    -----------------------------------------------------------------------------
    -- |
    -- 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