Support subscriptions

This is experimental support.
The implementation is based on conduit and is boring. There is a new
resolver data constructor that should create a source event stream. The
executor receives the events, pipes them through the normal execution
and puts them into the response stream which is returned to the user.

- Tests are missing.
- The executor should check field value resolver on subscription types.
- The graphql function should probably return (Either
  ResponseEventStream Response), but I'm not sure about this. It will
  make the usage more complicated if no subscriptions are involved, but
  with the current API implementing subscriptions is more
  difficult than it should be.

tests/Language/GraphQL/ExecuteSpec.hs

@@ -5,6 +5,7 @@ module Language.GraphQL.ExecuteSpec
 
 import Data.Aeson ((.=))
 import qualified Data.Aeson as Aeson
+import Data.Either (fromRight)
 import Data.Functor.Identity (Identity(..))
 import Data.HashMap.Strict (HashMap)
 import qualified Data.HashMap.Strict as HashMap
@@ -22,26 +23,27 @@ schema = Schema {query = queryType, mutation = Nothing}
 
 queryType :: Out.ObjectType Identity
 queryType = Out.ObjectType "Query" Nothing []
-    $ HashMap.singleton "philosopher" philosopherField
+    $ HashMap.singleton "philosopher" 
+    $ ValueResolver philosopherField
+    $ pure $ Type.Object mempty
   where
-    philosopherField
-        = Out.Field Nothing (Out.NonNullObjectType philosopherType) HashMap.empty
-        $ pure $ Type.Object mempty
+    philosopherField =
+        Out.Field Nothing (Out.NonNullObjectType philosopherType) HashMap.empty
 
 philosopherType :: Out.ObjectType Identity
 philosopherType = Out.ObjectType "Philosopher" Nothing []
     $ HashMap.fromList resolvers
   where
     resolvers =
-        [ ("firstName", firstNameField)
-        , ("lastName", lastNameField)
+        [ ("firstName", ValueResolver firstNameField firstNameResolver)
+        , ("lastName", ValueResolver lastNameField lastNameResolver)
         ]
-    firstNameField
-        = Out.Field Nothing (Out.NonNullScalarType string) HashMap.empty
-        $ pure $ Type.String "Friedrich"
+    firstNameField =
+        Out.Field Nothing (Out.NonNullScalarType string) HashMap.empty
+    firstNameResolver = pure $ Type.String "Friedrich"
     lastNameField
         = Out.Field Nothing (Out.NonNullScalarType string) HashMap.empty
-        $ pure $ Type.String "Nietzsche"
+    lastNameResolver = pure $ Type.String "Nietzsche"
 
 spec :: Spec
 spec =
@@ -54,8 +56,9 @@ spec =
                     ]
                 expected = Response data'' mempty
                 execute' = execute schema Nothing (mempty :: HashMap Name Aeson.Value)
-                actual = runIdentity
-                    $ either parseError execute'
+                actual = fromRight (singleError "")
+                    $ runIdentity
+                    $ either (pure . parseError) execute'
                     $ parse document "" "{ philosopher { firstName surname } }"
              in actual `shouldBe` expected
         it "merges selections" $
@@ -67,7 +70,8 @@ spec =
                     ]
                 expected = Response data'' mempty
                 execute' = execute schema Nothing (mempty :: HashMap Name Aeson.Value)
-                actual = runIdentity
-                    $ either parseError execute'
+                actual = fromRight (singleError "")
+                    $ runIdentity
+                    $ either (pure . parseError) execute'
                     $ parse document "" "{ philosopher { firstName } philosopher { lastName } }"
              in actual `shouldBe` expected