Hey all. I am trying to declare a read-only state monad and a read-write state monad, so as to distinguish between methods on a data type that are read-only vs. read-write. This is the best I could come up with: newtype ST s a = ST ( s -> (s,a) ) -- read-only newtype SW s a = SW ( s -> (s,a) ) -- read-write class ReadM m s a where readM :: m s s runM :: s -> m s a -> (a,s) class WriteM m r s where updateM :: (s -> s) -> m s () instance ReadM ST s a where readM = ST (\s -> (s,s)) runM s (ST c) = c s -- Doesn't work instance ReadM SW s a where readM = SW (\s -> (s,s)) runM s (SW c) = c s -- Doesn't work And later on ... updateM s (SW c) = c s Does that make sense? If not, how do I do it? If so, is there a simpler means of getting there? Thanks. ;Amit ------------------------------------------------------------------- Amit Garg | Office: ACES 6SEo4E Graduate Student | Phone : (512) 232-7875 Computer Sciences | Res : 2000 Pearl St. #209 University of Texas at Austin | Phone : (512) 659-2532 Homepage: http://www.cs.utexas.edu/~amitji -------------------------------------------------------------------