1 -----------------------------------------------------------------------------
3 -- Module : Control.Monad.Fix
4 -- Copyright : (c) Andy Gill 2001,
5 -- (c) Oregon Graduate Institute of Science and Technology, 2002
6 -- License : BSD-style (see the file libraries/base/LICENSE)
8 -- Maintainer : libraries@haskell.org
9 -- Stability : experimental
10 -- Portability : portable
14 -- Inspired by the paper
15 -- /Functional Programming with Overloading and
16 -- Higher-Order Polymorphism/,
17 -- Mark P Jones (<http://www.cse.ogi.edu/~mpj/>)
18 -- Advanced School of Functional Programming, 1995.
20 -----------------------------------------------------------------------------
22 module Control.Monad.Fix (
24 mfix -- :: (a -> m a) -> m a
26 fix -- :: (a -> a) -> a
33 fix f = let x = f x in x
35 class (Monad m) => MonadFix m where
36 mfix :: (a -> m a) -> m a
38 -- Instances of MonadFix for Prelude monads
41 instance MonadFix Maybe where
42 mfix f = let a = f (unJust a) in a
43 where unJust (Just x) = x
46 instance MonadFix [] where
47 mfix f = case fix (f . head) of
49 (x:_) -> x : mfix (tail . f)
52 instance MonadFix IO where