{- |

Module : ./Common/Lib/Maybe.hs

Description : MaybeT monad transformer without the non-portable features

Copyright : C. Maeder and Uni Bremen 2002-2005

License : GPLv2 or higher, see LICENSE.txt

Maintainer : jonathan.von_schroeder@dfki.de

Stability : experimental

Portability : portable

This module is a replacement of module Control.Monad.Maybe

and only contains the Monad instance for the newtype MaybeT m.

-}

module Common.Lib.Maybe (MaybeT (..), liftToMaybeT) where

import Control.Applicative

import Control.Monad

-- | A monad transformer which adds Maybe semantics to an existing monad.

newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) }

instance Monad m => Functor (MaybeT m) where

fmap = liftM

instance Monad m => Applicative (MaybeT m) where

pure = return

(<*>) = ap

instance Monad m => Monad (MaybeT m) where

fail _ = MaybeT $ return Nothing

return = MaybeT . return . Just

x >>= f = MaybeT $ runMaybeT x >>= maybe (return Nothing) (runMaybeT . f)

liftToMaybeT :: Monad m => m a -> MaybeT m a

liftToMaybeT = MaybeT . liftM Just