Add ToGraphQL and FromGraphQL typeclasses

With instances for basic types.

tests/Language/GraphQL/ClassSpec.hs

@@ -0,0 +1,47 @@
+{- 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 #-}
+module Language.GraphQL.ClassSpec
+    ( spec
+    ) where
+
+import Data.Text (Text)
+import qualified Language.GraphQL.Type as Type
+import Language.GraphQL.Class (FromGraphQL(..), ToGraphQL(..))
+import Test.Hspec (Spec, describe, it, shouldBe)
+
+spec :: Spec
+spec = do
+    describe "ToGraphQL" $ do
+        it "converts integers" $
+            toGraphQL (5 :: Int) `shouldBe` Type.Int 5
+
+        it "converts text" $
+            toGraphQL ("String" :: Text) `shouldBe` Type.String "String"
+
+        it "converts booleans" $
+            toGraphQL True `shouldBe` Type.Boolean True
+
+        it "converts Nothing to Null" $
+            toGraphQL (Nothing :: Maybe Int) `shouldBe` Type.Null
+
+        it "converts singleton lists" $
+            toGraphQL [True] `shouldBe` Type.List [Type.Boolean True]
+
+    describe "FromGraphQL" $ do
+        it "converts integers" $
+            fromGraphQL (Type.Int 5) `shouldBe` Just (5 :: Int)
+
+        it "converts text" $
+            fromGraphQL (Type.String "String") `shouldBe` Just ("String" :: Text)
+
+        it "converts booleans" $
+            fromGraphQL (Type.Boolean True) `shouldBe` Just True
+
+        it "converts Null to Nothing" $
+            fromGraphQL Type.Null `shouldBe` Just (Nothing :: Maybe Int)
+
+        it "converts singleton lists" $
+            fromGraphQL (Type.List [Type.Boolean True]) `shouldBe` Just [True]