Provide a custom Show instance for output Value

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]
+### Fixed
+- Provide a custom `Show` instance for `Type.Definition.Value` (for error
+  messages).
+
 ## [1.0.0.0]
 ### Added
 - `Language.GraphQL.Execute.OrderedMap` is a map data structure, that preserves
@@ -443,6 +448,7 @@ and this project adheres to
 ### Added
 - Data types for the GraphQL language.
 
+[Unreleased]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v1.0.0.0&rev_to=master
 [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
 [0.11.0.0]: https://www.caraus.tech/projects/pub-graphql/repository/23/diff?rev=v0.11.0.0&rev_to=v0.10.0.0

src/Language/GraphQL/Type/Definition.hs

@@ -20,10 +20,12 @@ module Language.GraphQL.Type.Definition
 import Data.Int (Int32)
 import Data.HashMap.Strict (HashMap)
 import qualified Data.HashMap.Strict as HashMap
+import Data.List (intercalate)
 import Data.String (IsString(..))
 import Data.Text (Text)
 import qualified Data.Text as Text
-import Language.GraphQL.AST (Name)
+import Language.GraphQL.AST (Name, escape)
+import Numeric (showFloat)
 import Prelude hiding (id)
 
 -- | Represents accordingly typed GraphQL values.
@@ -36,7 +38,27 @@ data Value
     | Enum Name
     | List [Value] -- ^ Arbitrary nested list.
     | Object (HashMap Name Value)
-    deriving (Eq, Show)
+    deriving Eq
+
+instance Show Value where
+    showList = mappend . showList'
+      where
+        showList' list = "[" ++ intercalate ", " (show <$> list) ++ "]"
+    show (Int integer) = show integer
+    show (Float float') = showFloat float' mempty
+    show (String text) = "\"" <> Text.foldr (mappend . escape) "\"" text
+    show (Boolean boolean') = show boolean'
+    show Null = "null"
+    show (Enum name) = Text.unpack name
+    show (List list) = show list
+    show (Object fields) = unwords
+        [ "{"
+        , intercalate ", " (HashMap.foldrWithKey showObject [] fields)
+        , "}"
+        ]
+      where
+        showObject key value accumulator =
+            concat [Text.unpack key, ": ", show value] : accumulator
 
 instance IsString Value where
     fromString = String . fromString