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 -- Oct. 1st, 2002: Added instances for Lazy ST, ST, and List monads
22 -----------------------------------------------------------------------------
24 module Control.Monad.Fix (
26 mfix -- :: (a -> m a) -> m a
28 fix -- :: (a -> a) -> a
32 import qualified Control.Monad.ST.Lazy as LazyST
33 import qualified Control.Monad.ST as ST
37 fix f = let x = f x in x
39 class (Monad m) => MonadFix m where
40 mfix :: (a -> m a) -> m a
42 -- Instances of MonadFix
45 instance MonadFix Maybe where
46 mfix f = let a = f (unJust a) in a
47 where unJust (Just x) = x
50 instance MonadFix [] where
51 mfix f = case fix (f . head) of
53 (x:_) -> x : mfix (tail . f)
56 instance MonadFix IO where
60 instance MonadFix (LazyST.ST s) where
64 instance MonadFix (ST.ST s) where