Add `unionsWith`?

[hdgarrood]

This function doesn't currently exist, but I think it could, and would potentially be useful:

unionsWith :: forall f k v. (Foldable f, Ord k) => (v -> v -> v) -> f (Map k v) -> Map k v

It's probably very simple to define this using what's already exported, but I think it's still useful to have these sorts of things defined for you.

[paf31]

:+1:

[Cmdv]

@paf31 just trying to pick up a ticket to get started with contributing, is this being worked on?

I assume this would need the unionsWith function added then exposed and tests?

[paf31]

@Cmdv Yes, that's right, thanks!

[Cmdv]

-- edit: am I wrong in thinking that it is already exposed? here

@paf31 cool I'll give it a go 😄

[hdgarrood]

unionWith is slightly different, it only combines two maps. unionsWith could be used to take the union of an array with any number of maps inside it, for example.

[Cmdv]

@hdgarrood just got pointed out to me on IRC by garyb as well!! I'll probably get stuck when it comes to testing but I'll crack on :)

[garyb]

For tests, one property to check off the top of my head: the keys of all the maps that were unioned should exist in the resulting map.

[garyb]

Also just to note, we can drop Ord from the signature with the very latest -maps release (it's about 10 minutes old):

unionsWith :: forall f k v. Foldable f => (v -> v -> v) -> f (Map k v) -> Map k v

[LiamGoodacre]

@garyb how would that work??? 😕 With traverse it's fine as we're reconstructing a map in the same way we deconstruct it. But here we're having to combine maps - which means we need to insert the keys in the right place relative to each other.

[garyb]

Uh, yeah, I'm talking nonsense 😆... disregard that.