Constrain base monad to MonadCatch

Let's try MonadThrow/MonadCatch. It looks nice at a first glance. The
monad transformer stack contains only the ReaderT, less lifts are
required. Exception subtyping is easier, the user can (and should)
define custom error types and throw them. And it is still possible to
use pure error handling, if someone doesn't like runtime exceptions or
need to run a query in a pure environment.

Fixes #42.

4 files changed, 50 insertions, 42 deletions

diff --git a/tests/Language/GraphQL/ExecuteSpec.hs b/tests/Language/GraphQL/ExecuteSpec.hs
index 7aab6c5..8fbb55b 100644
--- a/tests/Language/GraphQL/ExecuteSpec.hs
+++ b/tests/Language/GraphQL/ExecuteSpec.hs
@@ -7,11 +7,10 @@ module Language.GraphQL.ExecuteSpec
     ( spec
     ) where
 
+import Control.Exception (SomeException)
 import Data.Aeson ((.=))
 import qualified Data.Aeson as Aeson
 import Data.Conduit
-import Data.Either (fromRight)
-import Data.Functor.Identity (Identity(..))
 import Data.HashMap.Strict (HashMap)
 import qualified Data.HashMap.Strict as HashMap
 import Language.GraphQL.AST (Name)
@@ -23,14 +22,14 @@ import Language.GraphQL.Type.Out as Out
 import Test.Hspec (Spec, context, describe, it, shouldBe)
 import Text.Megaparsec (parse)
 
-schema :: Schema Identity
+schema :: Schema (Either SomeException)
 schema = Schema
     { query = queryType
     , mutation = Nothing
     , subscription = Just subscriptionType
     }
 
-queryType :: Out.ObjectType Identity
+queryType :: Out.ObjectType (Either SomeException)
 queryType = Out.ObjectType "Query" Nothing []
     $ HashMap.singleton "philosopher" 
     $ ValueResolver philosopherField
@@ -39,7 +38,7 @@ queryType = Out.ObjectType "Query" Nothing []
     philosopherField =
         Out.Field Nothing (Out.NonNullObjectType philosopherType) HashMap.empty
 
-philosopherType :: Out.ObjectType Identity
+philosopherType :: Out.ObjectType (Either SomeException)
 philosopherType = Out.ObjectType "Philosopher" Nothing []
     $ HashMap.fromList resolvers
   where
@@ -54,7 +53,7 @@ philosopherType = Out.ObjectType "Philosopher" Nothing []
         = Out.Field Nothing (Out.NonNullScalarType string) HashMap.empty
     lastNameResolver = pure $ Type.String "Nietzsche"
 
-subscriptionType :: Out.ObjectType Identity
+subscriptionType :: Out.ObjectType (Either SomeException)
 subscriptionType = Out.ObjectType "Subscription" Nothing []
     $ HashMap.singleton "newQuote"
     $ EventStreamResolver quoteField (pure $ Type.Object mempty)
@@ -63,7 +62,7 @@ subscriptionType = Out.ObjectType "Subscription" Nothing []
     quoteField =
         Out.Field Nothing (Out.NonNullObjectType quoteType) HashMap.empty
 
-quoteType :: Out.ObjectType Identity
+quoteType :: Out.ObjectType (Either SomeException)
 quoteType = Out.ObjectType "Quote" Nothing []
     $ HashMap.singleton "quote"
     $ ValueResolver quoteField
@@ -84,9 +83,7 @@ spec =
                         ]
                     expected = Response data'' mempty
                     execute' = execute schema Nothing (mempty :: HashMap Name Aeson.Value)
-                    actual = fromRight (singleError "")
-                        $ runIdentity
-                        $ either (pure . parseError) execute'
+                    Right (Right actual) = either (pure . parseError) execute'
                         $ parse document "" "{ philosopher { firstName surname } }"
                 in actual `shouldBe` expected
             it "merges selections" $
@@ -98,9 +95,7 @@ spec =
                         ]
                     expected = Response data'' mempty
                     execute' = execute schema Nothing (mempty :: HashMap Name Aeson.Value)
-                    actual = fromRight (singleError "")
-                        $ runIdentity
-                        $ either (pure . parseError) execute'
+                    Right (Right actual) = either (pure . parseError) execute'
                         $ parse document "" "{ philosopher { firstName } philosopher { lastName } }"
                 in actual `shouldBe` expected
         context "Subscription" $
@@ -112,8 +107,7 @@ spec =
                         ]
                     expected = Response data'' mempty
                     execute' = execute schema Nothing (mempty :: HashMap Name Aeson.Value)
-                    Left stream = runIdentity
-                        $ either (pure . parseError) execute'
+                    Right (Left stream) = either (pure . parseError) execute'
                         $ parse document "" "subscription { newQuote { quote } }"
-                    Just actual = runConduitPure $ stream .| await
+                    Right (Just actual) = runConduit $ stream .| await
                 in actual `shouldBe` expected
diff --git a/tests/Test/StarWars/Data.hs b/tests/Test/StarWars/Data.hs
index 00a89d9..e3dd696 100644
--- a/tests/Test/StarWars/Data.hs
+++ b/tests/Test/StarWars/Data.hs
@@ -1,6 +1,7 @@
 {-# LANGUAGE OverloadedStrings #-}
 module Test.StarWars.Data
     ( Character
+    , StarWarsException(..)
     , appearsIn
     , artoo
     , getDroid
@@ -16,11 +17,12 @@ module Test.StarWars.Data
     , typeName
     ) where
 
