# New monads/MaybeT

### From HaskellWiki

< New monads(Difference between revisions)

(added MonadIO and MonadState instances) |
(Add Reader and Writer instances) |
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instance Monad m => Monad (MaybeT m) where |
instance Monad m => Monad (MaybeT m) where |
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− | return = MaybeT . return . return |
+ | return = MaybeT . return . Just |

x >>= f = MaybeT $ runMaybeT x >>= maybe (return Nothing) (runMaybeT . f) |
x >>= f = MaybeT $ runMaybeT x >>= maybe (return Nothing) (runMaybeT . f) |
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fail _ = MaybeT $ return Nothing |
fail _ = MaybeT $ return Nothing |
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instance Monad m => MonadPlus (MaybeT m) where |
instance Monad m => MonadPlus (MaybeT m) where |
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− | mzero = MaybeT $ return mzero |
+ | mzero = MaybeT $ return Nothing |

mplus x y = MaybeT $ liftM2 mplus (runMaybeT x) (runMaybeT y) |
mplus x y = MaybeT $ liftM2 mplus (runMaybeT x) (runMaybeT y) |
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instance MonadTrans MaybeT where |
instance MonadTrans MaybeT where |
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− | lift = MaybeT . liftM return |
+ | lift = MaybeT . liftM Just |

-- (Add other MTL instances, and a MonadFix instance) |
-- (Add other MTL instances, and a MonadFix instance) |
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liftIO = lift . liftIO |
liftIO = lift . liftIO |
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− | instance (MonadState s m) => MonadState s (MaybeT m) where |
+ | instance MonadState s m => MonadState s (MaybeT m) where |

get = lift get |
get = lift get |
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put = lift . put |
put = lift . put |
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+ | |||

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

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

</haskell> |
</haskell> |

## Revision as of 18:54, 28 July 2007

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 . 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 $ liftM2 mplus (runMaybeT x) (runMaybeT y) -- 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)))