Add Serialize and VariableValue value instances

- `Serialize` instance for `Type.Definition.Value`.
- `VariableValue` instance for `Type.Definition.Value`.

It makes it possible to use the library without an additional
serialization format like JSON.

CHANGELOG.md

@@ -6,6 +6,11 @@ The format is based on
 and this project adheres to
 [Haskell Package Versioning Policy](https://pvp.haskell.org/).
 
+## [Unreleased]
+### Added
+- `Serialize` instance for `Type.Definition.Value`.
+- `VariableValue` instance for `Type.Definition.Value`.
+
 ## [1.0.1.0] - 2021-09-27
 ### Added
 - Custom `Show` instance for `Type.Definition.Value` (for error
@@ -459,6 +464,7 @@ and this project adheres to
 ### Added
 - Data types for the GraphQL language.
 
+[Unreleased]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=master&rev_to=v1.0.1.0
 [1.0.1.0]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v1.0.1.0&rev_to=v1.0.0.0
 [1.0.0.0]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v1.0.0.0&rev_to=v0.11.1.0
 [0.11.1.0]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v0.11.1.0&rev_to=v0.11.0.0

src/Language/GraphQL/Execute/Coerce.hs

@@ -3,6 +3,7 @@
    obtain one at https://mozilla.org/MPL/2.0/. -}
 
 {-# LANGUAGE ExplicitForAll #-}
+{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE ViewPatterns #-}
 
@@ -61,6 +62,9 @@ class VariableValue a where
         -> a -- ^ Variable value being coerced.
         -> Maybe Type.Value -- ^ Coerced value on success, 'Nothing' otherwise.
 
+instance VariableValue Type.Value where
+    coerceVariableValue = const Just
+
 instance VariableValue Aeson.Value where
     coerceVariableValue _ Aeson.Null = Just Type.Null
     coerceVariableValue (In.ScalarBaseType scalarType) value
@@ -216,6 +220,20 @@ data Output a
 instance forall a. IsString (Output a) where
     fromString = String . fromString
 
+instance Serialize Type.Value where
+    null = Type.Null
+    serialize _ = \case
+        Int int -> Just $ Type.Int int
+        Float float -> Just $ Type.Float float
+        String string -> Just $ Type.String string
+        Boolean boolean -> Just $ Type.Boolean boolean
+        Enum enum -> Just $ Type.Enum enum
+        List list -> Just $ Type.List list
+        Object object -> Just
+            $ Type.Object
+            $ HashMap.fromList
+            $ OrderedMap.toList object
+
 instance Serialize Aeson.Value where
     serialize (Out.ScalarBaseType scalarType) value
         | Type.ScalarType "Int" _ <- scalarType