23, AST.Location 5 23]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+            it "fails to merge same-named field and alias" $
+                let queryString = [r|
+                  {
+                    dog {
+                      doesKnowCommand(dogCommand: SIT)
+                      doesKnowCommand: isHousetrained(atOtherHomes: true)
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Fields \"doesKnowCommand\" conflict because \
+                            \\"doesKnowCommand\" and \"isHousetrained\" are \
+                            \different fields. Use different aliases on the \
+                            \fields to fetch both if this was intentional."
+                        , locations = [AST.Location 4 23, AST.Location 5 23]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+            it "looks for fields after a successfully merged field pair" $
+                let queryString = [r|
+                  {
+                    dog {
+                      name
+                      doesKnowCommand(dogCommand: SIT)
+                    }
+                    dog {
+                      name
+                      doesKnowCommand: isHousetrained(atOtherHomes: true)
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Fields \"doesKnowCommand\" conflict because \
+                            \\"doesKnowCommand\" and \"isHousetrained\" are \
+                            \different fields. Use different aliases on the \
+                            \fields to fetch both if this was intentional."
+                        , locations = [AST.Location 5 23, AST.Location 9 23]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "possibleFragmentSpreadsRule" $ do
+            it "rejects object inline spreads outside object scope" $
+                let queryString = [r|
+                  {
+                    dog {
+                      ... on Cat {
+                        meowVolume
+                      }
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Fragment cannot be spread here as objects of type \
+                            \\"Dog\" can never be of type \"Cat\"."
+                        , locations = [AST.Location 4 23]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+            it "rejects object named spreads outside object scope" $
+                let queryString = [r|
+                  {
+                    dog {
+                      ... catInDogFragmentInvalid
+                    }
+                  }
+
+                  fragment catInDogFragmentInvalid on Cat {
+                    meowVolume
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Fragment \"catInDogFragmentInvalid\" cannot be \
+                            \spread here as objects of type \"Dog\" can never \
+                            \be of type \"Cat\"."
+                        , locations = [AST.Location 4 23]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "providedRequiredInputFieldsRule" $
+            it "rejects missing required input fields" $
+                let queryString = [r|
+                  {
+                    findDog(complex: { name: null }) {
+                      name
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Input field \"name\" of type \"DogData\" is \
+                            \required, but it was not provided."
+                        , locations = [AST.Location 3 38]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "providedRequiredArgumentsRule" $
+            it "checks for (non-)nullable arguments" $
+                let queryString = [r|
+                  {
+                    dog {
+                      doesKnowCommand(dogCommand: null)
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Field \"doesKnowCommand\" argument \"dogCommand\" \
+                            \of type \"DogCommand\" is required, but it was \
+                            \not provided."
+                        , locations = [AST.Location 4 23]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "variablesInAllowedPositionRule" $ do
+            it "rejects wrongly typed variable arguments" $
+                let queryString = [r|
+                  query catCommandArgQuery($catCommandArg: CatCommand) {
+                    cat {
+                      doesKnowCommand(catCommand: $catCommandArg)
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Variable \"$catCommandArg\" of type \
+                            \\"CatCommand\" used in position expecting type \
+                            \\"!CatCommand\"."
+                        , locations = [AST.Location 2 44]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+            it "rejects wrongly typed variable arguments" $
+                let queryString = [r|
+                  query intCannotGoIntoBoolean($intArg: Int) {
+                    dog {
+                      isHousetrained(atOtherHomes: $intArg)
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Variable \"$intArg\" of type \"Int\" used in \
+                            \position expecting type \"Boolean\"."
+                        , locations = [AST.Location 2 48]
+                        }
+                 in validate queryString `shouldBe` [expected]
+
+        context "valuesOfCorrectTypeRule" $
+            it "rejects values of incorrect types" $
+                let queryString = [r|
+                  {
+                    dog {
+                      isHousetrained(atOtherHomes: 3)
+                    }
+                  }
+                |]
+                    expected = Error
+                        { message =
+                            "Value 3 cannot be coerced to type \"Boolean\"."
