X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Futils%2FIOEnv.hs;h=5f354f0e4d96ee84d14bce08271b1b3cc4d8008c;hb=8ffd91b6102f4ad3111cabdf6bdb1998f257887f;hp=61345ca246aa41243a7849838f5864b4830d48be;hpb=f4ce543cff19b797d54d435dc7c804acdefca9c8;p=ghc-hetmet.git

diff --git a/compiler/utils/IOEnv.hs b/compiler/utils/IOEnv.hs
index 61345ca..5f354f0 100644
--- a/compiler/utils/IOEnv.hs
+++ b/compiler/utils/IOEnv.hs
@@ -3,6 +3,7 @@
 --
 -- The IO Monad with an environment
 --
+{-# LANGUAGE UndecidableInstances #-}
 
 module IOEnv (
         IOEnv, -- Instance of Monad
@@ -31,6 +32,7 @@ import Data.IORef       ( IORef, newIORef, readIORef, writeIORef, modifyIORef )
 import Data.Typeable
 import System.IO.Unsafe ( unsafeInterleaveIO )
 import System.IO        ( fixIO )
+import Control.Monad
 import MonadUtils
 
 ----------------------------------------------------------------------
@@ -132,6 +134,22 @@ unsafeInterleaveM (IOEnv m) = IOEnv (\ env -> unsafeInterleaveIO (m env))
 
 
 ----------------------------------------------------------------------
+-- MonadPlus
+----------------------------------------------------------------------
+
+-- For use if the user has imported Control.Monad.Error from MTL
+-- Requires UndecidableInstances
+#if __GLASGOW_HASKELL__ > 606
+-- for some reason, this doesn't compile with GHC 6.6:
+-- utils/IOEnv.hs:144:33:
+--     No instance for (MonadPlus IO)
+--       arising from use of `mplus' at utils/IOEnv.hs:144:33-67
+instance MonadPlus IO => MonadPlus (IOEnv env) where
+    mzero = IOEnv (const mzero)
+    m `mplus` n = IOEnv (\env -> unIOEnv m env `mplus` unIOEnv n env)
+#endif
+
+----------------------------------------------------------------------
 -- Accessing input/output
 ----------------------------------------------------------------------