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.

CHANGELOG.md

@@ -24,12 +24,16 @@ and this project adheres to
 - Parsing subscriptions (the execution always fails yet).
 - `Error.ResponseEventStream`, `Type.Out.Resolve`, `Type.Out.Subscribe` and
   `Type.Out.SourceEventStream` define subscription resolvers.
+- `Error.ResolverException` is an exception that can be thrown by (field value
+  and event stream) resolvers to signalize an error. Other exceptions will
+  escape.
 
 ## Changed
 - `Type.Out.Resolver`: Interface fields don't have resolvers, object fields
   have value resolvers, root subscription type resolvers need an additional
   resolver that creates an event stream. `Resolver` represents these differences
-  now and pairs a field with the function(s).
+  now and pairs a field with the function(s). Resolvers don't have `ExceptT`,
+  errors are handled with `MonadThrow`/`MonadCatch`.
 - All code from `Trans` is moved to `Type.Out` and exported by `Type` and
   `Type.Out`.
 - `AST.Core` contained only `Arguments` which was moved to `Type.Definition`.
@@ -43,6 +47,8 @@ and this project adheres to
   variable names is changed back to JSON since it is a common format and it
   saves additional conversions. Custom format still can be used with the
   underlying functions (in the `Execute` module).
+- The constraint of the base monad was changed to `MonadCatch` (and it implies
+  `MonadThrow`).
 
 ## Removed
 - `Trans.ActionT` is an unneeded layer of complexity. `Type.Out.Resolver`

graphql.cabal

@@ -4,7 +4,7 @@ cabal-version: 1.12
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 38e16611476c6163a049a4ddbaef34cf3fdef8f85d25f7bcaed839372c9fdf75
+-- hash: f3469205f704a81ee0f55655758cf588a9e9eb52303dadd58def32a2eb207696
 
 name:           graphql
 version:        0.8.0.0
@@ -64,6 +64,7 @@ library
     , base >=4.7 && <5
     , conduit
     , containers
+    , exceptions
     , megaparsec
     , parser-combinators
     , scientific
@@ -100,6 +101,7 @@ test-suite tasty
     , base >=4.7 && <5
     , conduit
     , containers
+    , exceptions
     , graphql
     , hspec
     , hspec-expectations

package.yaml

@@ -30,6 +30,7 @@ dependencies:
 - base >= 4.7 && < 5
 - conduit
 - containers
+- exceptions
 - megaparsec
 - parser-combinators
 - scientific

src/Language/GraphQL.hs

@@ -7,6 +7,7 @@ module Language.GraphQL
     , graphqlSubs
     ) where
 
+import Control.Monad.Catch (MonadCatch)
 import qualified Data.Aeson as Aeson
 import qualified Data.Aeson.Types as Aeson
 import Data.Either (fromRight)
@@ -20,7 +21,7 @@ import Text.Megaparsec (parse)
 
 -- | If the text parses correctly as a @GraphQL@ query the query is
 -- executed using the given 'Schema'.
-graphql :: Monad m
+graphql :: MonadCatch m
     => Schema m -- ^ Resolvers.
     -> Text -- ^ Text representing a @GraphQL@ request document.
     -> m Aeson.Value -- ^ Response.
@@ -29,7 +30,7 @@ graphql schema = graphqlSubs schema mempty mempty
 -- | If the text parses correctly as a @GraphQL@ query the substitution is
 -- applied to the query and the query is then executed using to the given
 -- 'Schema'.
-graphqlSubs :: Monad m
+graphqlSubs :: MonadCatch m
     => Schema m -- ^ Resolvers.
     -> Maybe Text -- ^ Operation name.
     -> Aeson.Object -- ^ Variable substitution function.

src/Language/GraphQL/Error.hs

@@ -1,4 +1,5 @@
 {-# LANGUAGE DuplicateRecordFields #-}
+{-# LANGUAGE ExistentialQuantification #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE RecordWildCards #-}
 
