When argument is not found return null

The relevant test was restored too.

Data/GraphQL/Schema.hs

@@ -24,7 +24,7 @@ module Data.GraphQL.Schema
   , Value(..)
   ) where
 
-import Control.Applicative (Alternative(empty))
+import Control.Applicative (Alternative(empty), (<|>))
 import Data.Foldable (fold)
 import Data.List.NonEmpty (NonEmpty)
 import Data.Maybe (fromMaybe)
@@ -66,6 +66,7 @@ objectA
   => Name -> (Arguments -> Resolvers f) -> Resolver f
 objectA name f fld@(Field _ _ args flds) = withField name (resolve (f args) flds) fld
 
+
 -- | Create a named 'Resolver' from a list of 'Resolver's.
 object' :: (Alternative f, Monad f) => Text -> f [Resolver f] -> Resolver f
 object' name resolvs = objectA' name $ \case
@@ -136,9 +137,11 @@ enumA _ _ _ = empty
 withField
   :: (Alternative f, Aeson.ToJSON a)
   => Name -> f a -> Field -> f (HashMap Text Aeson.Value)
-withField name f (Field alias name' _ _) =
+withField name v (Field alias name' _ _) =
   if name == name'
-    then fmap (HashMap.singleton aliasOrName . Aeson.toJSON) f
+    then fmap (HashMap.singleton aliasOrName . Aeson.toJSON) v
+         -- TODO: Report error when Non-Nullable type for field argument.
+     <|> pure (HashMap.singleton aliasOrName Aeson.Null)
     else empty
   where
     aliasOrName = fromMaybe name alias

tests/Test/StarWars/QueryTests.hs

@@ -140,24 +140,19 @@ test = testGroup "Star Wars Query Tests"
       $ object [ "data" .= object [
           "human" .= object [hanName]
         ]]
-    -- TODO: Enable after Error handling restoration
-    -- , testCase "Invalid ID" . testQueryParams
-    --     (\v -> if v == "id"
-    --               then Just "Not a valid ID"
-    --               else Nothing)
-    --     [r| query humanQuery($id: String!) {
-    --           human(id: $id) {
-    --             name
-    --           }
-    --         }
-    --     |] $ object ["data" .= object ["human" .= object ["name" .= Aeson.Null]],
-    --                  "errors" .= Aeson.toJSON [object ["message" .= ("field name not resolved." :: Text)]]]
-        -- TODO: This test is directly ported from `graphql-js`, however do we want
-        -- to mimic the same behavior? Is this part of the spec? Once proper
-        -- exceptions are implemented this test might no longer be meaningful.
-        -- If the same behavior needs to be replicated, should it be implemented
-        -- when defining the `Schema` or when executing?
-        -- $ object [ "data" .= object ["human" .= Aeson.Null] ]
+    , testCase "Invalid ID" . testQueryParams
+        (\v -> if v == "id"
+               then Just "Not a valid ID"
+               else Nothing)
+        [r| query humanQuery($id: String!) {
+              human(id: $id) {
+                name
+              }
+            }
+        -- The GraphQL spec specifies that an error should be reported when the
+        -- type of the argument is Non-Nullable. However the equivalent test in
+        -- `graphql-js` doesn't check for any errors.
+        |] $ object ["data" .= object ["human" .= Aeson.Null]]
     , testCase "Luke aliased" . testQuery
         [r| query FetchLukeAliased {
               luke: human(id: "1000") {