Pointed and Traversable
jake.mcarthur at gmail.com
Sat Aug 31 00:14:10 CEST 2013
Are we even sure this law isn't already guaranteed by parametricity and
perhaps the existing laws of the other type classes involved? I for one
can't think of a pointed traversable which doesn't already obey this law.
On Aug 26, 2013 1:02 PM, "Henning Thielemann" <
schlepptop at henning-thielemann.de> wrote:
> There was a lot of discussion about separating "pure" from Applicative and
> putting it into a Pointed class. If I remember correctly, the main counter
> argument was that 'pure' alone does not satisfy interesting laws. There are
> only such laws in connection with the Applicative class.
> Now, in some situations I liked to have a generalized unfoldr. I can build
> this from "pure" and "sequenceA" using the State monad:
> unfoldr :: (Pointed t, Traversable t) => (s -> (a, s)) -> s -> t a
> unfoldr = evalState . sequenceA . pure . state
> One could state a law like:
> traverse f (pure a) == traverse id (pure (f a))
> Would this justify to move "pure" into a new Pointed class?
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> Libraries at haskell.org
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