@@ -8,6 +9,7 @@ module Language.GraphQL.Error
     , CollectErrsT
     , Error(..)
     , Resolution(..)
+    , ResolverException(..)
     , Response(..)
     , ResponseEventStream
     , addErr
@@ -17,6 +19,7 @@ module Language.GraphQL.Error
     ) where
 
 import Conduit
+import Control.Exception (Exception(..))
 import Control.Monad.Trans.State (StateT, modify, runStateT)
 import Data.HashMap.Strict (HashMap)
 import Data.Sequence (Seq(..), (|>))
@@ -102,6 +105,15 @@ data Response a = Response
 -- Stream.
 type ResponseEventStream m a = ConduitT () (Response a) m ()
 
+-- | Only exceptions that inherit from 'ResolverException' a cought by the
+-- executor.
+data ResolverException = forall e. Exception e => ResolverException e
+
+instance Show ResolverException where
+    show (ResolverException e) = show e
+
+instance Exception ResolverException
+
 -- | Runs the given query computation, but collects the errors into an error
 -- list, which is then sent back with the data.
 runCollectErrs :: (Monad m, Serialize a)

src/Language/GraphQL/Execute.hs

@@ -6,6 +6,7 @@ module Language.GraphQL.Execute
     , module Language.GraphQL.Execute.Coerce
     ) where
 
+import Control.Monad.Catch (MonadCatch)
 import Data.HashMap.Strict (HashMap)
 import Data.Sequence (Seq(..))
 import Data.Text (Text)
@@ -25,7 +26,7 @@ import Language.GraphQL.Type.Schema
 --
 -- Returns the result of the query against the schema wrapped in a /data/
 -- field, or errors wrapped in an /errors/ field.
-execute :: (Monad m, VariableValue a, Serialize b)
+execute :: (MonadCatch m, VariableValue a, Serialize b)
     => Schema m -- ^ Resolvers.
     -> Maybe Text -- ^ Operation name.
     -> HashMap Name a -- ^ Variable substitution function.
@@ -39,7 +40,7 @@ execute schema operationName subs document =
             $ Transform.queryError queryError
         Right transformed -> executeRequest transformed
 
-executeRequest :: (Monad m, Serialize a)
+executeRequest :: (MonadCatch m, Serialize a)
     => Transform.Document m
     -> m (Either (ResponseEventStream m a) (Response a))
 executeRequest (Transform.Document types' rootObjectType operation)
@@ -53,7 +54,7 @@ executeRequest (Transform.Document types' rootObjectType operation)
 
 -- This is actually executeMutation, but we don't distinguish between queries
 -- and mutations yet.
-executeOperation :: (Monad m, Serialize a)
+executeOperation :: (MonadCatch m, Serialize a)
     => HashMap Name (Type m)
     -> Out.ObjectType m
     -> Seq (Transform.Selection m)

src/Language/GraphQL/Execute/Execution.hs

@@ -8,8 +8,8 @@ module Language.GraphQL.Execute.Execution
     , executeSelectionSet
     ) where
 
+import Control.Monad.Catch (Exception(..), MonadCatch(..))
 import Control.Monad.Trans.Class (lift)
-import Control.Monad.Trans.Except (runExceptT)
 import Control.Monad.Trans.Reader (runReaderT)
 import Control.Monad.Trans.State (gets)
 import Data.List.NonEmpty (NonEmpty(..))
@@ -19,7 +19,7 @@ import qualified Data.HashMap.Strict as HashMap
 import qualified Data.Map.Strict as Map
 import Data.Maybe (fromMaybe)
 import Data.Sequence (Seq(..))
-import Data.Text (Text)
+import qualified Data.Text as Text
 import Language.GraphQL.AST (Name)
 import Language.GraphQL.Error
 import Language.GraphQL.Execute.Coerce
@@ -31,14 +31,19 @@ import Language.GraphQL.Type.Internal
 import Language.GraphQL.Type.Schema
 import Prelude hiding (null)
 
-resolveFieldValue :: Monad m
+resolveFieldValue :: MonadCatch m
     => Type.Value
     -> Type.Subs
     -> Type.Resolve m
-    -> m (Either Text Type.Value)
+    -> CollectErrsT m Type.Value
 resolveFieldValue result args resolver =
-    flip runReaderT context $ runExceptT resolver
+    catch (lift $ runReaderT resolver context) handleFieldError
   where
+    handleFieldError :: MonadCatch m
+        => ResolverException
+        -> CollectErrsT m Type.Value
+    handleFieldError e =
+        addErr (Error (Text.pack $ displayException e) []) >> pure Type.Null
     context = Type.Context
         { Type.arguments = Type.Arguments args
         , Type.values = result
@@ -102,7 +107,7 @@ instanceOf objectType (AbstractUnionType unionType) =
   where
     go unionMemberType acc = acc || objectType == unionMemberType
 
