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.

3 files changed, 36 insertions, 31 deletions

diff --git a/src/Language/GraphQL.hs b/src/Language/GraphQL.hs
index 8ed29cf..74052d9 100644
--- a/src/Language/GraphQL.hs
+++ b/src/Language/GraphQL.hs
@@ -1,8 +1,7 @@
 -- | This module provides the functions to parse and execute @GraphQL@ queries.
 module Language.GraphQL where
 
-import Control.Monad (MonadPlus)
-
+import Control.Monad.IO.Class (MonadIO)
 import qualified Data.Text as T
 
 import qualified Data.Aeson as Aeson
@@ -21,7 +20,7 @@ import Language.GraphQL.Error
 --   executed according to the given 'Schema'.
 --
 --   Returns the response as an @Aeson.@'Aeson.Value'.
-graphql :: MonadPlus m => Schema m -> T.Text -> m Aeson.Value
+graphql :: MonadIO m => Schema m -> T.Text -> m Aeson.Value
 graphql = flip graphqlSubs $ const Nothing
 
 -- | Takes a 'Schema', a variable substitution function and text
@@ -30,7 +29,7 @@ graphql = flip graphqlSubs $ const Nothing
 --   query and the query is then executed according to the given 'Schema'.
 --
 --   Returns the response as an @Aeson.@'Aeson.Value'.
-graphqlSubs :: MonadPlus m => Schema m -> Subs -> T.Text -> m Aeson.Value
+graphqlSubs :: MonadIO m => Schema m -> Subs -> T.Text -> m Aeson.Value
 graphqlSubs schema f =
     either (parseError . errorBundlePretty) (execute schema f)
     . parse document ""
diff --git a/src/Language/GraphQL/Execute.hs b/src/Language/GraphQL/Execute.hs
index eb53bba..7416167 100644
--- a/src/Language/GraphQL/Execute.hs
+++ b/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)
diff --git a/src/Language/GraphQL/Schema.hs b/src/Language/GraphQL/Schema.hs
index 5569155..1efdf0d 100644
--- a/src/Language/GraphQL/Schema.hs
+++ b/src/Language/GraphQL/Schema.hs
@@ -25,7 +25,7 @@ module Language.GraphQL.Schema
   , Value(..)
   ) where
 
-import Control.Monad (MonadPlus(..))
+import Control.Monad.IO.Class (MonadIO(..))
 import Control.Monad.Trans.Class (lift)
 import Control.Monad.Trans.Except (runExceptT)
 import Data.Foldable ( find
@@ -44,11 +44,11 @@ import Language.GraphQL.Type
 import Language.GraphQL.AST.Core
 
 -- | A GraphQL schema.
---   @f@ is usually expected to be an instance of 'Alternative'.
+--   @m@ is usually expected to be an instance of 'MonadIO'.
 type Schema m = NonEmpty (Resolver m)
 
 -- | Resolves a 'Field' into an @Aeson.@'Aeson.Object' with error information
---   (or 'empty'). @m@ is usually expected to be an instance of 'MonadPlus'.
+--   (or 'empty'). @m@ is usually expected to be an instance of 'MonadIO.
 data Resolver m = Resolver
     Text -- ^ Name
     (Field -> CollectErrsT m Aeson.Object) -- ^ Resolver
@@ -61,18 +61,18 @@ type Arguments = [Argument]
 type Subs = Name -> Maybe Value
 
 -- | Create a new 'Resolver' with the given 'Name' from the given 'Resolver's.
-object :: MonadPlus m => Name -> ActionT m [Resolver m] -> Resolver m
+object :: MonadIO m => Name -> ActionT m [Resolver m] -> Resolver m
 object name = objectA name . const
 
 -- | Like 'object' but also taking 'Argument's.
-objectA :: MonadPlus m
+objectA :: MonadIO m
     => Name -> (Arguments -> ActionT m [Resolver m]) -> Resolver m
 objectA name f = Resolver name $ resolveFieldValue f resolveRight
   where
     resolveRight fld@(Field _ _ _ flds) resolver = withField (resolve resolver flds) fld
 
 -- | Like 'object' but also taking 'Argument's and can be null or a list of objects.
-wrappedObjectA :: MonadPlus m
+wrappedObjectA :: MonadIO m
     => Name -> (Arguments -> ActionT m (Wrapping [Resolver m])) -> Resolver m
 wrappedObjectA name f = Resolver name $ resolveFieldValue f resolveRight
   where
@@ -80,23 +80,23 @@ wrappedObjectA name f = Resolver name $ resolveFieldValue f resolveRight
         = withField (traverse (`resolve` sels) resolver) fld
 
 -- | Like 'object' but can be null or a list of objects.
-wrappedObject :: MonadPlus m
+wrappedObject :: MonadIO m
     => Name -> ActionT m (Wrapping [Resolver m]) -> Resolver m
 wrappedObject name = wrappedObjectA name . const
 
