2009/1/24 Olex P <hoknam...@gmail.com>: > What I want to ask you guys can we define a function with arbitrary number > of parameters? Actually not really arbitrary but just several possibilities > (as we have with value constructors). > For instance cross product can have 1, 2 or 3 vectors as inputs depends on > the dimension. Is it 2d, 3d or 4d case.
You need type classes plus some language extension to do this properly. Here's an example using type families: class Vector v => Cross v where type CrossResult v cross :: v -> CrossResult v instance Cross Vector2 where type CrossResult v = Vector2 cross (Vector2 x y) = Vector2 (-y) x -- is this the right operation? instance Cross Vector3 where type CrossResult v = Vector3 -> Vector3 cross (Vector3 x1 y1 z1) (Vector3 x2 y2 z2) = Vector3 (y1*z2 - z1*y2) (z1*x2 - x1*z2) (x1*y2 - y1*x2) etc. A difficult exercise is to define these operations in terms of each other inductively so that it works for arbitrary vector types; you need vectors indexed on some type-level natural, plus a bunch of hackery in the instances to make it work. -- ryan _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