+                        , locations = [AST.Location 4 52]
+                        }
+                 in validate queryString `shouldBe` [expected]
diff --git a/tests/Language/GraphQL/ValidateSpec.hs b/tests/Language/GraphQL/ValidateSpec.hs
deleted file mode 100644
index b47149d..0000000
--- a/tests/Language/GraphQL/ValidateSpec.hs
+++ /dev/null
@@ -1,874 +0,0 @@
-{- This Source Code Form is subject to the terms of the Mozilla Public License,
-   v. 2.0. If a copy of the MPL was not distributed with this file, You can
-   obtain one at https://mozilla.org/MPL/2.0/. -}
-
-{-# LANGUAGE OverloadedStrings #-}
-{-# LANGUAGE QuasiQuotes #-}
-
-module Language.GraphQL.ValidateSpec
-    ( spec
-    ) where
-
-import Data.Foldable (toList)
-import qualified Data.HashMap.Strict as HashMap
-import Data.Text (Text)
-import qualified Language.GraphQL.AST as AST
-import Language.GraphQL.Type
-import qualified Language.GraphQL.Type.In as In
-import qualified Language.GraphQL.Type.Out as Out
-import Language.GraphQL.Validate
-import Test.Hspec (Spec, describe, it, shouldBe, shouldContain)
-import Text.Megaparsec (parse, errorBundlePretty)
-import Text.RawString.QQ (r)
-
-petSchema :: Schema IO
-petSchema = schema queryType Nothing (Just subscriptionType) mempty
-
-queryType :: ObjectType IO
-queryType = ObjectType "Query" Nothing [] $ HashMap.fromList
-    [ ("dog", dogResolver)
-    , ("cat", catResolver)
-    , ("findDog", findDogResolver)
-    ]
-  where
-    dogField = Field Nothing (Out.NamedObjectType dogType) mempty
-    dogResolver = ValueResolver dogField $ pure Null
-    findDogArguments = HashMap.singleton "complex"
-        $ In.Argument Nothing (In.NonNullInputObjectType dogDataType) Nothing
-    findDogField = Field Nothing (Out.NamedObjectType dogType) findDogArguments
-    findDogResolver = ValueResolver findDogField $ pure Null
-    catField = Field Nothing (Out.NamedObjectType catType) mempty
-    catResolver = ValueResolver catField $ pure Null
-
-catCommandType :: EnumType
-catCommandType = EnumType "CatCommand" Nothing $ HashMap.fromList
-    [ ("JUMP", EnumValue Nothing)
-    ]
-
-catType :: ObjectType IO
-catType = ObjectType "Cat" Nothing [petType] $ HashMap.fromList
-    [ ("name", nameResolver)
-    , ("nickname", nicknameResolver)
-    , ("doesKnowCommand", doesKnowCommandResolver)
-    , ("meowVolume", meowVolumeResolver)
-    ]
-  where
-    meowVolumeField = Field Nothing (Out.NamedScalarType int) mempty
-    meowVolumeResolver = ValueResolver meowVolumeField $ pure $ Int 3
-    doesKnowCommandField = Field Nothing (Out.NonNullScalarType boolean)
-        $ HashMap.singleton "catCommand"
-        $ In.Argument Nothing (In.NonNullEnumType catCommandType) Nothing
-    doesKnowCommandResolver = ValueResolver doesKnowCommandField
-        $ pure $ Boolean True
-
-nameResolver :: Resolver IO
-nameResolver = ValueResolver nameField $ pure "Name"
-  where
-    nameField = Field Nothing (Out.NonNullScalarType string) mempty
-
-nicknameResolver :: Resolver IO
-nicknameResolver = ValueResolver nicknameField $ pure "Nickname"
-  where
-    nicknameField = Field Nothing (Out.NamedScalarType string) mempty
-
-dogCommandType :: EnumType
-dogCommandType = EnumType "DogCommand" Nothing $ HashMap.fromList
-    [ ("SIT", EnumValue Nothing)
-    , ("DOWN", EnumValue Nothing)
-    , ("HEEL", EnumValue Nothing)
-    ]
-
-dogType :: ObjectType IO
-dogType = ObjectType "Dog" Nothing [petType] $ HashMap.fromList
-    [ ("name", nameResolver)
-    , ("nickname", nicknameResolver)
-    , ("barkVolume", barkVolumeResolver)
-    , ("doesKnowCommand", doesKnowCommandResolver)
-    , ("isHousetrained", isHousetrainedResolver)
-    , ("owner", ownerResolver)
-    ]
-  where
-    barkVolumeField = Field Nothing (Out.NamedScalarType int) mempty
-    barkVolumeResolver = ValueResolver barkVolumeField $ pure $ Int 3
-    doesKnowCommandField = Field Nothing (Out.NonNullScalarType boolean)
-        $ HashMap.singleton "dogCommand"
-        $ In.Argument Nothing (In.NonNullEnumType dogCommandType) Nothing
-    doesKnowCommandResolver = ValueResolver doesKnowCommandField
-        $ pure $ Boolean True
-    isHousetrainedField = Field Nothing (Out.NonNullScalarType boolean)
-        $ HashMap.singleton "atOtherHomes"
-        $ In.Argument Nothing (In.NamedScalarType boolean) Nothing
-    isHousetrainedResolver = ValueResolver isHousetrainedField
-        $ pure $ Boolean True
-    ownerField = Field Nothing (Out.NamedObjectType humanType) mempty
