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New monads/MaybeT

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(fixed so the code compiles.)

Revision as of 15:44, 26 August 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
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 $ 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)))