Simon Marlow
Dan Licata wrote:
Simon PJ and I are implementing view patterns, a way of pattern matching against abstract datatypes, in GHC.
At the risk of being a spoil-sport, I have a somewhat negative take on view patterns. Not because I think they're particularly bad, but because I don't think they're significantly useful enough to warrant adding to the language, at least if we also have pattern guards.
I wholeheartedly agree. I'd rather see a slightly different question addressed: how to permit the definition of overloaded functions using pattern matching (and I mean pattern matching with exactly the same syntax as anywhere else). In other words, if I write
f [] = e f (a:b) g a b
I currently only get f :: [t] -> something, so if I later discover that I need to change the input representation to be more efficient than lists, I have to rewrite f. Wouldn't it be so much nicer if I could simply add a declaration
f:: Stream s => s t -> something
and get a function that works on anything in the Stream class? The core of the idea would be to allow classes to include constructors (and associated destructors) so the definition of Stream would include something for ":" and "[]" and their inverses, though I've no real idea of the details; can anyone come up with a plan? * * * It's essential to this idea that it doesn't involve any new pattern matching syntax; the meaning of pattern matching for overloaded functions should be just as transparent as for non-overloaded ones. -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk