proposal: maps: add analogues to Keys/Values that return slices

[kwjw]

Proposal Details

If a program requires a slice of a map's keys or values, there are some options:

1. Call the nice-looking but poorly-performing slices.Collect(maps.Values(m)) (which has unnecessary allocations).
2. Call the accurate but ugly slices.AppendSeq(make([]K, 0, len(m)), maps.Values(m)) (or, more realistically, write this as a function). Edited to add: Even better, you'd write a function that iterates over the map manually instead of calling the iter-based methods at all.
3. Use x/exp/maps instead, where Keys/Values return slices (obviously not ideal).

Proposal: add standard library functions (maybe call them maps.KeysSlice or maps.ValuesSlice) to return a slice of map keys or values respectively.

[DeedleFake]

Another option would be to add a slices.CollectN[T any](seq iter.Seq[T], n int) that would preallocate a slice of a specific capacity for you to give an in-between for options 1 and 2. I'm not thrilled with the name because it kind of sounds like it limits how many it collects, but the idea is the point.

[kwjw]

To be sure, I am ambivalent about my own proposal.

On one hand, this is such a straightforward function to implement that it might not be worth expanding API surface for.

On the other hand, I generally think it's a good idea for the stdlib to make it easy to do the right thing.

[kwjw]

Another option would be to add a slices.CollectN[T any](seq iter.Seq[T], n int) that would preallocate a slice of a specific capacity for you to give an in-between for options 1 and 2.

This would still have some overhead from the repeated function calls involved in iterators. A pre-allocated version of Collect could be a nice middle ground, but I see value in allowing a totally cost-free migration path from the x/exp/maps sliced-based functions for callers who value its performance.

[gabyhelp]

Related Issues

• proposal: maps: add maps.KeysSlice and maps.ValuesSlice #61626 (closed)
• maps: add keys and values #65919 (closed)
• maps: remove Keys and Values for Go 1.21 #61538 (closed)
• proposal: slices: add CollectN with preallocated size. #68261
• proposal: x/exp/slices: add AsKeys #56441 (closed)
• proposal: spec: keys/vals function for map types #7057 (closed)
• maps: add iterator-related functions #61900 (closed)

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[icholy]

@DeedleFake what about something like AppendCollect where you can pass the destination slice?

[DeedleFake]

@icholy

That already exists under the name slices.AppendSeq(). The original proposal mentions it as option 2.