-executeField :: (Monad m, Serialize a)
+executeField :: (MonadCatch m, Serialize a)
     => Out.Resolver m
     -> Type.Value
     -> NonEmpty (Transform.Field m)
@@ -119,12 +124,10 @@ executeField fieldResolver prev fields
         case coerceArgumentValues argumentDefinitions arguments' of
             Nothing -> addErrMsg "Argument coercing failed."
             Just argumentValues -> do
-                answer <- lift $ resolveFieldValue prev argumentValues resolver
+                answer <- resolveFieldValue prev argumentValues resolver
-                case answer of
+                completeValue fieldType fields answer
-                    Right result -> completeValue fieldType fields result
-                    Left errorMessage -> addErrMsg errorMessage
 
-completeValue :: (Monad m, Serialize a)
+completeValue :: (MonadCatch m, Serialize a)
     => Out.Type m
     -> NonEmpty (Transform.Field m)
     -> Type.Value
@@ -166,13 +169,15 @@ completeValue (Out.UnionBaseType unionType) fields result
             Nothing -> addErrMsg "Value completion failed."
 completeValue _ _ _ = addErrMsg "Value completion failed."
 
-mergeSelectionSets :: Monad m => NonEmpty (Transform.Field m) -> Seq (Transform.Selection m)
+mergeSelectionSets :: MonadCatch m
+    => NonEmpty (Transform.Field m)
+    -> Seq (Transform.Selection m)
 mergeSelectionSets = foldr forEach mempty
   where
     forEach (Transform.Field _ _ _ fieldSelectionSet) selectionSet =
         selectionSet <> fieldSelectionSet
 
-coerceResult :: (Monad m, Serialize a)
+coerceResult :: (MonadCatch m, Serialize a)
     => Out.Type m
     -> Output a
     -> CollectErrsT m a
@@ -183,7 +188,7 @@ coerceResult outputType result
 -- | Takes an 'Out.ObjectType' and a list of 'Transform.Selection's and applies
 -- each field to each 'Transform.Selection'. Resolves into a value containing
 -- the resolved 'Transform.Selection', or a null value and error information.
-executeSelectionSet :: (Monad m, Serialize a)
+executeSelectionSet :: (MonadCatch m, Serialize a)
     => Type.Value
     -> Out.ObjectType m
     -> Seq (Transform.Selection m)

src/Language/GraphQL/Execute/Subscribe.hs

@@ -9,7 +9,7 @@ module Language.GraphQL.Execute.Subscribe
     ) where
 
 import Conduit
-import Control.Monad.Trans.Except (ExceptT(..), runExceptT)
+import Control.Monad.Catch (Exception(..), MonadCatch(..))
 import Control.Monad.Trans.Reader (ReaderT(..), runReaderT)
 import Data.HashMap.Strict (HashMap)
 import qualified Data.HashMap.Strict as HashMap
@@ -17,6 +17,7 @@ import qualified Data.Map.Strict as Map
 import qualified Data.List.NonEmpty as NonEmpty
 import Data.Sequence (Seq(..))
 import Data.Text (Text)
+import qualified Data.Text as Text
 import Language.GraphQL.AST (Name)
 import Language.GraphQL.Execute.Coerce
 import Language.GraphQL.Execute.Execution
@@ -29,7 +30,7 @@ import Language.GraphQL.Type.Schema
 
 -- This is actually executeMutation, but we don't distinguish between queries
 -- and mutations yet.
-subscribe :: (Monad m, Serialize a)
+subscribe :: (MonadCatch m, Serialize a)
     => HashMap Name (Type m)
     -> Out.ObjectType m
     -> Seq (Transform.Selection m)
@@ -38,7 +39,7 @@ subscribe types' objectType fields = do
     sourceStream <- createSourceEventStream types' objectType fields
     traverse (mapSourceToResponseEvent types' objectType fields) sourceStream
 
