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.

tests/Test/StarWars/Data.hs

@@ -2,8 +2,10 @@
 module Test.StarWars.Data where
 
 import Data.Monoid (mempty)
-import Control.Applicative (liftA2)
-import Control.Monad (MonadPlus(..))
+import Control.Applicative ( Alternative(..)
+                           , liftA2 
+                           )
+import Control.Monad.IO.Class (MonadIO(..))
 import Control.Monad.Trans.Except (throwE)
 import Data.Maybe (catMaybes)
 import Data.Text (Text)
@@ -52,7 +54,7 @@ appearsIn :: Character -> [Int]
 appearsIn (Left  x) = _appearsIn . _droidChar $ x
 appearsIn (Right x) = _appearsIn . _humanChar $ x
 
-secretBackstory :: MonadPlus m => Character -> ActionT m Text
+secretBackstory :: MonadIO m => Character -> ActionT m Text
 secretBackstory = const $ ActionT $ throwE "secretBackstory is secret."
 
 typeName :: Character -> Text
@@ -150,30 +152,30 @@ getHero _ = artoo
 getHeroIO :: Int -> IO Character
 getHeroIO = pure . getHero
 
-getHuman :: MonadPlus m => ID -> m Character
+getHuman :: Alternative f => ID -> f Character
 getHuman = fmap Right . getHuman'
 
-getHuman' :: MonadPlus m => ID -> m Human
+getHuman' :: Alternative f => ID -> f Human
 getHuman' "1000" = pure luke'
 getHuman' "1001" = pure vader
 getHuman' "1002" = pure han
 getHuman' "1003" = pure leia
 getHuman' "1004" = pure tarkin
-getHuman' _      = mzero
+getHuman' _      = empty
 
-getDroid :: MonadPlus m => ID -> m Character
+getDroid :: Alternative f => ID -> f Character
 getDroid = fmap Left . getDroid'
 
-getDroid' :: MonadPlus m => ID -> m Droid
+getDroid' :: Alternative f => ID -> f Droid
 getDroid' "2000" = pure threepio
 getDroid' "2001" = pure artoo'
-getDroid' _      = mzero
+getDroid' _      = empty
 
 getFriends :: Character -> [Character]
-getFriends char = catMaybes $ liftA2 mplus getDroid getHuman <$> friends char
+getFriends char = catMaybes $ liftA2 (<|>) getDroid getHuman <$> friends char
 
-getEpisode :: MonadPlus m => Int -> m Text
+getEpisode :: Alternative f => Int -> f Text
 getEpisode 4 = pure "NEWHOPE"
 getEpisode 5 = pure "EMPIRE"
 getEpisode 6 = pure "JEDI"
-getEpisode _ = mzero
+getEpisode _ = empty

tests/Test/StarWars/QueryTests.hs

@@ -11,7 +11,10 @@ import Language.GraphQL
 import Language.GraphQL.Schema (Subs)
 import Text.RawString.QQ (r)
 import Test.Tasty (TestTree, testGroup)
-import Test.Tasty.HUnit (Assertion, testCase, (@?=))
+import Test.Tasty.HUnit ( Assertion
+                        , testCase
+                        , (@?=)
+                        )
 import Test.StarWars.Schema
 
 -- * Test
@@ -344,13 +347,7 @@ test = testGroup "Star Wars Query Tests"
     alderaan = "homePlanet" .= ("Alderaan" :: Text)
 
 testQuery :: Text -> Aeson.Value -> Assertion
-testQuery q expected = graphql schema q @?= Just expected
-
--- testFail :: Text -> Assertion
--- testFail q = graphql schema q @?= Nothing
+testQuery q expected = graphql schema q >>= (@?= expected)
 
 testQueryParams :: Subs -> Text -> Aeson.Value -> Assertion
-testQueryParams f q expected = graphqlSubs schema f q @?= Just expected
-
--- testFailParams :: Subs -> Text -> Assertion
--- testFailParams f q = graphqlSubs schema f q @?= Nothing
+testQueryParams f q expected = graphqlSubs schema f q >>= (@?= expected)

tests/Test/StarWars/Schema.hs

@@ -2,9 +2,9 @@
 {-# LANGUAGE OverloadedStrings #-}
 module Test.StarWars.Schema where
 
-import Control.Monad (MonadPlus(..))
 import Control.Monad.Trans.Except (throwE)
 import Control.Monad.Trans.Class (lift)
+import Control.Monad.IO.Class (MonadIO(..))
 import Data.List.NonEmpty (NonEmpty((:|)))
 import Language.GraphQL.Schema ( Schema
                                , Resolver
@@ -19,10 +19,10 @@ import Test.StarWars.Data
 -- * Schema
 -- See https://github.com/graphql/graphql-js/blob/master/src/__tests__/starWarsSchema.js
 
-schema :: MonadPlus m => Schema m
+schema :: MonadIO m => Schema m
 schema = hero :| [human, droid]
 
-hero :: MonadPlus m => Resolver m
+hero :: MonadIO m => Resolver m
 hero = Schema.objectA "hero" $ \case
   [] -> character artoo
   [Argument "episode" (ValueEnum "NEWHOPE")] -> character $ getHero 4
@@ -30,7 +30,7 @@ hero = Schema.objectA "hero" $ \case
   [Argument "episode" (ValueEnum "JEDI"   )] -> character $ getHero 6
   _ -> ActionT $ throwE "Invalid arguments."
 
-human :: MonadPlus m => Resolver m
+human :: MonadIO m => Resolver m
 human = Schema.wrappedObjectA "human" $ \case
   [Argument "id" (ValueString i)] -> do
       humanCharacter <- lift $ return $ getHuman i >>= Just
@@ -39,12 +39,12 @@ human = Schema.wrappedObjectA "human" $ \case
         Just e -> Named <$> character e
   _ -> ActionT $ throwE "Invalid arguments."
 
-droid :: MonadPlus m => Resolver m
+droid :: MonadIO m => Resolver m
 droid = Schema.objectA "droid" $ \case
-   [Argument "id" (ValueString i)] -> character =<< lift (getDroid i)
+   [Argument "id" (ValueString i)] -> character =<< liftIO (getDroid i)
    _ -> ActionT $ throwE "Invalid arguments."
 
-character :: MonadPlus m => Character -> ActionT m [Resolver m]
+character :: MonadIO m => Character -> ActionT m [Resolver m]
 character char = return
     [ Schema.scalar "id" $ return $ id_ char
     , Schema.scalar "name" $ return $ name char