New monads/MaybeT
Jump to navigation
Jump to search
The printable version is no longer supported and may have rendering errors. Please update your browser bookmarks and please use the default browser print function instead.
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