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.

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
-                case answer of
-                    Right result -> completeValue fieldType fields result
-                    Left errorMessage -> addErrMsg errorMessage
+                answer <- resolveFieldValue prev argumentValues resolver
+                completeValue fieldType fields answer
 
-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)