How to convert iter.Seq2[int, E] to iter.Seq[E] in go 1.23

Question

For instance slices.Backward has the signature

func Backward[Slice ~[]E, E any](s Slice) iter.Seq2[int, E]

when slices.Collect has the signature

func Collect[E any](seq iter.Seq[E]) []E

How can i use them together?

it := slices.Backward([]int{1, 2, 3})
// collect back `it` to a slice 
// slices.Collect(it) -> in call to slices.Collect, type iter.Seq2[int, int] of it does not match iter.Seq[E] (cannot infer E)

Answer 1

iter.Seq2 is an iterator which provides both keys (or indicies) and values. There's no builtin conversion to drop the keys (so we get an iter.Seq as a result), but it's pretty simple to create one:

func Values[K, V any](seq iter.Seq2[K, V]) iter.Seq[V] {
    return func(yield func(V) bool) {
        for _, v := range seq {
            if !yield(v) {
                return
            }
        }
    }
}

Testing it:

is := []int{1, 2, 3, 4}

seq2 := slices.Backward(is)

for i, v := range seq2 {
    fmt.Println(i, v)
}

seq := Values(seq2)

values := slices.Collect(seq)
fmt.Println(values)

for v := range seq {
    fmt.Println(v)
}

Output (try it on the Go Playground):

3 4
2 3
1 2
0 1
[4 3 2 1]
4
3
2
1