Simon Peyton-Jones wrote:
http://hackage.haskell.org/trac/haskell-prime/wiki/ViewPatterns
I'm thinking of implementing it in GHC, so I'd be interested in feedback of the form - how desirable is it to have a feature of this general form? - can this particular proposal be improved?
Regarding the syntax, I don't like the use of ->, because I think it is confusing to write even -> n when even is the function itself. In all other places in Haskell, A -> B represents a function from A to B whereas in the proposal A -> B represents the function A applied to the thing currently being matched which may return a value wrapped up in Maybe which matches B. There are 2 things - the inconsistent use of -> and the implicit Maybe. I'd rather the Maybe was not implicit (as described in extension 2), and a more consistent syntax was found. A possible syntax could represent the value being matched explicitly, say using ? to represent the value currently being matched, then the pattern could be written as an equation: f (prodSize ? = Small) = ... f (prodSize ? = Medium) = ... f (prodSize ? = Big) = ... or simulating an n + k pattern: g (? - 3 = n) = ... or a function which checks its arg satisfies an arbitrary expression: h (? - (3 * ?) = 10) = ... or an "n+k" against the second element of a list j ( toList ? = (_ : (? - 3 = n : _))) = ... Regarding the problem itself, I'm still not sure why views are needed. For example in (f) above, why not just write as: f :: Product -> ... f prod = k (prodSize prod) where k = ... Best regards, Brian. -- http://www.metamilk.com