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.

src/Language/GraphQL.hs

@@ -9,6 +9,7 @@ module Language.GraphQL
 
 import qualified Data.Aeson as Aeson
 import qualified Data.Aeson.Types as Aeson
+import Data.Either (fromRight)
 import qualified Data.Sequence as Seq
 import Data.Text (Text)
 import Language.GraphQL.AST
@@ -34,10 +35,14 @@ graphqlSubs :: Monad m
     -> Aeson.Object -- ^ Variable substitution function.
     -> Text -- ^ Text representing a @GraphQL@ request document.
     -> m Aeson.Value -- ^ Response.
-graphqlSubs schema operationName variableValues document' =
-    either parseError executeRequest parsed >>= formatResponse
+graphqlSubs schema operationName variableValues document'
+    = either parseError executeRequest (parse document "" document')
+    >>= formatResponse
   where
-    parsed = parse document "" document'
+    executeRequest parsed
+      = fromRight streamReturned
+      <$> execute schema operationName variableValues parsed
+    streamReturned = singleError "This service does not support subscriptions."
     formatResponse (Response data'' Seq.Empty) =
         pure $ Aeson.object [("data", data'')]
     formatResponse (Response data'' errors') = pure $ Aeson.object
@@ -54,4 +59,3 @@ graphqlSubs schema operationName variableValues document' =
         [ ("line", Aeson.toJSON line)
         , ("column", Aeson.toJSON column)
         ]
-    executeRequest = execute schema operationName variableValues