Re: [Haskell-cafe] Num instances for 2-dimensional types

Am Montag 05 Oktober 2009 16:29:02 schrieb Job Vranish:

In what way is it not a number?

If there's a natural[1] implementation of fromInteger, good. If there isn't, *don't provide one*. fromInteger _ = error "Not sensible" is better than doing something strange. [1] In the case of residue class rings, you may choose between restricting the range of legitimate arguments for fromInteger or doing a modulo operation on the argument, both ways are natural and meet expectations sufficiently well.

Sönke Hahn: btw, I forgot to mention in my first email, but fromInteger n = (r, r) where r = fromInteger n is better than: fromInteger n = (fromInteger n, 0) as you get a lot of corner cases otherwise.

I use fromInteger = pure . fromInteger, which when combined with my Applicative instance, is effectively the same as your: fromInteger n = (r, r) where r = fromInteger n

- Job

On Mon, Oct 5, 2009 at 9:12 AM, Miguel Mitrofanov wrote:

...

Sönke Hahn wrote:

I used to implement

...

fromInteger n = (r, r) where r = fromInteger n

, but thinking about it, fromInteger n = (fromInteger n, 0)

seems very reasonable, too.

Stop pretending something is a number when it's not.