encode null value as "null"

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to
 [Haskell Package Versioning Policy](https://pvp.haskell.org/).
 
 ## [Unreleased]
+### Fixed
+- Result of null encoding
+
 ### Added
 - AST for the GraphQL schema.
 - Type system definition parser.

src/Language/GraphQL/AST/Encoder.hs

@@ -205,7 +205,7 @@ value _ (Full.Variable x) = variable x
 value _ (Full.Int x) = Builder.toLazyText $ decimal x
 value _ (Full.Float x) = Builder.toLazyText $ realFloat x
 value _ (Full.Boolean  x) = booleanValue x
-value _ Full.Null = mempty
+value _ Full.Null = "null"
 value formatter (Full.String string) = stringValue formatter string
 value _ (Full.Enum x) = Lazy.Text.fromStrict x
 value formatter (Full.List x) = listValue formatter x

tests/Language/GraphQL/AST/EncoderSpec.hs

@@ -12,6 +12,11 @@ import Text.RawString.QQ (r)
 spec :: Spec
 spec = do
     describe "value" $ do
+        context "null value" $ do
+            let testNull formatter = value formatter Null `shouldBe` "null"
+            it "minified" $ testNull minified
+            it "pretty" $ testNull pretty
+
         context "minified" $ do
             it "escapes \\" $
                 value minified (String "\\") `shouldBe` "\"\\\\\""