Trans.hs

-----------------------------------------------------------------------------
-- |
-- Module      :  Control.Monad.Trans
-- Copyright   :  (c) Mauro Jaskelioff 2008,
--                (c) Andy Gill 2001,
--                (c) Oregon Graduate Institute of Science and Technology, 2001
-- License     :  BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer  :  mjj@cs.nott.ac.uk
-- Stability   :  experimental
-- Portability :  portable
--
-- The MonadTrans class.
--
--      Inspired by the paper
--      /Functional Programming with Overloading and
--          Higher-Order Polymorphism/,
--        Mark P Jones (<http://www.cse.ogi.edu/~mpj/>)
--          Advanced School of Functional Programming, 1995.
-----------------------------------------------------------------------------

module Control.Monad.Trans (
    MonadTrans(..),
    MonadIO(..),
  ) where

import System.IO

-- ---------------------------------------------------------------------------
-- MonadTrans class
--
-- Monad to facilitate stackable Monads.
-- Provides a way of digging into an outer
-- monad, giving access to (lifting) the inner monad.

class MonadTrans t where
    lift :: Monad m => m a -> t m a
    tmap :: (Monad m, Monad n) => 
            (forall a.  m a -> n a) ->   
            (forall b.  n b -> m b) ->
            t m c -> t n c

class (Monad m) => MonadIO m where
    liftIO :: IO a -> m a

instance MonadIO IO where
    liftIO = id