Add `filterKeys`

[emilypi]

Hi All,

A more general form of restrictKeys would be filterKeys, implemented thusly:

filterKeys :: (k -> Bool) -> Map k a -> Map k a 
filterKeys p = filterWithKey (const . p)

Which would allow restrictKeys to be implemented simply as

restrictKeys :: Set k -> Map k a -> Map k a 
restrictKeys s = filterKeys (`Set.member` s)

Is there an appetite for this kind of change? filterKeys is a pretty generally useful function, and restrictKeys tends to be a little too specialized too suit my needs. Since the change would arguably improve the existing functions, I think it's worth a thought.

[treeowl]

restrictKeys exists because it can be particularly efficient.

[emilypi]

alright, but what about the addition of filterKeys then? (assuming we leave restrictKeys alone)

[treeowl]

I don't feel strongly about it. I guess it adds symmetry, balancing filter.

[emilypi]

I'll go do the needful then. Additionally, I'm finding somethings are used internally, with full haddocks that would be useful to export, but are not (e.g. filterWithKeyA). Can we export those as well?

[treeowl]

filterWithKeyA would be a good one, yeah. Maybe some more mergeA machinery too.

[emilypi]

I'll raise separate PRs to not dirty the history