strings: please add `UnwriteByte` to `Builder`

[jech]

Go version 1.13.

There does not seem to be a way to remove data written to a strings.Builder.

I'm currently doing the following:

func pathUrl(p []string) string {
	var b []byte
	for _, s := range p {
		t := url.PathEscape(s)
		b = append(b, t...)
		b = append(b, '/')
	}
	return string(b[0 : len(b)-1])
}

The last line removes the trailing slash from the accumulated buffer. I don't see an obvious way of doing the same with a strings.Builder. Perhaps it would be a good idea to add UnwriteByte, UnwriteRune and perhaps UnwriteString functions to strings.Builder?

[ianlancetaylor]

It seems to me easy enough to write

func pathUrl(p []string) string {
	var b []byte
	for i, s := range p {
		if i > 0 {
			b = append(b, '/')
		}
		t := url.PathEscape(s)
		b = append(b, t...)
	}
	return string(b)
}

I'm not convinced it's worth the complexity of adding Unwrite methods to strings.Builder.

[josharian]

If you are unwriting only the final byte, you could use strings.Builder and then slice away the final byte of the string.

[jech]

@ianlancetaylor, I agree that it's a matter of taste. It becomes a little more messy when i is not readily available (when the components are not coming from a slice).

Feel free to close if you feel that this proposal is unlikely to gain consensus.

[renthraysk]

I'd personally deal with p[0] outside the loop, and range over p[1:], and never have to unwrite.

https://play.golang.org/p/LODpCtD3mjh

[ianlancetaylor]

I think it's always going to be straightforward to not write data, and very few io.Writer implementations implement UnwriteByte. So I do think this is unlikely to gain consensus.