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 a24d9af..a1309fa 100644 (file)
--- a/Control/Monad/Fix.hs
+++ b/Control/Monad/Fix.hs
@@ -1,8 +1,10 @@
+{-# LANGUAGE CPP #-}
+
 -----------------------------------------------------------------------------
 -- |
 -- 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
@@ -16,44 +18,49 @@
 -----------------------------------------------------------------------------
 
 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.Instances ()
+import Data.Function (fix)
+#ifdef __HUGS__
+import Hugs.Prelude (MonadFix(mfix))
+#endif
+#if defined(__GLASGOW_HASKELL__)
+import GHC.ST
+#endif
 
--- | @'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
-
+#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)@
+--      @'mfix' ('return' . h)  =  'return' ('fix' h)@
 --
 -- [/left shrinking/ (or /tightening/)]
---     @'mfix' (\\x -> a >>= \\y -> f x y)  =  a >>= \\y -> 'mfix' (\\x -> f x y)@
+--      @'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@.
+--      @'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)@
+--      @'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
+        -- | 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
 
@@ -61,6 +68,7 @@ class (Monad m) => MonadFix m where
 instance MonadFix Maybe where
     mfix f = let a = f (unJust a) in a
              where unJust (Just x) = x
+                   unJust Nothing  = error "mfix Maybe: Nothing"
 
 -- List:
 instance MonadFix [] where
@@ -71,3 +79,19 @@ instance MonadFix [] where
 -- IO:
 instance MonadFix IO where
     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
+#endif
+