[Haskell-cafe] How to define an operation in terms of itself (but of different type)?

[Dan Piponi]

2009/1/23 Olex P <hoknamahn at gmail.com>:

> class Vector v where
>     (^+^)       :: v -> v -> v
>
> data Vector3 = V3 !Double !Double !Double
>
> instance Vector Vector3 where
>     (V3 x1 y1 z1) ^+^ (V3 x2 y2 z2) = V3 (x1 + x2) (y1 + y2) (z1 + z2)
>
> class Matrix m where
>     (^+^)     :: m -> m -> m
>
> data Matrix3 = M3 !Vector3 !Vector3 !Vector3
>
> instance Matrix Matrix3 where
>     (M3 r11 r12 r13) ^+^ (M3 r21 r22 r23) = M3 (r11 ^+^ r21)
>                                                                   (r12 ^+^
> r22)
>                                                                   (r13 ^+^
> r23)
>
> Hope this is better :)
> So yeah... r11 ^+^ r21 just doesn't work.

It works fine if you make Matrix3 an instance of Vector.
--
Dan