On 1/24/07, John Meacham
I am having a hard time figuring out what views gain you, if anything.
If you know from the start you want abstract deconstructors, then you can do that now and it has never been an issue, just mechanical and verbose:
date Term = Ap Term Term | Let Id Term Term
fromAp :: Term -> Maybe (Term,Term) fromAp (Ap a b) = Just (a,b) fromAp _ = Nothing
foo t | Just (a,b) <- fromAp t = ...
now views give you some (perhaps) nicer syntax for matching these sort of functions, but they don't make declaring them any less mechanical and verbose, but more importantly, you still have to know a priori whether you will need abstract deconstructors or not.
I agree completely, although with the caveat that this applies more to Peyton-Jones's view patterns than Wadler's original views, at least as I understood them. I had thought the advantages to views (and the reason I've always been hoping they would eventually be implemented) were: 1) Provide type-class like behavior for type constructors, and 2) Reuse Haskell's existing extremely nice syntax for construction and nested pattern matching. This proposal seems to do neither, and I think the troubles it gets into as soon as you want to match multiple values (extension 1, the Just2 type, etc) demonstrate a somewhat limited applicability. I personally like pattern guards a great deal, as light-weight extensions go, but I'm not sure what view patterns would add over and above them. /g -- It is myself I have never met, whose face is pasted on the underside of my mind.