New monads/MaybeT

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

The code

{-# OPTIONS_GHC -fglasgow-exts  -fallow-undecidable-instances #-}

module Control.Monad.Maybe
  (MaybeT(runMaybeT),
   module Control.Monad,
   module Control.Monad.Trans)
where

import Control.Monad
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)

instance MonadIO m => MonadIO (maybeT m) where
  liftIO = lift . liftIO

instance (MonadState s m) => MonadState s (MaybeT m) where
  get = lift get
  put = lift . put