1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
|
-----------------------------------------------------------------------------
-- |
-- Module : Control.Monad.Fix
-- Copyright : (c) Andy Gill 2001,
-- (c) Oregon Graduate Institute of Science and Technology, 2002
-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : portable
--
-- The Fix monad.
--
-- 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.Fix (
MonadFix(
mfix -- :: (a -> m a) -> m a
),
fix -- :: (a -> a) -> a
) where
import Prelude
import System.IO
fix :: (a -> a) -> a
fix f = let x = f x in x
class (Monad m) => MonadFix m where
mfix :: (a -> m a) -> m a
-- Instances of MonadFix for Prelude monads
-- Maybe:
instance MonadFix Maybe where
mfix f = let a = f (unJust a) in a
where unJust (Just x) = x
-- List:
instance MonadFix [] where
mfix f = case fix (f . head) of
[] -> []
(x:_) -> x : mfix (tail . f)
-- IO:
instance MonadFix IO where
mfix = fixIO
|