-    ownerResolver = ValueResolver ownerField $ pure Null
-
-dogDataType :: InputObjectType
-dogDataType = InputObjectType "DogData" Nothing
-    $ HashMap.singleton "name" nameInputField
-  where
-    nameInputField = InputField Nothing (In.NonNullScalarType string) Nothing
-
-sentientType :: InterfaceType IO
-sentientType = InterfaceType "Sentient" Nothing []
-    $ HashMap.singleton "name"
-    $ Field Nothing (Out.NonNullScalarType string) mempty
-
-petType :: InterfaceType IO
-petType = InterfaceType "Pet" Nothing []
-    $ HashMap.singleton "name"
-    $ Field Nothing (Out.NonNullScalarType string) mempty
-
-subscriptionType :: ObjectType IO
-subscriptionType = ObjectType "Subscription" Nothing [] $ HashMap.fromList
-    [ ("newMessage", newMessageResolver)
-    , ("disallowedSecondRootField", newMessageResolver)
-    ]
-  where
-    newMessageField = Field Nothing (Out.NonNullObjectType messageType) mempty
-    newMessageResolver = ValueResolver newMessageField
-        $ pure $ Object HashMap.empty
-
-messageType :: ObjectType IO
-messageType = ObjectType "Message" Nothing [] $ HashMap.fromList
-    [ ("sender", senderResolver)
-    , ("body", bodyResolver)
-    ]
-  where
-    senderField = Field Nothing (Out.NonNullScalarType string) mempty
-    senderResolver = ValueResolver senderField $ pure "Sender"
-    bodyField = Field Nothing (Out.NonNullScalarType string) mempty
-    bodyResolver = ValueResolver bodyField $ pure "Message body."
-
-humanType :: ObjectType IO
-humanType = ObjectType "Human" Nothing [sentientType] $ HashMap.fromList
-    [ ("name", nameResolver)
-    , ("pets", petsResolver)
-    ]
-  where
-    petsField =
-        Field Nothing (Out.ListType $ Out.NonNullInterfaceType petType) mempty
-    petsResolver = ValueResolver petsField $ pure $ List []
-{-
-catOrDogType :: UnionType IO
-catOrDogType = UnionType "CatOrDog" Nothing [catType, dogType]
--}
-validate :: Text -> [Error]
-validate queryString =
-    case parse AST.document "" queryString of
-        Left parseErrors -> error $ errorBundlePretty parseErrors
-        Right ast -> toList $ document petSchema specifiedRules ast
-
-spec :: Spec
-spec =
-    describe "document" $ do
-        it "rejects type definitions" $
-            let queryString = [r|
-              query getDogName {
-                dog {
-                  name
-                  color
-                }
-              }
-
-              extend type Dog {
-                color: String
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Definition must be OperationDefinition or FragmentDefinition."
-                    , locations = [AST.Location 9 15]
-                    }
-               in validate queryString `shouldContain` [expected]
-
-        it "rejects multiple subscription root fields" $
-            let queryString = [r|
-              subscription sub {
-                newMessage {
-                  body
-                  sender
-                }
-                disallowedSecondRootField
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Subscription \"sub\" must select only one top level \
-                        \field."
-                    , locations = [AST.Location 2 15]
-                    }
-             in validate queryString `shouldContain` [expected]
-
-        it "rejects multiple subscription root fields coming from a fragment" $
-            let queryString = [r|
-              subscription sub {
-                ...multipleSubscriptions
-              }
-
-              fragment multipleSubscriptions on Subscription {
-                newMessage {
-                  body
-                  sender
-                }
-                disallowedSecondRootField
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Subscription \"sub\" must select only one top level \
-                        \field."
-                    , locations = [AST.Location 2 15]
-                    }
-             in validate queryString `shouldContain` [expected]
-
-        it "rejects multiple anonymous operations" $
-            let queryString = [r|
-              {
-                dog {
-                  name
-                }
-              }
-
-              query getName {
-                dog {
-                  owner {
-                    name
-                  }
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "This anonymous operation must be the only defined operation."
