# More type design questions

**Andre Pang
**
ozone@algorithm.com.au

*Tue, 19 Aug 2003 03:10:54 +1000*

On Monday, August 18, 2003, at 04:56 PM, Konrad Hinsen wrote:
>* Continuing in my quest to understand type design in Haskell, here's
*>* another
*>* episode that leaves me scratching my head:
*>*
*>* module Foo where
*>*
*>* class Vect v a where
*>* (<+>) :: Floating a => v a -> v a -> v a
*>*
*>* data Vector a = Vector a a a
*>*
*>* instance Vect Vector a where
*>* (<+>) (Vector x1 y1 z1) (Vector x2 y2 z2)
*>* = Vector (x1+x2) (y1+y2) (z1+z2)
*>*
*>* instance Vect [Vector a] a where
*>* (<+>) l1 l2 = zipWith (<+>) l1 l2
*
This seems to work (with -fglasgow-exts):
module Foo where
class Vect v where
(<+>) :: v -> v -> v
data Vector a = Vector a a a
deriving (Show, Eq)
instance Floating a => Vect (Vector a) where
(<+>) (Vector x1 y1 z1) (Vector x2 y2 z2)
= Vector (x1+x2) (y1+y2) (z1+z2)
instance Floating a => Vect [Vector a] where
(<+>) l1 l2 = zipWith (<+>) l1 l2
*Foo> (Vector 5 6 7) <+> (Vector 1 2 3)
Vector 6.0 8.0 10.0
*Foo> [Vector 1 2 3, Vector 10 20 30] <+> [Vector 100 200 300, Vector 4
5 6]
[Vector 101.0 202.0 303.0,Vector 14.0 25.0 36.0]
... or does example not do something which you want it to do?
--
% Andre Pang : trust.in.love.to.save