[project @ 2001-06-28 14:15:04 by simonmar]
[packages/old-time.git] / Control / Monad / Fix.hs
1 -----------------------------------------------------------------------------
2 --
3 -- Module : Control.Monad.Fix
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 : non-portable (reqruires multi-param type classes)
11 --
12 -- $Id: Fix.hs,v 1.1 2001/06/28 14:15:02 simonmar Exp $
13 --
14 -- The Fix monad.
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
24 module Control.Monad.Fix (
25 MonadFix(
26 mfix -- :: (a -> m a) -> m a
27 ),
28 fix -- :: (a -> a) -> a
29 ) where
30
31 import Prelude
32
33 import System.IO
34 import Control.Monad.ST
35
36
37 fix :: (a -> a) -> a
38 fix f = let x = f x in x
39
40 class (Monad m) => MonadFix m where
41 mfix :: (a -> m a) -> m a
42
43 -- Perhaps these should live beside (the ST & IO) definition.
44 instance MonadFix IO where
45 mfix = fixIO
46
47 instance MonadFix (ST s) where
48 mfix = fixST
49
50 instance MonadFix Maybe where
51 mfix f = let
52 a = f $ case a of
53 Just x -> x
54 _ -> error "empty mfix argument"
55 in a