I use these instances and similar ones from NumInstances [1] quite a bit. I
also frequently use another pattern for applicative functors, via
applicative-numbers [2]. I don't mind pulling these instances from a
package, though they are orphans.
It does trouble me a bit that the two patterns overlap and disagree on
pairs.
For pairs, I like fromInteger n = (fromInteger n, fromInteger n). Then, for
instance, 1 * (a,b) = (a,b).
-- Conal
[1] http://hackage.haskell.org/package/NumInstances
[2] http://hackage.haskell.org/package/applicative-numbers
On Thu, Oct 18, 2012 at 2:58 AM, Twan van Laarhoven
Dear list,
One piece of utility code that I find myself writing over and over again is the pointwise Num instance for tuples, i.e.
instance (Num a, Num b) => Num (a,b) where fromInteger x = (fromInteger x, fromInteger x) (a,b) + (a',b') = (a + a', b + b') (a,b) - (a',b') = (a - a', b - b') (a,b) * (a',b') = (a * a', b * b') negate (a,b) = (negate a, negate b) abs (a,b) = (abs a, abs b) signum (a,b) = (signum a, signum b)
I therefore propose that this instance be added to the base library. If we do that, the equivalent instances for (,,) (,,,) etc. should perhaps also be added, as well as instances for other numeric classes such as Fractional and Floating.
Discussion period: 2 weeks, ends 2012-11-01.
Twan
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