New monads/MaybeT

The Maybe monad deserves a transformer, just like the other classic monads.

The code

{-# OPTIONS_GHC -fglasgow-exts #-}

module Control.Monad.Maybe

 (MaybeT(runMaybeT),
  module Control.Monad,
  module Control.Monad.Fix,
  module Control.Monad.Trans)

where

import Control.Monad import Control.Monad.Fix import Control.Monad.Trans import Control.Monad.State

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

instance Functor m => Functor (MaybeT m) where

 fmap f x = MaybeT $ fmap (fmap f) $ runMaybeT x

instance Monad m => Monad (MaybeT m) where

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

instance Monad m => MonadPlus (MaybeT m) where

 mzero = MaybeT $ return mzero
 mplus x y = MaybeT $ liftM2 mplus (runMaybeT x) (runMaybeT y)

-- Provide other MTL instances, for convenience

instance MonadTrans MaybeT where

 lift = MaybeT . liftM return

-- (Add other MTL instances, and a MonadFix instance)