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New monads/MaybeT

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(Import/export of Fix and State not needed yet.)
(added MonadIO and MonadState instances)
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<haskell>
 
<haskell>
{-# OPTIONS_GHC -fglasgow-exts #-}
+
{-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances #-}
   
 
module Control.Monad.Maybe
 
module Control.Monad.Maybe
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import Control.Monad
 
import Control.Monad
 
import Control.Monad.Trans
 
import Control.Monad.Trans
  +
import Control.Monad.State
   
 
newtype MaybeT m a = MaybeT {runMaybeT :: m (Maybe a)}
 
newtype MaybeT m a = MaybeT {runMaybeT :: m (Maybe a)}
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-- (Add other MTL instances, and a MonadFix instance)
 
-- (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
  +
 
</haskell>
 
</haskell>
   

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
  (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