Constrain the resolvers with MonadIO

This replaces the most usages of MonadPlus, which is not appropriate for
the resolvers, since a resolver is unambiguously chosen by the name (no
need for 'mplus'), and the resolvers are often doing IO.

src/Language/GraphQL/Execute.hs

@@ -1,9 +1,10 @@
 {-# LANGUAGE OverloadedStrings #-}
+
 -- | This module provides the function to execute a @GraphQL@ request --
 --   according to a 'Schema'.
 module Language.GraphQL.Execute (execute) where
 
-import Control.Monad (MonadPlus(..))
+import Control.Monad.IO.Class (MonadIO)
 import qualified Data.List.NonEmpty as NE
 import Data.List.NonEmpty (NonEmpty((:|)))
 import qualified Data.Aeson as Aeson
@@ -21,17 +22,22 @@ import qualified Language.GraphQL.Schema as Schema
 --   Returns the result of the query against the 'Schema' wrapped in a /data/ field, or
 --   errors wrapped in an /errors/ field.
 execute
-  :: (MonadPlus m)
-  => Schema m -> Schema.Subs -> AST.Document -> m Aeson.Value
-execute schema subs doc = do
-    coreDocument <- maybe mzero pure (Transform.document subs doc)
-    document schema coreDocument
+    :: MonadIO m
+    => Schema m -> Schema.Subs -> AST.Document -> m Aeson.Value
+execute schema subs doc =
+    maybe transformError (document schema) $ Transform.document subs doc
+  where
+    transformError = return $ Aeson.object
+        [("errors", Aeson.toJSON
+            [ Aeson.object [("message", "Schema transformation error.")]
+            ]
+        )]
 
-document :: MonadPlus m => Schema m -> AST.Core.Document -> m Aeson.Value
+document :: MonadIO m => Schema m -> AST.Core.Document -> m Aeson.Value
 document schema (op :| []) = operation schema op
 document _ _ = error "Multiple operations not supported yet"
 
-operation :: MonadPlus m => Schema m -> AST.Core.Operation -> m Aeson.Value
+operation :: MonadIO m => Schema m -> AST.Core.Operation -> m Aeson.Value
 operation schema (AST.Core.Query flds)
     = runCollectErrs (Schema.resolve (NE.toList schema) (NE.toList flds))
 operation schema (AST.Core.Mutation flds)