Define resolvers on type fields

Returning resolvers from other resolvers isn't supported anymore. Since
we have a type system now, we define the resolvers in the object type
fields and pass an object with the previous result to them.

src/Language/GraphQL/Error.hs

@@ -5,21 +5,20 @@
 module Language.GraphQL.Error
     ( parseError
     , CollectErrsT
+    , Resolution(..)
     , addErr
     , addErrMsg
     , runCollectErrs
-    , runAppendErrs
     , singleError
     ) where
 
+import Control.Monad.Trans.State (StateT, modify, runStateT)
 import qualified Data.Aeson as Aeson
+import Data.HashMap.Strict (HashMap)
 import Data.Text (Text)
 import Data.Void (Void)
-import Control.Monad.Trans.Class (lift)
-import Control.Monad.Trans.State ( StateT
-                                 , modify
-                                 , runStateT
-                                 )
+import Language.GraphQL.AST.Document (Name)
+import Language.GraphQL.Type.Schema
 import Text.Megaparsec
     ( ParseErrorBundle(..)
     , PosState(..)
@@ -30,6 +29,11 @@ import Text.Megaparsec
     , unPos
     )
 
+data Resolution m = Resolution
+    { errors :: [Aeson.Value]
+    , types :: HashMap Name (Type m)
+    }
+
 -- | Wraps a parse error into a list of errors.
 parseError :: Applicative f => ParseErrorBundle Text Void -> f Aeson.Value
 parseError ParseErrorBundle{..}  =
@@ -46,11 +50,13 @@ parseError ParseErrorBundle{..}  =
          in (errorObject x sourcePosition : result, newState)
 
 -- | A wrapper to pass error messages around.
-type CollectErrsT m = StateT [Aeson.Value] m
+type CollectErrsT m = StateT (Resolution m) m
 
 -- | Adds an error to the list of errors.
 addErr :: Monad m => Aeson.Value -> CollectErrsT m ()
-addErr v = modify (v :)
+addErr v = modify appender
+  where
+    appender resolution@Resolution{..} = resolution{ errors = v : errors }
 
 makeErrorMessage :: Text -> Aeson.Value
 makeErrorMessage s = Aeson.object [("message", Aeson.toJSON s)]
@@ -66,23 +72,17 @@ singleError message = Aeson.object
 addErrMsg :: Monad m => Text -> CollectErrsT m ()
 addErrMsg = addErr . makeErrorMessage
 
--- | Appends the given list of errors to the current list of errors.
-appendErrs :: Monad m => [Aeson.Value] -> CollectErrsT m ()
-appendErrs errs = modify (errs ++)
-
 -- | Runs the given query computation, but collects the errors into an error
 --   list, which is then sent back with the data.
-runCollectErrs :: Monad m => CollectErrsT m Aeson.Value -> m Aeson.Value
-runCollectErrs res = do
-    (dat, errs) <- runStateT res []
-    if null errs
+runCollectErrs :: Monad m
+    => HashMap Name (Type m)
+    -> CollectErrsT m Aeson.Value
+    -> m Aeson.Value
+runCollectErrs types' res = do
+    (dat, Resolution{..}) <- runStateT res $ Resolution{ errors = [], types = types' }
+    if null errors
        then return $ Aeson.object [("data", dat)]
-       else return $ Aeson.object [("data", dat), ("errors", Aeson.toJSON $ reverse errs)]
-
--- | Runs the given computation, collecting the errors and appending them
---   to the previous list of errors.
-runAppendErrs :: Monad m => CollectErrsT m a -> CollectErrsT m a
-runAppendErrs f = do
-    (v, errs) <- lift $ runStateT f []
-    appendErrs errs
-    return v
+       else return $ Aeson.object
+           [ ("data", dat)
+           , ("errors", Aeson.toJSON $ reverse errors)
+           ]