{- |

Module : $Header$

Description : ResultT type and a monadic transformer instance

Copyright : (c) T. Mossakowski, C. Maeder, Uni Bremen 2006

License : GPLv2 or higher

Maintainer : Christian.Maeder@dfki.de

Stability : provisional

Portability : portable

'ResultT' type and a monadic transformer instance

-}

module Common.ResultT where

import Common.Result

import Control.Monad.Trans

newtype ResultT m a = ResultT { runResultT :: m (Result a) }

instance Monad m => Functor (ResultT m) where

fmap f m = ResultT $ do

r <- runResultT m

return $ fmap f r

instance Monad m => Monad (ResultT m) where

return = ResultT . return . return

m >>= k = ResultT $ do

r@(Result e v) <- runResultT m

case v of

Nothing -> return $ Result e Nothing

Just a -> do

s <- runResultT $ k a

return $ joinResult r s

fail = ResultT . return . fail

instance MonadTrans ResultT where

lift m = ResultT $ do

a <- m

return $ return a

liftR :: Monad m => Result a -> ResultT m a

liftR = ResultT . return