[Haskell-cafe] Num instances for 2-dimensional types
daniel.is.fischer at web.de
Mon Oct 5 11:02:19 EDT 2009
Am Montag 05 Oktober 2009 16:29:02 schrieb Job Vranish:
> In what way is it not a number?
If there's a natural implementation of fromInteger, good.
If there isn't, *don't provide one*.
fromInteger _ = error "Not sensible" is better than doing something strange.
 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 <miguelimo38 at yandex.ru>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.
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