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 c1b4fe1..a1309fa 100644 (file)
--- 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.
@@ -75,5 +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
+