New monads/MaybeT

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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
   module Control.Monad,
   module Control.Monad.Trans)

import Control.Monad
import Control.Monad.Trans
import Control.Monad.State
import Control.Monad.Reader
import Control.Monad.Writer

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

instance Monad m => MonadPlus (MaybeT m) where
  mzero = MaybeT $ return Nothing
  mplus x y = MaybeT $ do
    mx <- runMaybeT x
    case mx of
      Nothing -> runMaybeT y
      Just _  -> return mx

-- Provide other MTL instances, for convenience

instance MonadTrans MaybeT where
  lift = MaybeT . liftM Just

-- (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

instance MonadReader r m => MonadReader r (MaybeT m) where
  ask = lift ask
  local f = MaybeT . local f . runMaybeT

instance (Monoid w, MonadWriter w m) => MonadWriter w (MaybeT m) where
  tell = lift . tell
  listen m = MaybeT (do (mv,w) <- listen (runMaybeT m)
                        case mv of
                          Nothing -> return Nothing
                          Just v -> return (Just (v,w)))
  pass m = MaybeT (do mvf <- runMaybeT m
                      case mvf of
                        Nothing -> return Nothing
                        Just (v,f) -> pass (return (Just v,f)))