diff options
Diffstat (limited to 'compiler/utils/IOEnv.hs')
| -rw-r--r-- | compiler/utils/IOEnv.hs | 12 |
1 files changed, 12 insertions, 0 deletions
diff --git a/compiler/utils/IOEnv.hs b/compiler/utils/IOEnv.hs index 61345ca246..305e30eed7 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 +134,16 @@ unsafeInterleaveM (IOEnv m) = IOEnv (\ env -> unsafeInterleaveIO (m env)) ---------------------------------------------------------------------- +-- 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 ---------------------------------------------------------------------- |