traverse: Apply -> Applicative

[KiaraGrouwstra]

afaict this was a typo. very confusing for learners like me. :<

[safareli]

2. `f` must return a value of an Applicative

Yes it's a typo, thanks!

[rpominov]

See also https://github.com/fantasyland/fantasy-land/pull/147/files#r79309177

[safareli]

@rpominov as you pointed out maybe we should change laws so it's consistent with type signature?

Or we could change the method to something like this?

traverse :: Applicative f, Traversable t => t a ~> (TypeRep f, a -> f b) -> f (t b)

Or we need to add FAQ.md 😄

[rpominov]

Yea, +1 to at lest make it consistent.

[safareli]

For Tuple z a user could provide function of form a -> Tuple z a (for example (a) => Tuple([], a)) and I think it would work properly. So we should change Applicative to Apply in law?
or use TypeRep f instead of passing a -> f a? @SimonRichardson @davidchambers @scott-christopher

[scott-christopher]

As per the comment @rpominov linked to above, my original thoughts were to use the weaker constraints of Apply given that of had to be explicitly passed anyway meaning it was already described in the signature.

Now that we have the language to describe Applicative f => TypeRep f, it could make it clearer to ask for the full TypeRep now rather than just the of function.

[rpominov]

Now that we have the language to describe Applicative f => TypeRep f, it could make it clearer to ask for the full TypeRep now rather than just the of function.

That a good idea for the future. Btw this is how it's done in static-land, so we should get this automatically if unification happen.

But for now we should at least fix the inconsistency: either make a change in the signature or in laws.

[scott-christopher]

But for now we should at least fix the inconsistency: either make a change in the signature or in laws.

Sure, in that case I'm personally happy with this PR as it is.

[davidchambers]

Now that we have the language to describe Applicative f => TypeRep f, it could make it clearer to ask for the full TypeRep now rather than just the of function.

I'm in favour of this change. It's beyond the scope of this pull request, though.

[safareli]

I think this is ready to be merged.

I have created #207 to discuss changing of a -> f a to TypeRep f.

[safareli]

@fantasyland/core if others think this is the way to go please approve this.

[joneshf]

Thanks so much @tycho01 and every involved!