Added exception handling with Alternative constraint according to spec.

1 files changed, 62 insertions, 0 deletions

diff --git a/Data/GraphQL/Error.hs b/Data/GraphQL/Error.hs
new file mode 100644
index 0000000..25632b4
--- /dev/null
+++ b/Data/GraphQL/Error.hs
@@ -0,0 +1,62 @@
+{-# LANGUAGE CPP #-}
+{-# LANGUAGE OverloadedStrings #-}
+module Data.GraphQL.Error (
+  parseError,
+  CollectErrsT,
+  addErr,
+  addErrMsg,
+  runCollectErrs,
+  joinErrs,
+  errWrap
+  ) where
+
+import qualified Data.Aeson as Aeson
+import Data.Text (Text, pack)
+
+import Control.Arrow ((&&&))
+
+#if !MIN_VERSION_base(4,8,0)
+import Control.Applicative (Applicative, pure)
+import Data.Foldable (Foldable, concatMap)
+import Prelude hiding (concatMap)
+#endif
+
+-- | Wraps a parse error into a list of errors.
+parseError :: Applicative f => String -> f Aeson.Value
+parseError s =
+  pure $ Aeson.object [("errors", Aeson.toJSON [makeErrorMsg $ pack s])]
+
+-- | A wrapper for an applicative functor, for passing around error messages.
+type CollectErrsT f a = f (a,[Aeson.Value])
+
+-- | Takes a (wrapped) list (foldable functor) of values and errors and
+--   joins the values into a list and concatenates the errors.
+joinErrs
+  :: (Functor m, Functor f, Foldable f)
+  => m (f (a,[Aeson.Value])) -> CollectErrsT m (f a)
+joinErrs = fmap $ fmap fst &&& concatMap snd
+
+-- | Wraps the given applicative to handle errors
+errWrap :: Functor f => f a -> f (a, [Aeson.Value])
+errWrap = fmap (flip (,) [])
+
+-- | Adds an error to the list of errors.
+addErr :: Functor f => Aeson.Value -> CollectErrsT f a -> CollectErrsT f a
+addErr v = (fmap . fmap) (v :)
+
+makeErrorMsg :: Text -> Aeson.Value
+makeErrorMsg s = Aeson.object [("message",Aeson.toJSON s)]
+
+-- | Convenience function for just wrapping an error message.
+addErrMsg :: Functor f => Text -> CollectErrsT f a -> CollectErrsT f a
+addErrMsg = addErr . makeErrorMsg
+
+-- | Runs the given query computation, but collects the errors into an error
+--  list, which is then sent back with the data.
+runCollectErrs :: Functor f => CollectErrsT f Aeson.Value -> f Aeson.Value
+runCollectErrs = fmap finalD
+  where finalD (dat,errs) =
+          Aeson.object $
+          if null errs
+            then [("data",dat)]
+            else [("data",dat),("errors",Aeson.toJSON $ reverse errs)]