# 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
(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)))
```