proposal: x/exp/xiter: Add transformers between iter.Seq and iter.Seq2

[eikemeier]

Proposal Details

Sometimes one has a iter.Seq and needs a iter.Seq2 or vice versa. On example could be slices.Backward, which results in a iter.Seq2 and slices.Collect which takes an iter.Seq

Proposed are two mapping functions:

// Map21 returns an iterator over f applied to seq.
func Map21[KIn, VIn, Out any](f func(KIn, VIn) Out, seq iter.Seq2[KIn, VIn]) iter.Seq[Out]

// Map12 returns an iterator over f applied to seq.
func Map12[In, KOut, VOut any](f func(In) (KOut, VOut), seq iter.Seq[In]) iter.Seq2[KOut, VOut]

and two convenience functions:

// Values returns an iterator over seq  with the first elements dropped.
func Values[KIn, VIn any](seq iter.Seq2[KIn, VIn]) iter.Seq[VIn]

// Sequence2 returns a counted iterator over seq.
func Sequence2[In any](seq iter.Seq[In]) iter.Seq2[int, In]

With the following implementation ( Go playground):

// Map21 returns an iterator over f applied to seq.
func Map21[KIn, VIn, Out any](f func(KIn, VIn) Out, seq iter.Seq2[KIn, VIn]) iter.Seq[Out] {
	return func(yield func(Out) bool) {
		for k, v := range seq {
			if !yield(f(k, v)) {
				return
			}
		}
	}
}

// Map12 returns an iterator over f applied to seq.
func Map12[In, KOut, VOut any](f func(In) (KOut, VOut), seq iter.Seq[In]) iter.Seq2[KOut, VOut] {
	return func(yield func(KOut, VOut) bool) {
		for in := range seq {
			if !yield(f(in)) {
				return
			}
		}
	}
}

// Values returns an iterator over seq  with the first elements dropped.
func Values[KIn, VIn any](seq iter.Seq2[KIn, VIn]) iter.Seq[VIn] {
	return Map21(func(_ KIn, v VIn) VIn { return v }, seq)
}

// Sequence2 returns a counted iterator over seq.
func Sequence2[In any](seq iter.Seq[In]) iter.Seq2[int, In] {
	return func(yield func(int, In) bool) {
		k := 0
		for in := range seq {
			if !yield(k, in) {
				return
			}
			k++
		}
	}
}

[gabyhelp]

Related Issues and Documentation

• proposal: x/exp/xiter: new package with iterator adapters #61898
• proposal: iter: Enumerate, Keys and Values functions #68887 (closed)
• proposal: iter: convert between Seq and Seq2 #67334

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[eikemeier]

Related Issues and Documentation

• proposal: iter: convert between Seq and Seq2 #67334

Thanks @gabyhelp. Also, I realized that @bcmills already suggested this.

Also, already discussed.