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-- Based on the mtl Control.Monad.State, which is
--
-- Copyright : (c) Andy Gill 2001,
-- (c) Oregon Graduate Institute of Science and Technology, 2001
-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- We'd like to just use the library, but for nhc98 compatibility we can't
module StateM (
StateM, evalStateM, get, put
) where
newtype StateM s a = StateM { runStateM :: s -> (a, s) }
evalStateM :: StateM s a -> s -> a
evalStateM m s = fst (runStateM m s)
instance Monad (StateM s) where
return a = StateM $ \s -> (a, s)
m >>= k = StateM $ \s -> let
(a, s') = runStateM m s
in runStateM (k a) s'
get :: StateM s s
get = StateM $ \s -> (s, s)
put :: s -> StateM s ()
put s = StateM $ \_ -> ((), s)
|
