# New monads/MaybeT

### From HaskellWiki

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 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)))