[Haskell-cafe] mapM and Control.Arrow
zemyla at gmail.com
Tue Jul 9 22:16:19 UTC 2019
Because that's what Edward Kmett does. If he didn't exist, the Haskell
community would have had to invent him.
On Tue, Jul 9, 2019, 15:04 Olaf Klinke <olf at aatal-apotheke.de> wrote:
> Cool, thanks for the pointer. But why is it that whenever something mildly
> categorical pops into my head, Edward Kmett has already published a package
> containing it?
> Am 09.07.2019 um 01:07 schrieb Zemyla <zemyla at gmail.com>:
> > It needs to be an ArrowChoice, not just an Arrow, but it can be done.
> > On Mon, Jul 8, 2019, 15:37 Olaf Klinke <olf at aatal-apotheke.de> wrote:
> > Dear cafe,
> > consider the declaration
> > mapM :: (SomeArrow a, Traversable t) => a b c -> a (t b) (t c)
> > For the Arrow a = (->), the above mapM is just an over-constrained fmap,
> since Functor is a superclass of Traversable. For the arrow a = Kleisi m,
> the above mapM is the Control.Monad.mapM we know. Currently I fail to see
> how it could be defined with any of the existing sub-classes in
> > One might say that fmap and mapM are on the same spectrum: If you
> specialize SomeArrow to (->) you can generalize Traversable to Functor, if
> you specialize Functor to Traversable you can generalize (->) to SomeArrow.
> > Cheers,
> > Olaf
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