[Haskell-cafe] mapM and Control.Arrow
olf at aatal-apotheke.de
Tue Jul 9 20:03:58 UTC 2019
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. https://github.com/ekmett/profunctors/pull/40
> 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 Control.Arrow.
> 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.
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