Control/Monad/IO/Class.hs

-----------------------------------------------------------------------------
-- |
-- Module : Control.Monad.IO.Class
-- Copyright : (c) Andy Gill 2001,
-- (c) Oregon Graduate Institute of Science and Technology, 2001
-- License : BSD-style (see the file LICENSE)
--
-- Maintainer : ross@soi.city.ac.uk
-- Stability : experimental
-- Portability : portable
--
-- Class of monads based on @IO@.
-----------------------------------------------------------------------------

module Control.Monad.IO.Class (
 MonadIO(..)
 ) where

import System.IO (IO)

-- | Monads in which 'IO' computations may be embedded.
-- Any monad built by applying a sequence of monad transformers to the
-- 'IO' monad will be an instance of this class.
--
-- Instances should satisfy the following laws, which state that 'liftIO'
-- is a transformer of monads:
--
-- * @'liftIO' . 'return' = 'return'@
--
-- * @'liftIO' (m >>= f) = 'liftIO' m >>= ('liftIO' . f)@

class (Monad m) => MonadIO m where
 -- | Lift a computation from the 'IO' monad.
 liftIO :: IO a -> m a

instance MonadIO IO where
 liftIO = id