-                    , locations = [AST.Location 2 15]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects operations with the same name" $
-            let queryString = [r|
-              query dogOperation {
-                dog {
-                  name
-                }
-              }
-
-              mutation dogOperation {
-                mutateDog {
-                  id
-                }
-            }
-            |]
-                expected = Error
-                    { message =
-                        "There can be only one operation named \"dogOperation\"."
-                    , locations = [AST.Location 2 15, AST.Location 8 15]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects fragments with the same name" $
-            let queryString = [r|
-              {
-                dog {
-                  ...fragmentOne
-                }
-              }
-
-              fragment fragmentOne on Dog {
-                name
-              }
-
-              fragment fragmentOne on Dog {
-                owner {
-                  name
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "There can be only one fragment named \"fragmentOne\"."
-                    , locations = [AST.Location 8 15, AST.Location 12 15]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects the fragment spread without a target" $
-            let queryString = [r|
-              {
-                dog {
-                  ...undefinedFragment
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Fragment target \"undefinedFragment\" is undefined."
-                    , locations = [AST.Location 4 19]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects fragment spreads without an unknown target type" $
-            let queryString = [r|
-              {
-                dog {
-                  ...notOnExistingType
-                }
-              }
-              fragment notOnExistingType on NotInSchema {
-                name
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Fragment \"notOnExistingType\" is specified on type \
-                        \\"NotInSchema\" which doesn't exist in the schema."
-                    , locations = [AST.Location 4 19]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects inline fragments without a target" $
-            let queryString = [r|
-              {
-                ... on NotInSchema {
-                  name
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Inline fragment is specified on type \"NotInSchema\" \
-                        \which doesn't exist in the schema."
-                    , locations = [AST.Location 3 17]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects fragments on scalar types" $
-            let queryString = [r|
-              {
-                dog {
-                  ...fragOnScalar
-                }
-              }
-              fragment fragOnScalar on Int {
-                name
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Fragment cannot condition on non composite type \
-                        \\"Int\"."
-                    , locations = [AST.Location 7 15]
-                    }
-             in validate queryString `shouldContain` [expected]
-
-        it "rejects inline fragments on scalar types" $
-            let queryString = [r|
-              {
-                ... on Boolean {
-                  name
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Fragment cannot condition on non composite type \
-                        \\"Boolean\"."
-                    , locations = [AST.Location 3 17]
-                    }
-             in validate queryString `shouldContain` [expected]
-
-        it "rejects unused fragments" $
-            let queryString = [r|
-              fragment nameFragment on Dog { # unused
-                name
-              }
-
-              {
-                dog {
-                  name
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Fragment \"nameFragment\" is never used."
-                    , locations = [AST.Location 2 15]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects spreads that form cycles" $
-            let queryString = [r|
-              {
-                dog {
-                  ...nameFragment
-                }
-              }
-              fragment nameFragment on Dog {
-                name
-                ...barkVolumeFragment
-              }
-              fragment barkVolumeFragment on Dog {
-                barkVolume
-                ...nameFragment
-              }
-            |]
-                error1 = Error
-                    { message =
-                        "Cannot spread fragment \"barkVolumeFragment\" within \
-                        \itself (via barkVolumeFragment -> nameFragment -> \
-                        \barkVolumeFragment)."
-                    , locations = [AST.Location 11 15]
-                    }
-                error2 = Error
-                    { message =
-                        "Cannot spread fragment \"nameFragment\" within itself \
-                        \(via nameFragment -> barkVolumeFragment -> \
-                        \nameFragment)."
-                    , locations = [AST.Location 7 15]
-                    }
-             in validate queryString `shouldBe` [error1, error2]
-
-        it "rejects duplicate field arguments" $ do
-            let queryString = [r|
-              {
-                dog {
-                  isHousetrained(atOtherHomes: true, atOtherHomes: true)
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "There can be only one argument named \"atOtherHomes\"."
-                    , locations = [AST.Location 4 34, AST.Location 4 54]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects more than one directive per location" $ do
-            let queryString = [r|
-              query ($foo: Boolean = true, $bar: Boolean = false) {
-                dog @skip(if: $foo) @skip(if: $bar) {
-                  name
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "There can be only one directive named \"skip\"."
