4751N/A{- |

4751N/AModule : ./Common/ResultT.hs

4751N/ADescription : ResultT type and a monadic transformer instance

4751N/ACopyright : (c) T. Mossakowski, C. Maeder, Uni Bremen 2006

4751N/ALicense : GPLv2 or higher, see LICENSE.txt

4751N/A

4751N/AMaintainer : Christian.Maeder@dfki.de

4751N/AStability : provisional

4751N/APortability : portable

4751N/A

4751N/A'ResultT' type and a monadic transformer instance

4751N/A-}

4751N/A

4751N/Amodule Common.ResultT where

4751N/A

4751N/Aimport Common.Result

4751N/Aimport Control.Applicative

4751N/Aimport Control.Monad

4751N/Aimport Control.Monad.Trans

4751N/A

4751N/Anewtype ResultT m a = ResultT { runResultT :: m (Result a) }

4751N/A

4751N/Ainstance Monad m => Functor (ResultT m) where

4751N/A fmap f m = ResultT $ do

4751N/A r <- runResultT m

5061N/A return $ fmap f r

6184N/A

4751N/Ainstance Monad m => Applicative (ResultT m) where

4751N/A pure = return

4751N/A (<*>) = ap

4751N/A

4751N/Ainstance Monad m => Monad (ResultT m) where

4751N/A return = ResultT . return . return

4751N/A m >>= k = ResultT $ do

4751N/A r@(Result e v) <- runResultT m

4751N/A case v of

4751N/A Nothing -> return $ Result e Nothing

4751N/A Just a -> do

4751N/A s <- runResultT $ k a

4751N/A return $ joinResult r s

4751N/A fail = ResultT . return . fail

4751N/A

4751N/Ainstance MonadTrans ResultT where

4751N/A lift m = ResultT $ do

4751N/A a <- m

4751N/A return $ return a

4751N/A

4751N/A-- | Inspired by the MonadIO-class

4751N/Aclass Monad m => MonadResult m where

4751N/A liftR :: Result a -> m a

4751N/A

4751N/A

4751N/Ainstance Monad m => MonadResult (ResultT m) where

4751N/A liftR = ResultT . return

4751N/A

4751N/Ainstance MonadIO m => MonadIO (ResultT m) where

4751N/A liftIO = ResultT . liftM return . liftIO

4751N/A