proposal: x/exp/slices: addition of Count function

[Noah-Wagner]

API

// Count reports the number of elements in s that are equal to v.
func Count[E comparable](s []E, v E) int

For inputs of nil or an empty slice, the expected output is 0. For an input where v is not in s, the expected output is 0. For all other (happy path) cases, the expected output is equal to the number of elements in s that are equal to v.

Rationale

No nuanced rationale here, a couple times in the near past I have been looking for such a Count method when working with slices. Interested in gauging interest of whether such an additional API is worth adding to the exp/slices library. It is trivial to implement when needed, but also a nice convenience function to have.

Note that this proposal can also be easily extended to cover CountFunc if we wish to maintain the usual symmetry that is found in the exp/slices library.

[ianlancetaylor]

It would help to point to some existing code that would benefit from this function. Thanks.

[icholy]

This type of usecase seems like it would be better covered by a x/exp/set package. Something like:

count := set.Intersect(set.Of(a...), set.Of(b...)).Size()

[rsc]

I think we are still waiting on a compelling use case for this.

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

Based on the discussion above, this proposal seems like a likely decline.
— rsc for the proposal review group

[rsc]

No change in consensus, so declined.
— rsc for the proposal review group