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
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.