2009/1/24 Olex P
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