{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}

{- |

Module : $Header$

Description : An IO State Monad implementation

Copyright : (c) Ewaryst Schulz, DFKI Bremen 2010

License : similar to LGPL

Maintainer : Ewaryst.Schulz@dfki.de

Stability : provisional

Portability : non-portable (various -fglasgow-exts extensions)

An IO State Monad implementation taken from

Appearently the IOS type comes from John O'Donnell as you can see in

-}

module Common.IOS where

import Control.Monad.Trans (MonadIO (..))

import Control.Monad.State (MonadState (..))

-- * IO State Monad

-- | An IO State Monad

data IOS s a = IOS { unIOS :: s -> IO (a, s) }

getIOS :: IOS s s

getIOS = IOS (\s -> return (s,s))

setIOS :: s -> IOS s ()

setIOS s = IOS (\_ -> return ((),s))

modifyIOS :: (s->s) -> IOS s s

modifyIOS f = IOS (\s -> return (s, f s))

runIOS :: IOS s a -> s -> IO (a, s)

runIOS = unIOS

instance Monad (IOS s) where

return x = IOS (\s -> return (x,s))

m >>= f = IOS (\s ->

do{ (x,s1) <- unIOS m s

; unIOS (f x) s1 } )

instance MonadIO (IOS s) where

liftIO m = IOS (\s -> do{ a <- m; return (a,s) })

instance MonadState s (IOS s) where

get = getIOS

put = setIOS