--
-- The IO Monad with an environment
--
+{-# LANGUAGE UndecidableInstances #-}
module IOEnv (
IOEnv, -- Instance of Monad
import Data.Typeable
import System.IO.Unsafe ( unsafeInterleaveIO )
import System.IO ( fixIO )
+import Control.Monad
import MonadUtils
----------------------------------------------------------------------
unIOEnv (f r) env })
thenM_ :: IOEnv env a -> IOEnv env b -> IOEnv env b
-thenM_ (IOEnv m) f = IOEnv (\ env -> do { m env ; unIOEnv f env })
+thenM_ (IOEnv m) f = IOEnv (\ env -> do { _ <- m env ; unIOEnv f env })
failM :: IOEnv env a
failM = IOEnv (\ _ -> throwIO IOEnvFailure)
----------------------------------------------------------------------
+-- MonadPlus
+----------------------------------------------------------------------
+
+-- For use if the user has imported Control.Monad.Error from MTL
+-- Requires UndecidableInstances
+instance MonadPlus IO => MonadPlus (IOEnv env) where
+ mzero = IOEnv (const mzero)
+ m `mplus` n = IOEnv (\env -> unIOEnv m env `mplus` unIOEnv n env)
+
+----------------------------------------------------------------------
-- Accessing input/output
----------------------------------------------------------------------