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.

1 files changed, 3 insertions, 4 deletions

diff --git a/src/Language/GraphQL/Type/Out.hs b/src/Language/GraphQL/Type/Out.hs
index d094b4d..89bbf1d 100644
--- a/src/Language/GraphQL/Type/Out.hs
+++ b/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) =