-import Data.Functor.Identity (Identity)
+import Control.Monad.Catch (Exception(..), MonadThrow(..), SomeException)
 import Control.Applicative (Alternative(..), liftA2)
-import Control.Monad.Trans.Except (throwE)
 import Data.Maybe (catMaybes)
 import Data.Text (Text)
+import Data.Typeable (cast)
+import Language.GraphQL.Error
 import Language.GraphQL.Type
 
 -- * Data
@@ -66,8 +68,20 @@ appearsIn :: Character -> [Int]
 appearsIn (Left  x) = _appearsIn . _droidChar $ x
 appearsIn (Right x) = _appearsIn . _humanChar $ x
 
-secretBackstory :: Resolve Identity
-secretBackstory = throwE "secretBackstory is secret."
+data StarWarsException = SecretBackstory | InvalidArguments
+
+instance Show StarWarsException where
+    show SecretBackstory = "secretBackstory is secret."
+    show InvalidArguments = "Invalid arguments."
+
+instance Exception StarWarsException where
+    toException = toException . ResolverException
+    fromException e = do
+        ResolverException resolverException <- fromException e
+        cast resolverException
+
+secretBackstory :: Resolve (Either SomeException)
+secretBackstory = throwM SecretBackstory
 
 typeName :: Character -> Text
 typeName = either (const "Droid") (const "Human")
diff --git a/tests/Test/StarWars/QuerySpec.hs b/tests/Test/StarWars/QuerySpec.hs
index 4e48dbf..301fb7c 100644
--- a/tests/Test/StarWars/QuerySpec.hs
+++ b/tests/Test/StarWars/QuerySpec.hs
@@ -6,7 +6,6 @@ module Test.StarWars.QuerySpec
 
 import qualified Data.Aeson as Aeson
 import Data.Aeson ((.=))
-import Data.Functor.Identity (Identity(..))
 import qualified Data.HashMap.Strict as HashMap
 import Data.Text (Text)
 import Language.GraphQL
@@ -357,8 +356,11 @@ spec = describe "Star Wars Query Tests" $ do
     alderaan = "homePlanet" .= ("Alderaan" :: Text)
 
 testQuery :: Text -> Aeson.Value -> Expectation
-testQuery q expected = runIdentity (graphql schema q) `shouldBe` expected
+testQuery q expected =
+    let Right actual = graphql schema q
+     in actual `shouldBe` expected
 
 testQueryParams :: Aeson.Object -> Text -> Aeson.Value -> Expectation
 testQueryParams f q expected =
-    runIdentity (graphqlSubs schema Nothing f q) `shouldBe` expected
+    let Right actual = graphqlSubs schema Nothing f q
+     in actual `shouldBe` expected
diff --git a/tests/Test/StarWars/Schema.hs b/tests/Test/StarWars/Schema.hs
index 99200ff..cecd8eb 100644
--- a/tests/Test/StarWars/Schema.hs
+++ b/tests/Test/StarWars/Schema.hs
@@ -4,10 +4,8 @@ module Test.StarWars.Schema
     ( schema
     ) where
 
+import Control.Monad.Catch (MonadThrow(..), SomeException)
 import Control.Monad.Trans.Reader (asks)
-import Control.Monad.Trans.Except (throwE)
-import Control.Monad.Trans.Class (lift)
-import Data.Functor.Identity (Identity)
 import qualified Data.HashMap.Strict as HashMap
 import Data.Maybe (catMaybes)
 import Data.Text (Text)
@@ -19,7 +17,7 @@ import Prelude hiding (id)
 
 -- See https://github.com/graphql/graphql-js/blob/master/src/__tests__/starWarsSchema.js
 
