Hello Jeffrey, Jeffrey Yasskin wrote:
Is there a reason I'm missing that it's a bad idea to use GHS.Prim.reallyUnsafePtrEquality# or a similar low-level function in the alter implementation?
I think, given the fuss about unboxed Ints, most people would recoil in horror at this :-) I guess what you could do would be to have the function argument of alter and friends return a type (Maybe (Maybe a)) where.. Nothing means delete it Just Nothing means leave it alone Just (Just a) means use a as the new associated value. ..then to pass the change/nochange information back up the call chain you could use an unboxed pair result containing an appropriate Bool in addition to the (possibly modified) subtree. But I can't help wondering if all the functions like this are really the wrong abstraction. They might be what you want sometimes, but often you'd want to combine them with a lookup (without searching the map twice in quick succession using the same key). Something like the proposed zipper based focus, OMap or whatever seems much more general purpose, flexible and easy to use. It's just a question of whether or not it can be implemented efficiently enough to make alter and friends obsolete (at least as class methods), or should it be an addition to an API that still retains alter (deleteMaybe etc..). Regards -- Adrian Hey