[project @ 2002-04-24 16:31:37 by simonmar]
[packages/base.git] / Control / Monad / Trans.hs
1 -----------------------------------------------------------------------------
2 -- |
3 -- Module : Control.Monad.Trans
4 -- Copyright : (c) Andy Gill 2001,
5 -- (c) Oregon Graduate Institute of Science and Technology, 2001
6 -- License : BSD-style (see the file libraries/core/LICENSE)
7 --
8 -- Maintainer : libraries@haskell.org
9 -- Stability : experimental
10 -- Portability : portable
11 --
12 -- $Id: Trans.hs,v 1.2 2002/04/24 16:31:38 simonmar Exp $
13 --
14 -- The MonadTrans class.
15 --
16 -- Inspired by the paper
17 -- \em{Functional Programming with Overloading and
18 -- Higher-Order Polymorphism},
19 -- \A[HREF="http://www.cse.ogi.edu/~mpj"]{Mark P Jones},
20 -- Advanced School of Functional Programming, 1995.}
21 -----------------------------------------------------------------------------
22
23 module Control.Monad.Trans (
24 MonadTrans(..),
25 MonadIO(..),
26 ) where
27
28 import Prelude
29
30 import System.IO
31
32 -- ---------------------------------------------------------------------------
33 -- MonadTrans class
34 --
35 -- Monad to facilitate stackable Monads.
36 -- Provides a way of digging into an outer
37 -- monad, giving access to (lifting) the inner monad.
38
39 class MonadTrans t where
40 lift :: Monad m => m a -> t m a
41
42 class (Monad m) => MonadIO m where
43 liftIO :: IO a -> m a
44
45 instance MonadIO IO where
46 liftIO = id