x/exp/slices: add Replace

[DeedleFake]

I have on multiple occasions had a need to replace elements of a slice. This is easily accomplished by calling slices.Delete() followed by slices.Insert(), but this results in more copying than is necessary. Well, unless the compiler's capable of eliminating the extra work involved, but I have no idea if it can or not.

Regardless, my proposal's simple: Add the following function to the slices package:

func Replace[S ~[]E, E any](s S, i, j int, v ...E) S

This would function exactly like

s = slices.Delete(s, i, j)
s = slices.Inset(s, i, v...)

except that it would be a bit more efficient.

[zigo101]

I often want a combine(s ...[]T) function, which is helpful for this too:

combine(s[:i], v, s[j:])

[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

[earthboundkid]

@go101 I suggested slices.Concat during some phase of this process. :-) slices.Concat is still my preferred spelling for this. func Concat[S ~[]E, E any](dst S, slices ...S) S s = slices.Concat(s[:0], s[:i], v, s[j:])

[earthboundkid]

Concat would also be able to work in place or allocating a new slice.

[rsc]

Given that we have Delete and Insert, Replace seems like it belongs.

[rsc]

It sounds like people are in favor of:

func Replace[S ~[]E, E any](s S, i, j int, v ...E) S

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

[zigo101]

It looks Concat gets more votes.

[earthboundkid]

Should I open a separate issue for Concat?

[rsc]

This issue is about Replace. Feel free to open a separate issue about Concat.

Votes alone are not really the metric either - Replace fits next to the existing Insert and Delete.

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group

[gopherbot]

Change https://go.dev/cl/444682 mentions this issue: slices: add Replace

[DeedleFake]

At a glance, it looks like the implementation above could allocate twice because of the double append(). I know what append() allocates extra space, but I'm not familiar with how exactly it works, especially when appending multiple items in a single call. Are there actually any circumstances where it does two allocations?

Edit: The implementation at the time of writing:

t := s[j:]
s = append(s[:i], v...)
s = append(s, t...)
return s

[earthboundkid]

Probably better to take that to the CL comments.