-schema :: Schema Identity
+schema :: Schema (Either SomeException)
 schema = Schema
     { query = queryType
     , mutation = Nothing
@@ -42,7 +40,7 @@ schema = Schema
     droidField = Out.Field Nothing (Out.NamedObjectType droidObject) mempty
     droidFieldResolver = ValueResolver droidField droid
 
-heroObject :: Out.ObjectType Identity
+heroObject :: Out.ObjectType (Either SomeException)
 heroObject = Out.ObjectType "Human" Nothing [] $ HashMap.fromList
     [ ("id", idFieldType)
     , ("name", nameFieldType)
@@ -57,7 +55,7 @@ heroObject = Out.ObjectType "Human" Nothing [] $ HashMap.fromList
       = ValueResolver (Out.Field Nothing (Out.NamedScalarType string) mempty)
       $ idField "homePlanet"
 
-droidObject :: Out.ObjectType Identity
+droidObject :: Out.ObjectType (Either SomeException)
 droidObject = Out.ObjectType "Droid" Nothing [] $ HashMap.fromList
     [ ("id", idFieldType)
     , ("name", nameFieldType)
@@ -72,29 +70,29 @@ droidObject = Out.ObjectType "Droid" Nothing [] $ HashMap.fromList
         = ValueResolver (Out.Field Nothing (Out.NamedScalarType string) mempty)
         $ idField "primaryFunction"
 
-typenameFieldType :: Resolver Identity
+typenameFieldType :: Resolver (Either SomeException)
 typenameFieldType
     = ValueResolver (Out.Field Nothing (Out.NamedScalarType string) mempty)
     $ idField "__typename"
 
-idFieldType :: Resolver Identity
+idFieldType :: Resolver (Either SomeException)
 idFieldType
     = ValueResolver (Out.Field Nothing (Out.NamedScalarType id) mempty)
     $ idField "id"
 
-nameFieldType :: Resolver  Identity
+nameFieldType :: Resolver (Either SomeException)
 nameFieldType
     = ValueResolver (Out.Field Nothing (Out.NamedScalarType string) mempty)
     $ idField "name"
 
-friendsFieldType :: Resolver Identity
+friendsFieldType :: Resolver (Either SomeException)
 friendsFieldType
     = ValueResolver (Out.Field Nothing fieldType mempty)
     $ idField "friends"
   where
     fieldType = Out.ListType $ Out.NamedObjectType droidObject
 
-appearsInField :: Resolver Identity
+appearsInField :: Resolver (Either SomeException)
 appearsInField
     = ValueResolver (Out.Field (Just description) fieldType mempty)
     $ idField "appearsIn"
@@ -102,14 +100,14 @@ appearsInField
     fieldType = Out.ListType $ Out.NamedEnumType episodeEnum
     description = "Which movies they appear in."
 
-secretBackstoryFieldType :: Resolver Identity
+secretBackstoryFieldType :: Resolver (Either SomeException)
 secretBackstoryFieldType = ValueResolver field secretBackstory
   where
     field = Out.Field Nothing (Out.NamedScalarType string) mempty
 
-idField :: Text -> Resolve Identity
+idField :: Text -> Resolve (Either SomeException)
 idField f = do
-    v <- lift $ asks values
+    v <- asks values
     let (Object v') = v
     pure $ v' HashMap.! f
 
@@ -122,7 +120,7 @@ episodeEnum = EnumType "Episode" (Just description)
     empire = ("EMPIRE", EnumValue $ Just "Released in 1980.")
     jedi = ("JEDI", EnumValue $ Just "Released in 1983.")
 
-hero :: Resolve Identity
+hero :: Resolve (Either SomeException)
 hero = do
   episode <- argument "episode"
   pure $ character $ case episode of
@@ -131,19 +129,19 @@ hero = do
       Enum "JEDI" -> getHero 6
       _ -> artoo
 
-human :: Resolve Identity
+human :: Resolve (Either SomeException)
 human = do
     id' <- argument "id"
     case id' of
         String i -> pure $ maybe Null character $ getHuman i >>= Just
-        _ -> throwE "Invalid arguments."
+        _ -> throwM InvalidArguments
 
-droid :: Resolve Identity
+droid :: Resolve (Either SomeException)
 droid = do
     id' <- argument "id"
     case id' of
         String i -> pure $ maybe Null character $ getDroid i >>= Just
-        _ -> throwE "Invalid arguments."
+        _ -> throwM InvalidArguments
 
 character :: Character -> Value
 character char = Object $ HashMap.fromList