Pointed and Traversable

Dan Burton danburton.email at gmail.com
Mon Aug 26 19:54:19 CEST 2013

What are some example data types which have a "pure" which obeys this law,
but no corresponding law-abiding "ap"? The only compelling reason to split
it off is if the separation of abstractions will give us the power to reuse
code which was previously not as reusable.

-- Dan Burton

On Mon, Aug 26, 2013 at 10:16 AM, Edward Kmett <ekmett at gmail.com> wrote:

> There are several uses of Pointed as a separate beast from Applicative. In
> particular it comes up when we talk about "affine traversals", and would
> let us refine the type hierarchy of lens, so you'd think I'd be for it.
> However, to move it into its own class would require literally everyone
> who currently has an Applicative instance to clutter their code with CPPs.
> Even as the author of the Pointed class, I personally find that the
> benefit of the change doesn't warrant the impact of the change.
> -Edward
> On Mon, Aug 26, 2013 at 12:59 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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