-                    , locations = [AST.Location 3 21, AST.Location 3 37]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects duplicate variables" $
-            let queryString = [r|
-              query houseTrainedQuery($atOtherHomes: Boolean, $atOtherHomes: Boolean) {
-                dog {
-                  isHousetrained(atOtherHomes: $atOtherHomes)
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "There can be only one variable named \"atOtherHomes\"."
-                    , locations = [AST.Location 2 39, AST.Location 2 63]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects non-input types as variables" $
-            let queryString = [r|
-              query takesDogBang($dog: Dog!) {
-                dog {
-                  isHousetrained(atOtherHomes: $dog)
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Variable \"$dog\" cannot be non-input type \"Dog\"."
-                    , locations = [AST.Location 2 34]
-                    }
-             in validate queryString `shouldContain` [expected]
-
-        it "rejects undefined variables" $
-            let queryString = [r|
-              query variableIsNotDefinedUsedInSingleFragment {
-                dog {
-                  ...isHousetrainedFragment
-                }
-              }
-
-              fragment isHousetrainedFragment on Dog {
-                isHousetrained(atOtherHomes: $atOtherHomes)
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Variable \"$atOtherHomes\" is not defined by \
-                        \operation \
-                        \\"variableIsNotDefinedUsedInSingleFragment\"."
-                    , locations = [AST.Location 9 46]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects unused variables" $
-            let queryString = [r|
-              query variableUnused($atOtherHomes: Boolean) {
-                dog {
-                  isHousetrained
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Variable \"$atOtherHomes\" is never used in operation \
-                        \\"variableUnused\"."
-                    , locations = [AST.Location 2 36]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects duplicate fields in input objects" $
-            let queryString = [r|
-              {
-                findDog(complex: { name: "Fido", name: "Jack" }) {
-                  name
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "There can be only one input field named \"name\"."
-                    , locations = [AST.Location 3 36, AST.Location 3 50]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects undefined fields" $
-            let queryString = [r|
-              {
-                dog {
-                  meowVolume
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Cannot query field \"meowVolume\" on type \"Dog\"."
-                    , locations = [AST.Location 4 19]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects scalar fields with not empty selection set" $
-            let queryString = [r|
-              {
-                dog {
-                  barkVolume {
-                    sinceWhen
-                  }
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Field \"barkVolume\" must not have a selection since \
-                        \type \"Int\" has no subfields."
-                    , locations = [AST.Location 4 19]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects field arguments missing in the type" $
-            let queryString = [r|
-              {
-                dog {
-                  doesKnowCommand(command: CLEAN_UP_HOUSE, dogCommand: SIT)
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Unknown argument \"command\" on field \
-                        \\"Dog.doesKnowCommand\"."
-                    , locations = [AST.Location 4 35]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects directive arguments missing in the definition" $
-            let queryString = [r|
-              {
-                dog {
-                  isHousetrained(atOtherHomes: true) @include(unless: false, if: true)
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Unknown argument \"unless\" on directive \"@include\"."
-                    , locations = [AST.Location 4 63]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects undefined directives" $
-            let queryString = [r|
-              {
-                dog {
-                  isHousetrained(atOtherHomes: true) @ignore(if: true)
-                }
-              }
-            |]
-                expected = Error
-                    { message = "Unknown directive \"@ignore\"."
-                    , locations = [AST.Location 4 54]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects undefined input object fields" $
-            let queryString = [r|
-              {
-                findDog(complex: { favoriteCookieFlavor: "Bacon", name: "Jack" }) {
-                  name
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Field \"favoriteCookieFlavor\" is not defined \
-                        \by type \"DogData\"."
-                    , locations = [AST.Location 3 36]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects directives in invalid locations" $
-            let queryString = [r|
-              query @skip(if: $foo) {
-                dog {
-                  name
-                }
-              }
-            |]
-                expected = Error
-                    { message = "Directive \"@skip\" may not be used on QUERY."
-                    , locations = [AST.Location 2 21]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects missing required input fields" $
-            let queryString = [r|
-              {
-                findDog(complex: { name: null }) {
-                  name
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Input field \"name\" of type \"DogData\" is required, \
-                        \but it was not provided."
-                    , locations = [AST.Location 3 34]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "finds corresponding subscription fragment" $
-            let queryString = [r|
-              subscription sub {
-                ...anotherSubscription
-                ...multipleSubscriptions
-              }
-              fragment multipleSubscriptions on Subscription {
-                newMessage {
-                  body
-                }
-                disallowedSecondRootField {
-                  sender
-                }
-              }
-              fragment anotherSubscription on Subscription {
-                newMessage {
-                  body
-                  sender
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Subscription \"sub\" must select only one top level \
-                        \field."
