X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=Control%2FMonad%2FFix.hs;h=c2b1669f836d9c5aa0c2510b5deed8a8b5aaf73f;hb=202065fe56bd604d26d1924cbc9c0959266ca7ea;hp=6c9ac7f25166ed9f4377b40f442286ff1fbc2e39;hpb=40c9fe1edc154b17c380408f10f5991663735dd2;p=ghc-base.git

diff --git a/Control/Monad/Fix.hs b/Control/Monad/Fix.hs
index 6c9ac7f..c2b1669 100644
--- a/Control/Monad/Fix.hs
+++ b/Control/Monad/Fix.hs
@@ -2,20 +2,16 @@
 -- |
 -- Module      :  Control.Monad.Fix
 -- Copyright   :  (c) Andy Gill 2001,
---		  (c) Oregon Graduate Institute of Science and Technology, 2002
+-- 		  (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.
+-- Monadic fixpoints.
 --
---	  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.
+-- For a detailed discussion, see Levent Erkok's thesis,
+-- /Value Recursion in Monadic Computations/, Oregon Graduate Institute, 2002.
 --
 -----------------------------------------------------------------------------
 
@@ -29,10 +25,34 @@ module Control.Monad.Fix (
 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