# New monads/MaybeT

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< New monads(Difference between revisions)

(Add Reader and Writer instances) |
(Changed mplus to shortcut as one would expect) |
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(2 intermediate revisions by one user not shown) | |||

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module Control.Monad.Maybe | module Control.Monad.Maybe | ||

− | (MaybeT | + | (MaybeT, |

+ | runMaybeT, | ||

module Control.Monad, | module Control.Monad, | ||

module Control.Monad.Trans) | module Control.Monad.Trans) | ||

Line 15: | Line 16: | ||

import Control.Monad.Trans | import Control.Monad.Trans | ||

import Control.Monad.State | import Control.Monad.State | ||

+ | import Control.Monad.Reader | ||

+ | import Control.Monad.Writer | ||

newtype MaybeT m a = MaybeT {runMaybeT :: m (Maybe a)} | newtype MaybeT m a = MaybeT {runMaybeT :: m (Maybe a)} | ||

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instance Monad m => MonadPlus (MaybeT m) where | instance Monad m => MonadPlus (MaybeT m) where | ||

mzero = MaybeT $ return Nothing | mzero = MaybeT $ return Nothing | ||

− | mplus x y = MaybeT $ | + | mplus x y = MaybeT $ do |

+ | mx <- runMaybeT x | ||

+ | case mx of | ||

+ | Nothing -> runMaybeT y | ||

+ | Just _ -> return mx | ||

-- Provide other MTL instances, for convenience | -- Provide other MTL instances, for convenience | ||

Line 37: | Line 44: | ||

-- (Add other MTL instances, and a MonadFix instance) | -- (Add other MTL instances, and a MonadFix instance) | ||

− | instance MonadIO m => MonadIO ( | + | instance MonadIO m => MonadIO (MaybeT m) where |

liftIO = lift . liftIO | liftIO = lift . liftIO | ||

## Latest revision as of 02:24, 18 March 2008

The Maybe monad deserves a transformer, just like the other classic monads.

## [edit] 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)))