proposal: slices: add `Swap` and `Swapper` functions

[mdempsky]

I propose adding to package slices (or package sort):

// Swap swaps s[i] and s[j].
func Swap[S ~[]E, E any](s S, i, j int) { s[i], s[j] = s[j], s[i] }

// Swapper returns a function that swaps s[i] and s[j].
func Swapper[S ~[]E, E any](s S) func(i, j int) { return func(i, j int) { Swap(s, i, j) } }

This is a common operation when implementing sort.Interface (e.g., appears 164 times within $GOROOT). It's not hard to write out by hand, but it's tedious writing s 4 times when it's a long expression, or making sure to get the i/j/j/i ordering right.

[ianlancetaylor]

I'm not opposed to this. Still, given that this is restricted to slices, how often do we expect people to implement sort.Interface in new code, as opposed to writing a comparison function that can be passed to slices.Sort?

[DeedleFake]

This could be made more general by taking pointers instead:

func Swap[E any](v1, v2 *E) { *v1, *v2 = *v2, *v1 }

Then you could do

func (ex *Example) Swap(i1, i2 int) {
  pkg.Swap(&ex.s[i1], &ex.s[i2])
}

Ah, beat me to it, @ianlancetaylor. Your reply appeared as I was typing this.

[mdempsky]

how often do we expect people to implement sort.Interface in new code, as opposed to writing a comparison function that can be passed to slices.Sort?

sort.Data is more general than slices.Sort. For example, it allows sorting when your records are organized in columns.

[mdempsky]

pkg.Swap(&ex.s[i1], &ex.s[i2])

That's much noisier than slices.Swap(ex.S, i1, i2), IMO. It requires extra &, [, and ] tokens and requires repeating the ex.s expression.

[mdempsky]

slices.Swap would also be useful for using rand.Shuffle. E.g., https://pkg.go.dev/math/rand#example-Shuffle-SlicesInUnison

[mdempsky]

I added a Swapper suggestion too, to further simplify the common case of just wanting to curry the first argument to Swap.