On Mon, Sep 26, 2011 at 9:25 AM, Twan van Laarhoven
Hello list,
Currently the 'state' function is only defined for the State type (aka StateT Identity):
state :: (s -> (a, s)) -> State s a state f = StateT (Identity . f)
But this function makes sense for all MonadState instances. I therefore propose to either add it to the MonadState class:
class MonadState s m | m -> s where ... state :: (s -> (a,s)) -> m a state f = do s <- get let (a,s') = f s set s' return a
or as a stand alone function like modify. I personally prefer to add it to the class, since that allows for a more efficient implementation. In particular:
instance Monad m => MonadState s (StateT s m) where .... state = StateT (return . f)
I think that "mtl" has a real maintainer now - unless something changed from the thread back in June I think Edward K. is doing the duty, so I don't know that the libraries process strictly applies. But that doesn't mean we can't have a discussion in public! Would we make the corresponding change to the "monads-tf" package? I think it is worth keeping their public API in sync to the extent practical, but I don't know if the maintainers of the two packages have a policy on that. Antoine