Cool! Thanks Ryan! On Sat, Jan 24, 2009 at 8:31 PM, Ryan Ingram <ryani.s...@gmail.com> wrote:
> 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 >
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