X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=Control%2FMonad%2FFix.hs;h=a1309faccd41db6574dc8bd5f36297708d635b35;hb=41839f14458d79ac02ad11087885c7f4f1144a73;hp=d0582200ed459df42c0c961196909c473b478701;hpb=c7d2a7507a72b02be171fd1087a2105d66defb6a;p=ghc-base.git

diff --git a/Control/Monad/Fix.hs b/Control/Monad/Fix.hs
index d058220..a1309fa 100644
--- a/Control/Monad/Fix.hs
+++ b/Control/Monad/Fix.hs
@@ -1,3 +1,5 @@
+{-# LANGUAGE CPP #-}
+
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Control.Monad.Fix
@@ -29,6 +31,9 @@ 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.
@@ -63,6 +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
@@ -74,5 +80,18 @@ instance MonadFix [] where
 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
+