slices: add MatchedThen and MatchedFuncThen by iter

[weilaaa]

Background

slices provides slices.Contains() and slices.ContainsFunc(), both are convenient for determining wether a slice contains a certain element. That's great but seems not enough for common scene that need filter some matched elements in slices and do something with them. Based on this, I want to introduce two new APIs MatchedThen and MatchedFuncThen into slices/iter.

Proposal

I propose adding the following APIs to the slices package as a part of slices/iter.go.

// MatchedThen returns an iterator that yields the clements matched.
func MatchedThen[S ~[]E, E comparable](s S, v E) iter.Seq[E] {...}

// MatchedFuncThen returns an iterator that yields the clements of s satisfies f(e).
func MatchedFuncThen[S ~[]E, E comparable](s S, f func(E) bool) iter.Seq[E] {...}

Both MatchedThen and MatchedFuncThen are based on iterator, we can easily use it according to our preference, pull or push mode.

s1 := []int{1, 2, 3, 3, 4, 5}
for v := range slices.MatchedThen(s1, 3) {
	fmt.Println(v)
}
// Output:
// 3
// 3

type inr struct {
	foo int
	bar string
}
s2 := []inr{
	{1, "1"},
	{2, "2"},
	{3, "3"},
	{4, "4"},
	{5, "5"},
	{6, "6"},
}
next, stop := iter.Pull(slices.MatchedFuncThen(s2, func(inr inr) bool {
	return inr.foo%2 == 0
}))
defer stop()
for {
	v, ok := next()
	if !ok {
		break
	}
	fmt.Println(v)
}
// Output:
// {2 2}
// {4 4}
// {6 6}

Design discussion

Why iter.Seq[E] and not iter.Seq2[E]?

Iter.Seq[E] was chosen instead of iter.Seq2[E] because the resulting index after filtering would be confusing which looks discontinuous. And users usually focus on the objects themselves in the match, not the index.

[randall77]

I think this is a dup of #61898, particularly its Filter function. Please reopen if you disagree.

[weilaaa]

I think this is a dup of #61898, particularly its Filter function. Please reopen if you disagree.

Agree with Filter function, it can be used not only for slices, but also for all iterable objects such as map, array, slice, etc, which provide iter. But if you need a dedicated filter capability used in the slices library, just like slices.All(), slices.Values(), please let me know and I will reopen this issue.