Add specifiedBy directive

src/Language/GraphQL/Type/Schema.hs

@@ -85,6 +85,7 @@ schemaWithTypes description' queryRoot mutationRoot subscriptionRoot types' dire
         [ ("skip", skipDirective)
         , ("include", includeDirective)
         , ("deprecated", deprecatedDirective)
+        , ("specifiedBy", specifiedByDirective)
         ]
     includeDirective =
         Directive includeDescription skipIncludeLocations includeArguments
@@ -113,9 +114,8 @@ schemaWithTypes description' queryRoot mutationRoot subscriptionRoot types' dire
         \the Markdown syntax, as specified by \
         \[CommonMark](https://commonmark.org/).'"
     deprecatedArguments = HashMap.singleton "reason"
-        $ In.Argument reasonDescription reasonType
+        $ In.Argument reasonDescription (In.NamedScalarType Definition.string)
         $ Just "No longer supported"
-    reasonType = In.NamedScalarType Definition.string
     deprecatedDescription = Just
         "Marks an element of a GraphQL schema as no longer supported."
     deprecatedLocations =
@@ -124,6 +124,16 @@ schemaWithTypes description' queryRoot mutationRoot subscriptionRoot types' dire
         , TypeSystemDirectiveLocation DirectiveLocation.InputFieldDefinition
         , TypeSystemDirectiveLocation DirectiveLocation.EnumValue
         ]
+    specifiedByDirective =
+        Directive specifiedByDescription specifiedByLocations specifiedByArguments
+    urlDescription = Just
+        "The URL that specifies the behavior of this scalar."
+    specifiedByArguments = HashMap.singleton "url"
+        $ In.Argument urlDescription (In.NonNullScalarType Definition.string) Nothing
+    specifiedByDescription = Just
+        "Exposes a URL that specifies the behavior of this scalar."
+    specifiedByLocations =
+        [TypeSystemDirectiveLocation DirectiveLocation.Scalar]
 
 -- | Traverses the schema and finds all referenced types.
 collectReferencedTypes :: forall m