New monads/MaybeT

{-# 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