proposal: slices: add difference function

[cuishuang]

There are many scenarios where it is necessary to obtain the difference of two slices. At present, the more common method is to create a new utils package in the project and write the method implementation in it. Now that the Go standard library has a new slices package, it is recommended to add an API:

func Difference[E comparable](s1, s2 []E) []E {
	var result []E
	s2len := len(s2)
	s1len := len(s1)
	if s2len == 0 {
		return s1
	}
	s2Map := make(map[E]bool, s2len)
	for i := 0; i < s2len; i++ {
		el := s2[i]
		s2Map[el] = true
	}
	for i := 0; i < s1len; i++ {
		element := s1[i]
		if _, ok := s2Map[element]; !ok {
			result = append(result, element)
		}
	}
	return result
}

Similar as golang/exp#50

The full implementation golang/exp#60

[fzipp]

This sounds like an operation that would suit a set datatype better than slices.

[seankhliao]

This seems to have a very specific idea of "difference".
Slices aren't a set of objects, they also include ordering, and items can be duplicated.
How do you justify using this definition of difference vs others?

[DeedleFake]

If this isn't going to preserve ordering, I see no reason that this shouldn't just be handled, eventually, as something like

set1 := set.New(slice1...)
diff := set1.Sub(set.New(slice2)...)
diffslice := diff.Slice()

That's not much more difficult, is more obvious about what it's doing and all the implicit allocations, and gives the caller significantly more control.

[jimmyfrasche]

This is filter of s1 but using a set provided as a slice in s2 instead of a predicate function.

[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

[dsnet]

In addition to the good arguments about this really being a set operation...

For a function named Difference, I would have expected it to produce an edit script, which is essentially a sequence of insert and delete operations that converts the left argument into the right argument. But such a function is probably too complex to live in the slices package and should be a different package.

[rsc]

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