generalize type of Data.Set.unions from List to Foldable

[David Feuer]

It is fold, although fold is not so great for lists in this context. It's
also foldl' union Set.empty, which is better for lists, and probably also
for balanced trees. I initially thought that we should surely generalize,
but now another alternative comes to mind: remove. As a containers
maintainer, I believe we should either:

1. Generalize as proposed, or
2. Deprecate and remove.

I'm currently somewhat in favor of the second option.

On Feb 3, 2018 9:42 AM, "Henning Thielemann" <lemming at henning-thielemann.de>
wrote:


On Sat, 3 Feb 2018, Johannes Waldmann wrote:

I propose this change for containers:
>
> have:  unions ::               [ Set a ] -> Set a
> want:  unions :: Foldable f => f (Set a) -> Set a
>

That's Foldable.fold.

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