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

[Luke Palmer]

2009/1/23 Brandon S. Allbery KF8NH <allbery at ece.cmu.edu>

> On 2009 Jan 23, at 17:58, Olex P wrote:
>
> class Vector v where
>     (^+^)       :: v -> v -> v
>
> class Matrix m where
>     (^+^)     :: m -> m -> m
>
>
> You can't reuse the same operator in different classes.  Vector "owns"
> (^+^), so Matrix can't use it itself.  You could say
>
> > instance Matrix m => Vector m where
> >   (^+^) = ...
>

No you can't!  Stop thinking you can do that!

It would be sane to do:

class Vector m => Matrix m where
    -- matrix ops that don't make sense on vector

Thus anything that implements Matrix must first implement Vector.  Which is
sane because matrices are square vectors with some additional structure, in
some sense.

Luke
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