[Haskell-cafe] mapM and Control.Arrow

Olaf Klinke 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. 
> Cheers,
> Olaf
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.

More information about the Haskell-Cafe mailing list