proposal: strings: missing mirror functions

[colin-sitehost]

background

there does not appear to be a turnkey way to do a SplitN of a string starting at the tail and proceeding toward the head. similarly, the following do not appear to exist:

• SplitAfter, but before (put the separator on the next element: []string{"f" ",o" ",o"})
• SplitAfterN, but before
• Replace but start from the tail

proposal

add the following symbols to the strings package:

func SplitLastN(s, sep string, n int) []string
func SplitBefore(s, sep string) []string
func SplitBeforeN(s, sep string, n int) []string
func ReplaceLast(s, sep string, n int) []string

alternatives

we could say that these are weird and esoteric enough to not justify symbols for, but at least that decision is documented. there are workarounds to get this behaviour if you really need it:

rest, last := s[strings.LastIndex(s, ","):] // SplitLast(s, ",", 1)

I am very flexible on the names; so LastSplitN, ReplaceReverseN, or anything else is fine; I am just interested in the functionality.

[ianlancetaylor]

Can you point to some examples of where these would be used in existing code? Thanks.

[rsc]

Note that if we add CutLast as in #46336 then this would probably serve the need.

[colin-sitehost]

Note that if we add CutLast as in #46336 then this would probably serve the need.

this would serve the needs for SplitLast(s, sep, 1), but that is already just syntax sugar for s[strings.LastIndex(s, ","):], though the ok is a nice add. needing to chain or loop them together seems like a bodge.

Can you point to some examples of where these would be used in existing code? Thanks.

I can explain my use case, but I have not combed through the examples corpus to see if this would fit nicely into some existing logic, and this seems quite string parse(r/ing) specific.

for SplitLastN, we are tail first parsing some structured data, and want to split of the last N segments. there may be more separators in the head, so there is no way to index from there. after, we process the head separately.

[rsc]

These seem rare enough to have in other libraries to start.

[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]

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

[rsc]

No change in consensus, so declined.
— rsc for the proposal review group