hello, Andrew J Bromage wrote:
1. there are the map* functions for different transformers (mapReader, mapWriter, etc.). does anyone use them for anything? while there is something common between them, i am not quite sure exactly what it is.
I've used them once, and that was to swap the underlying monad of a transformer. It's occasionally handy, for example, when you want your state to be backtrackable for a small part of your code, to stick a backtracking monad _under_ the top monad transformer.
I don't think that you can duplicate this behaviour using the other operations on the monad.
A simple solution would be to introduce this typeclass:
class (MonadTrans t) => MonadMapTrans t where mapTrans :: (m a -> n b) -> (t m a -> t n b)
i think this type doesn't quite work as often the underlying computation returns a more complicated type (e.g. the state monad also returns a new state). in my other library i had a similar operation, corresponding to the fact that most monad transformers are functors on the category of monads. it works for all transformers except for continuations: class MondaTrans t => MapTrans t where mapTrans :: (Monad m, Monad n) => (forall a. m a -> n a) -> t m b -> t n b intuitively the "forall" constraint says that we are changing just the monad, and not touching the value. to get continuations to work, one needs a map (n a -> m a) as well, i think. any comments about that class? bye iavor -- ================================================== | Iavor S. Diatchki, Ph.D. student | | Department of Computer Science and Engineering | | School of OGI at OHSU | | http://www.cse.ogi.edu/~diatchki | ==================================================