G'day all. On Fri, Jul 25, 2003 at 12:40:22AM +0200, Iavor Diatchki 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'm not happy with the names, but I'm sure someone can think of something better. It might even make sense as a method of MonadTrans itself. Cheers, Andrew Bromage