-mapSourceToResponseEvent :: (Monad m, Serialize a)
+mapSourceToResponseEvent :: (MonadCatch m, Serialize a)
     => HashMap Name (Type m)
     -> Out.ObjectType m
     -> Seq (Transform.Selection m)
@@ -48,7 +49,7 @@ mapSourceToResponseEvent types' subscriptionType fields sourceStream = pure
     $ sourceStream
     .| mapMC (executeSubscriptionEvent types' subscriptionType fields)
 
-createSourceEventStream :: Monad m
+createSourceEventStream :: MonadCatch m
     => HashMap Name (Type m)
     -> Out.ObjectType m
     -> Seq (Transform.Selection m)
@@ -67,14 +68,18 @@ createSourceEventStream _types subscriptionType@(Out.ObjectType _ _ _ fieldTypes
   where
     groupedFieldSet = collectFields subscriptionType fields
 
-resolveFieldEventStream :: Monad m
+resolveFieldEventStream :: MonadCatch m
     => Type.Value
     -> Type.Subs
-    -> ExceptT Text (ReaderT Out.Context m) (Out.SourceEventStream m)
+    -> Out.Subscribe m
     -> m (Either Text (Out.SourceEventStream m))
 resolveFieldEventStream result args resolver =
-    flip runReaderT context $ runExceptT resolver
+    catch (Right <$> runReaderT resolver context) handleEventStreamError
   where
+    handleEventStreamError :: MonadCatch m
+        => ResolverException
+        -> m (Either Text (Out.SourceEventStream m))
+    handleEventStreamError = pure . Left . Text.pack . displayException
     context = Type.Context
         { Type.arguments = Type.Arguments args
         , Type.values = result
@@ -82,7 +87,7 @@ resolveFieldEventStream result args resolver =
 
 -- This is actually executeMutation, but we don't distinguish between queries
 -- and mutations yet.
-executeSubscriptionEvent :: (Monad m, Serialize a)
+executeSubscriptionEvent :: (MonadCatch m, Serialize a)
     => HashMap Name (Type m)
     -> Out.ObjectType m
     -> Seq (Transform.Selection m)

src/Language/GraphQL/Type/Out.hs

@@ -33,7 +33,6 @@ module Language.GraphQL.Type.Out
     ) where
 
 import Conduit
-import Control.Monad.Trans.Except (ExceptT)
 import Control.Monad.Trans.Reader (ReaderT, asks)
 import Data.HashMap.Strict (HashMap)
 import qualified Data.HashMap.Strict as HashMap
@@ -180,11 +179,11 @@ data Context = Context
 
 -- | Monad transformer stack used by the resolvers for determining the resolved
 -- value of a field.
-type Resolve m = ExceptT Text (ReaderT Context m) Value
+type Resolve m = ReaderT Context m Value
 
 -- | Monad transformer stack used by the resolvers for determining the resolved
 -- event stream of a subscription field.
-type Subscribe m = ExceptT Text (ReaderT Context m) (SourceEventStream m)
+type Subscribe m = ReaderT Context m (SourceEventStream m)
 
 -- | A source stream represents the sequence of events, each of which will
 -- trigger a GraphQL execution corresponding to that event.
@@ -206,7 +205,7 @@ data Resolver m
 -- be optional then).
 argument :: Monad m => Name -> Resolve m
 argument argumentName = do
-    argumentValue <- lift $ asks $ lookupArgument . arguments
+    argumentValue <- asks $ lookupArgument . arguments
     pure $ fromMaybe Null argumentValue
   where
     lookupArgument (Arguments argumentMap) =

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 "")
+                    Right (Right actual) = either (pure . parseError) execute'
-                        $ runIdentity
-                        $ 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 "")
+                    Right (Right actual) = either (pure . parseError) execute'
-                        $ runIdentity
-                        $ 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
+                    Right (Left stream) = either (pure . parseError) execute'
-                        $ either (pure . parseError) execute'
                         $ parse document "" "subscription { newQuote { quote } }"
-                    Just actual = runConduitPure $ stream .| await
+                    Right (Just actual) = runConduit $ stream .| await
                 in actual `shouldBe` expected

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
+data StarWarsException = SecretBackstory | InvalidArguments
-secretBackstory = throwE "secretBackstory is secret."
+
+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")

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

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