Personal tools

New monads/MaybeT

From HaskellWiki

< New monads(Difference between revisions)
Jump to: navigation, search
(Import/export of Fix and State not needed yet.)
(added MonadIO and MonadState instances)

Revision as of 13:34, 26 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
   module Control.Monad,
   module Control.Monad.Trans)
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