 -- | A scalar represents a primitive value, like a string or an integer.
-scalar :: (MonadPlus m, Aeson.ToJSON a) => Name -> ActionT m a -> Resolver m
+scalar :: (MonadIO m, Aeson.ToJSON a) => Name -> ActionT m a -> Resolver m
 scalar name = scalarA name . const
 
 -- | Like 'scalar' but also taking 'Argument's.
-scalarA :: (MonadPlus m, Aeson.ToJSON a)
+scalarA :: (MonadIO m, Aeson.ToJSON a)
     => Name -> (Arguments -> ActionT m a) -> Resolver m
 scalarA name f = Resolver name $ resolveFieldValue f resolveRight
   where
     resolveRight fld result = withField (return result) fld
 
 -- | Lika 'scalar' but also taking 'Argument's and can be null or a list of scalars.
-wrappedScalarA :: (MonadPlus m, Aeson.ToJSON a)
+wrappedScalarA :: (MonadIO m, Aeson.ToJSON a)
     => Name -> (Arguments -> ActionT m (Wrapping a)) -> Resolver m
 wrappedScalarA name f = Resolver name $ resolveFieldValue f resolveRight
   where
@@ -106,23 +106,23 @@ wrappedScalarA name f = Resolver name $ resolveFieldValue f resolveRight
     resolveRight fld (List result) = withField (return result) fld
 
 -- | Like 'scalar' but can be null or a list of scalars.
-wrappedScalar :: (MonadPlus m, Aeson.ToJSON a)
+wrappedScalar :: (MonadIO m, Aeson.ToJSON a)
     => Name -> ActionT m (Wrapping a) -> Resolver m
 wrappedScalar name = wrappedScalarA name . const
 
 -- | Represents one of a finite set of possible values.
 --   Used in place of a 'scalar' when the possible responses are easily enumerable.
-enum :: MonadPlus m => Name -> ActionT m [Text] -> Resolver m
+enum :: MonadIO m => Name -> ActionT m [Text] -> Resolver m
 enum name = enumA name . const
 
 -- | Like 'enum' but also taking 'Argument's.
-enumA :: MonadPlus m => Name -> (Arguments -> ActionT m [Text]) -> Resolver m
+enumA :: MonadIO m => Name -> (Arguments -> ActionT m [Text]) -> Resolver m
 enumA name f = Resolver name $ resolveFieldValue f resolveRight
   where
     resolveRight fld resolver = withField (return resolver) fld
 
 -- | Like 'enum' but also taking 'Argument's and can be null or a list of enums.
-wrappedEnumA :: MonadPlus m
+wrappedEnumA :: MonadIO m
     => Name -> (Arguments -> ActionT m (Wrapping [Text])) -> Resolver m
 wrappedEnumA name f = Resolver name $ resolveFieldValue f resolveRight
   where
@@ -132,10 +132,10 @@ wrappedEnumA name f = Resolver name $ resolveFieldValue f resolveRight
     resolveRight fld (List resolver) = withField (return resolver) fld
 
 -- | Like 'enum' but can be null or a list of enums.
-wrappedEnum :: MonadPlus m => Name -> ActionT m (Wrapping [Text]) -> Resolver m
+wrappedEnum :: MonadIO m => Name -> ActionT m (Wrapping [Text]) -> Resolver m
 wrappedEnum name = wrappedEnumA name . const
 
-resolveFieldValue :: MonadPlus m
+resolveFieldValue :: MonadIO m
     => ([Argument] -> ActionT m a)
     -> (Field -> a -> CollectErrsT m (HashMap Text Aeson.Value))
     -> Field
@@ -149,7 +149,7 @@ resolveFieldValue f resolveRight fld@(Field _ _ args _) = do
             return $ HashMap.singleton (aliasOrName fld) Aeson.Null
 
 -- | Helper function to facilitate 'Argument' handling.
-withField :: (MonadPlus m, Aeson.ToJSON a)
+withField :: (MonadIO m, Aeson.ToJSON a)
     => CollectErrsT m a -> Field -> CollectErrsT m (HashMap Text Aeson.Value)
 withField v fld
     = HashMap.singleton (aliasOrName fld) . Aeson.toJSON <$> runAppendErrs v
@@ -157,11 +157,11 @@ withField v fld
 -- | Takes a list of 'Resolver's and a list of 'Field's and applies each
 --   'Resolver' to each 'Field'. Resolves into a value containing the
 --   resolved 'Field', or a null value and error information.
-resolve :: MonadPlus m
+resolve :: MonadIO m
     => [Resolver m] -> Fields -> CollectErrsT m Aeson.Value
 resolve resolvers = fmap (Aeson.toJSON . fold) . traverse tryResolvers
   where
-    tryResolvers fld = mplus (maybe mzero (tryResolver fld) $ find (compareResolvers fld) resolvers) $ errmsg fld
+    tryResolvers fld = maybe (errmsg fld) (tryResolver fld) $ find (compareResolvers fld) resolvers
     compareResolvers (Field _ name _ _) (Resolver name' _) = name == name'
     tryResolver fld (Resolver _ resolver)  = resolver fld
     errmsg fld@(Field _ name _ _) = do