maps: add iterator-related functions

[rsc]

We propose to add the following functions to package maps, to provide good support for code using iterators.

This is one of a collection of proposals updating the standard library for the new 'range over function' feature (#61405). It would only be accepted if that proposal is accepted. See #61897 for a list of related proposals.

All serves as a “source” for iterators.

// All returns an iterator over key-value pairs from m.
func All[Map ~map[K]V, K comparable, V any](m Map) iter.Seq2[K, V] {
	return func(yield func(K, V) bool) bool {
		for k, v := range m {
			if !yield(k, v) {
				return false
			}
		}
		return true
	}
}

Keys and Values are like All: not terribly useful by themselves but useful as inputs to other iteration adapters.
In particular, we expect that x := slices.Sorted(maps.Keys(m)) will be a common pattern.

// Keys returns an iterator over keys in m.
func Keys[Map ~map[K]V, K comparable, V any](m Map) iter.Seq[K] {
	return func(yield func(K) bool) bool {
		for k := range m {
			if !yield(k) {
				return false
			}
		}
		return true
	}
}

// Values returns an iterator over values in m.
func Values[Map ~map[K]V, K comparable, V any](m Map) iter.Seq[V] {
	return func(yield func(V) bool) bool {
		for _, v := range m {
			if !yield(v) {
				return false
			}
		}
		return true
	}
}

Insert and Collect serve as “sinks” for iterators.

// Insert adds the key-value pairs from seq to m.
func Insert[Map ~map[K]V, K comparable, V any](m M, seq iter.Seq2[K, V]) {
	for k, v := range seq {
		m[k] = v
	}
	return m
}

// Collect collects key-value pairs from seq into a new map
// and returns it.
func Collect[K comparable, V any](seq iter.Seq2[K, V]) map[K]V {
	m := make(map[K]V)
	Insert(m, seq)
	return m
}

[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]

Finishing this proposal discussion is blocked on #61405.

[gopherbot]

Change https://go.dev/cl/558736 mentions this issue: maps: add All, Keys, Values, Insert, Collect

[earthboundkid]

It is unfortunate that slices.Sorted(maps.Keys(m)) won't get a length hint from m and may end up doing unnecessary allocations and copies. What if there were also maps.SortedKeys(m) that did the right thing?

[Merovius]

@earthboundkid ISTM if that is a concern, it should maybe rather be slices.SortedLength[E any](int, iter.Seq[E]) (modulo name), as any use of "make a sorted slice from an iterator" would suffer the same issue.

I get, FWIW, that "get the sorted keys from a map" is a fairly common use case, to try and get deterministic order from the specifically non-deterministic maps. But it still seems a more general problem.

[gophun]

maps.Keys could return a sequence implementation with a length hint and slices.Sorted could type assert for that, similar to what some io.Reader functions do.

[gophun]

Nevermind, iter.Seq is a concrete type, not an interface.