# 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 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