From 91679650b5fc387d59925f1c660af62ec3aa4b87 Mon Sep 17 00:00:00 2001
From: Eugen Wissner <belka@caraus.de>
Date: Tue, 2 Jul 2019 20:07:26 +0200
Subject: Introduce monad transformer for resolvers

Now the errors in the resolvers can be handled and 3 tests throwing
errors pass now. Another test fail but it requires distinguisching
nullable and non-nullable values.
---
 src/Language/GraphQL/Trans.hs | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)
 create mode 100644 src/Language/GraphQL/Trans.hs

(limited to 'src/Language/GraphQL/Trans.hs')

diff --git a/src/Language/GraphQL/Trans.hs b/src/Language/GraphQL/Trans.hs
new file mode 100644
index 0000000..f0fcefb
--- /dev/null
+++ b/src/Language/GraphQL/Trans.hs
@@ -0,0 +1,35 @@
+module Language.GraphQL.Trans where
+
+import Control.Applicative (Alternative(..))
+import Control.Monad (MonadPlus(..))
+import Control.Monad.IO.Class (MonadIO(..))
+import Control.Monad.Trans.Class (MonadTrans(..))
+import Control.Monad.Trans.Except (ExceptT)
+import Data.Text (Text)
+
+newtype ActionT m a = ActionT { runActionT :: ExceptT Text m a }
+
+instance Functor m => Functor (ActionT m) where
+    fmap f = ActionT . fmap f . runActionT
+
+instance Monad m => Applicative (ActionT m) where
+    pure = ActionT . pure
+    (ActionT f) <*> (ActionT x) = ActionT $ f <*> x
+
+instance Monad m => Monad (ActionT m) where
+    return = pure
+    (ActionT action) >>= f = ActionT $ action >>= runActionT . f
+
+instance MonadTrans ActionT where
+    lift = ActionT . lift
+
+instance MonadIO m => MonadIO (ActionT m) where
+    liftIO = lift . liftIO
+
+instance Monad m => Alternative (ActionT m) where
+    empty = ActionT empty
+    (ActionT x) <|> (ActionT y) = ActionT $ x <|> y
+
+instance Monad m => MonadPlus (ActionT m) where
+    mzero = empty
+    mplus = (<|>)
-- 
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