move Monad and MonadFix instances for Either from mtl (proposal #4159)

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

diff --git a/Control/Monad/Fix.hs b/Control/Monad/Fix.hs
index 57308bf..e8ba03d 100644 (file)
--- a/Control/Monad/Fix.hs
+++ b/Control/Monad/Fix.hs
@@ -78,9 +78,16 @@ 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