| However, as somebody pointed out, the Java version is polymorphic. | Assuming that length() is defined for multiple types of container, the | Java version works with lists, arrays, sets, etc. If you try to do this | in Haskell, you end up with A standard way to do it would be this: class ListLike c where lcase :: c a -> LCase a (c a) lcons :: a -> c a -> c a lnil :: c a data LCase a b = LNil | LCons a b f :: ListLike c => c (c Int) -> Int f (lcase -> LCons (lcase -> LCons x (lcase -> LNil)) (lcase -> LCons (lcase -> LCons y (lcase -> LCons _ (lcase -> LNil))) (lcase -> LNil))) = x+y Simon | -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On | Behalf Of Andrew Coppin | Sent: 27 February 2010 18:11 | To: haskell-cafe@haskell.org | Subject: [Haskell-cafe] View patterns | | One somewhat neat thing about Haskell is that you can say | | case list of | [[x], [y,_], [z,_,_]] -> x + y + z | _ -> 0 | | In Java, you'd have to write something like | | if (list.length() == 3) | { | List t1 = list.at(0); | if (t1.length() == 1) | { | int x = t1.at(0); | List t2 = list.at(1); | if (t2.length() == 2) | ... | | I can't even be bothered to finish typing all that lot! | | However, as somebody pointed out, the Java version is polymorphic. | Assuming that length() is defined for multiple types of container, the | Java version works with lists, arrays, sets, etc. If you try to do this | in Haskell, you end up with | | case size c of | 3 -> | case (c ! 0, c ! 1, c ! 2) of | (xs, ys, zs) | size x == 1 && size y == 2 & size z == 3 -> (xs ! | 0) + (ys ! 0) + (zs ! 0) | _ -> 0 | _ -> 0 | | or similar. Which is shorter than Java, but nowhere near as nice as the | original list-only version. | | Now I was under the impression that "view patterns" fix this problem, | but it seems they don't: | | case c of | (size -> 3) -> | case (c ! 0, c ! 1, c ! 2) of | (size -> 1, size -> 2, size -> 3) -> (c ! 0 ! 0) + (c ! 1 ! 0) + | (c ! 2 ! 0) | | Any suggestions? | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe