-----------------------------------------------------------------------------
-- |
-- 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
--
-- Monadic fixpoints.
--
-- For a detailed discussion, see Levent Erkok's thesis,
-- /Value Recursion in Monadic Computations/, Oregon Graduate Institute, 2002.
--
-----------------------------------------------------------------------------

module Control.Monad.Fix (
	MonadFix(
	   mfix	-- :: (a -> m a) -> m a
         ),
	fix	-- :: (a -> a) -> a
  ) where

import Prelude
import System.IO

-- | @'fix' f@ is the least fixed point of the function @f@,
-- i.e. the least defined @x@ such that @f x = x@.
fix :: (a -> a) -> a
fix f = let x = f x in x

-- | Monads having fixed points with a \'knot-tying\' semantics.
-- Instances of 'MonadFix' should satisfy the following laws:
--
-- [/purity/]
--	@'mfix' ('return' . h)  =  'return' ('fix' h)@
--
-- [/left shrinking/ (or /tightening/)]
--	@'mfix' (\\x -> a >>= \\y -> f x y)  =  \\y -> 'mfix' (\\x -> f x y)@
--
-- [/sliding/]
--	@'mfix' ('Control.Monad.liftM' h . f)  =  'Control.Monad.liftM' h ('mfix' (f . h))@,
--	for strict @h@.
--
-- [/nesting/]
--	@'mfix' (\\x -> 'mfix' (\\y -> f x y))  =  'mfix' (\\x -> f x x)@
--
-- This class is used in the translation of the recursive @do@ notation
-- supported by GHC and Hugs.
class (Monad m) => MonadFix m where
	-- | The fixed point of a monadic computation.
	-- @'mfix' f@ executes the action @f@ only once, with the eventual
	-- output fed back as the input.  Hence @f@ should not be strict,
	-- for then @'mfix' f@ would diverge.
	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 