-                    , locations = [AST.Location 2 15]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "fails to merge fields of mismatching types" $
-            let queryString = [r|
-              {
-                dog {
-                  name: nickname
-                  name
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Fields \"name\" conflict because \"nickname\" and \
-                        \\"name\" are different fields. Use different aliases \
-                        \on the fields to fetch both if this was intentional."
-                    , locations = [AST.Location 4 19, AST.Location 5 19]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "fails if the arguments of the same field don't match" $
-            let queryString = [r|
-              {
-                dog {
-                  doesKnowCommand(dogCommand: SIT)
-                  doesKnowCommand(dogCommand: HEEL)
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Fields \"doesKnowCommand\" conflict because they have \
-                        \different arguments. Use different aliases on the \
-                        \fields to fetch both if this was intentional."
-                    , locations = [AST.Location 4 19, AST.Location 5 19]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "fails to merge same-named field and alias" $
-            let queryString = [r|
-              {
-                dog {
-                  doesKnowCommand(dogCommand: SIT)
-                  doesKnowCommand: isHousetrained(atOtherHomes: true)
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Fields \"doesKnowCommand\" conflict because \
-                        \\"doesKnowCommand\" and \"isHousetrained\" are \
-                        \different fields. Use different aliases on the fields \
-                        \to fetch both if this was intentional."
-                    , locations = [AST.Location 4 19, AST.Location 5 19]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "looks for fields after a successfully merged field pair" $
-            let queryString = [r|
-              {
-                dog {
-                  name
-                  doesKnowCommand(dogCommand: SIT)
-                }
-                dog {
-                  name
-                  doesKnowCommand: isHousetrained(atOtherHomes: true)
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Fields \"doesKnowCommand\" conflict because \
-                        \\"doesKnowCommand\" and \"isHousetrained\" are \
-                        \different fields. Use different aliases on the fields \
-                        \to fetch both if this was intentional."
-                    , locations = [AST.Location 5 19, AST.Location 9 19]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects object inline spreads outside object scope" $
-            let queryString = [r|
-              {
-                dog {
-                  ... on Cat {
-                    meowVolume
-                  }
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Fragment cannot be spread here as objects of type \
-                        \\"Dog\" can never be of type \"Cat\"."
-                    , locations = [AST.Location 4 19]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects object named spreads outside object scope" $
-            let queryString = [r|
-              {
-                dog {
-                  ... catInDogFragmentInvalid
-                }
-              }
-
-              fragment catInDogFragmentInvalid on Cat {
-                meowVolume
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Fragment \"catInDogFragmentInvalid\" cannot be spread \
-                        \here as objects of type \"Dog\" can never be of type \
-                        \\"Cat\"."
-                    , locations = [AST.Location 4 19]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects wrongly typed variable arguments" $
-            let queryString = [r|
-              query catCommandArgQuery($catCommandArg: CatCommand) {
-                cat {
-                  doesKnowCommand(catCommand: $catCommandArg)
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Variable \"$catCommandArg\" of type \"CatCommand\" \
-                        \used in position expecting type \"!CatCommand\"."
-                    , locations = [AST.Location 2 40]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects wrongly typed variable arguments" $
-            let queryString = [r|
-              query intCannotGoIntoBoolean($intArg: Int) {
-                dog {
-                  isHousetrained(atOtherHomes: $intArg)
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Variable \"$intArg\" of type \"Int\" used in position \
-                        \expecting type \"Boolean\"."
-                    , locations = [AST.Location 2 44]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "rejects values of incorrect types" $
-            let queryString = [r|
-              {
-                dog {
-                  isHousetrained(atOtherHomes: 3)
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Value 3 cannot be coerced to type \"Boolean\"."
-                    , locations = [AST.Location 4 48]
-                    }
-             in validate queryString `shouldBe` [expected]
-
-        it "checks for (non-)nullable arguments" $
-            let queryString = [r|
-              {
-                dog {
-                  doesKnowCommand(dogCommand: null)
-                }
-              }
-            |]
-                expected = Error
-                    { message =
-                        "Field \"doesKnowCommand\" argument \"dogCommand\" of \
-                        \type \"DogCommand\" is required, but it was not \
-                        \provided."
-                    , locations = [AST.Location 4 19]
-                    }
-             in validate queryString `shouldBe` [expected]