{- |

Module : $Header$

Copyright : C. Maeder and Uni Bremen 2002-2005

License : similar to LGPL, see HetCATS/LICENSE.txt or LIZENZ.txt

Maintainer : maeder@tzi.de

Stability : experimental

Portability : portable

State type from Control.Monad.State but no class MonadState

This module may be replaced by the (non-nhc98 module) Control.Monad.State

-}

module Common.Lib.State where

-- ---------------------------------------------------------------------------

-- Our fixed state monad

newtype State s a = State { runState :: s -> (a, s) }

instance Functor (State s) where

fmap f m = State $ \s -> let

(a, s') = runState m s

in (f a, s')

instance Monad (State s) where

return a = State $ \s -> (a, s)

m >>= k = State $ \s -> let

(a, s') = runState m s

in runState (k a) s'

-- put and get are non-overloaded here!

get :: State s s

get = State $ \s -> (s, s)

put :: s -> State s ()

put s = State $ \_ -> ((), s)

modify :: (s -> s) -> State s ()

modify f = get >>= put . f

gets :: (s -> a) -> State s a

gets f = fmap f get

evalState :: State s a -> s -> a

evalState m = fst . runState m

execState :: State s a -> s -> s

execState m = snd . runState m

mapState :: ((a, s) -> (b, s)) -> State s a -> State s b

mapState f m = State $ f . runState m

withState :: (s -> s) -> State s a -> State s a

withState f m = State $ runState m . f