On Wed, Jan 31, 2007 at 05:53:08PM +0300, Bulat Ziganshin wrote: > something like this: > > data Coord = Coord Float Float > view of Coord = Polar Float Float where > Polar r d = Coord (r*d) (r+d) -- construction > Coord x y | x/=0 || y/=0 = Polar (x*y) (x+y) -- matching
This is somewhat pretty, but in spite of your desire to avoid creating new syntax, you have just done so, and in the process made views more limited. Pattern matching sytax remains the same, but a new declaration syntax has been added. And now in order to pattern match on a function it needs to explicitely be declared as a "view". And unless you are planning to allow one-way views (you don't give any examples of that), "view functions" must be invertible, which greatly weakens their power. If you choose to allow one-way views (non-invertible functions), then I'd vote for not allowing two-way views, as it adds complexity without adding any appreciable gain. I don't like your use of capital letters for ordinary functions, I enjoy having the syntax tell me whether (Foo 1) might or might not be an expensive operation. Finally, you've replaced Simon's explicit incomplete function using Maybe with an implicit incomplete function that returns _|_ when the view doesn't match. I find this rather unappealing. I certainly prefer *intentionally* incomplete functions to return Maybe somthing, rather than just bombing out when given invalid input. I suppose you'll point out that the view Coord is a function that you can never explicitely call, but to me that just makes things even more confusing. Now we're defining functions that we can only use in pattern matching, but can never call. -- David Roundy http://www.darcs.net _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
