Use explicit language extensions & remove extension fields from base.cabal

[ghc-base.git] / Control / Monad / Fix.hs

diff --git a/Control/Monad/Fix.hs b/Control/Monad/Fix.hs
index a596f44..a1309fa 100644 (file)
--- a/Control/Monad/Fix.hs
+++ b/Control/Monad/Fix.hs
@@ -1,55 +1,97 @@
+{-# LANGUAGE CPP #-}
+
 -----------------------------------------------------------------------------
--- 
+-- |
 -- Module      :  Control.Monad.Fix
 -- Copyright   :  (c) Andy Gill 2001,
---               (c) Oregon Graduate Institute of Science and Technology, 2001
--- License     :  BSD-style (see the file libraries/core/LICENSE)
--- 
+--                (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 :  non-portable (reqruires multi-param type classes)
---
--- $Id: Fix.hs,v 1.1 2001/06/28 14:15:02 simonmar Exp $
+-- Portability :  portable
 --
--- The Fix monad.
+-- Monadic fixpoints.
 --
---       Inspired by the paper:
---       \em{Functional Programming with Overloading and
---           Higher-Order Polymorphism},
---         \A[HREF="http://www.cse.ogi.edu/~mpj"]{Mark P Jones},
---               Advanced School of Functional Programming, 1995.}
+-- 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
+        MonadFix(
+           mfix -- :: (a -> m a) -> m a
          ),
-       fix     -- :: (a -> a) -> a
+        fix     -- :: (a -> a) -> a
   ) where
 
 import Prelude
-
 import System.IO
-import Control.Monad.ST
+import Control.Monad.Instances ()
+import Data.Function (fix)
+#ifdef __HUGS__
+import Hugs.Prelude (MonadFix(mfix))
+#endif
+#if defined(__GLASGOW_HASKELL__)
+import GHC.ST
+#endif
+
+#ifndef __HUGS__
+-- | 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)  =  a >>= \\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
+#endif /* !__HUGS__ */
 
+-- Instances of MonadFix for Prelude monads
 
-fix :: (a -> a) -> a
-fix f = let x = f x in x
+-- Maybe:
+instance MonadFix Maybe where
+    mfix f = let a = f (unJust a) in a
+             where unJust (Just x) = x
+                   unJust Nothing  = error "mfix Maybe: Nothing"
 
-class (Monad m) => MonadFix m where
-       mfix :: (a -> m a) -> m a
+-- List:
+instance MonadFix [] where
+    mfix f = case fix (f . head) of
+               []    -> []
+               (x:_) -> x : mfix (tail . f)
 
--- Perhaps these should live beside (the ST & IO) definition.
+-- IO:
 instance MonadFix IO where
-       mfix = fixIO
+    mfix = fixIO 
 
+-- Prelude types with Monad instances in Control.Monad.Instances
+
+instance MonadFix ((->) r) where
+    mfix f = \ r -> let a = f a r in a
+
+instance MonadFix (Either e) where
+    mfix f = let a = f (unRight a) in a
+             where unRight (Right x) = x
+                   unRight (Left  _) = error "mfix Either: Left"
+
+#if defined(__GLASGOW_HASKELL__)
 instance MonadFix (ST s) where
-       mfix = fixST
+        mfix = fixST
+#endif
 
-instance MonadFix Maybe where
-       mfix f = let
-               a = f $ case a of
-                       Just x -> x
-                       _      -> error "empty mfix argument"
-               in a