text/template: add "else with" like "else if"

[willfaught]

The text/template doc says:

{{if pipeline}} T1 {{else if pipeline}} T0 {{end}}
	To simplify the appearance of if-else chains, the else action
	of an if may include another if directly; the effect is exactly
	the same as writing
		{{if pipeline}} T1 {{else}}{{if pipeline}} T0 {{end}}{{end}}

However, no such "else with" counterpart exists:

{{with pipeline}} T1 {{else}} T0 {{end}}
	If the value of the pipeline is empty, dot is unaffected and T0
	is executed; otherwise, dot is set to the value of the pipeline
	and T1 is executed.

Which would be very useful:

{{ with .foo.bar.baz }}
  {{ . }}
{{ else with .foo.bar.bez }}
  {{ . }}
{{ else with .foo.bar.biz }}
  {{ . }}
{{ end }}

Currently, we must do the nesting that the else/if doc mentions:

{{ with .foo.bar.baz }}
  {{ . }}
{{ else }}
  {{ with .foo.bar.bez }}
    {{ . }}
  {{ else }}
    {{ with .foo.bar.biz }}
      {{ . }}
    {{ end }}
  {{ end }}
{{ end }}

This gets tiresome.

Mixing the two would also be useful:

{{ if .foo.bar.baz }}
  {{ .quux }}
{{ else with .foo.bar.bez }}
  {{ . }}
{{ else if .foo.bar.biz }}
  {{ .boz }}
{{ end }}

[ianlancetaylor]

CC @robpike

[robpike]

This adds no functionality, it is just a convenience. One may argue that it should be done for consistency, and it's not difficult although every change has costs (and for templates, they are doubled because of html/template). I am not convinced it is worth doing but I am not adamant.

[willfaught]

It's just a convenience for if/else too.

This is real code:

{{ with $page.Resources.GetMatch $url }}
    {{ $result = . }}
{{ else }}
    {{ with $page.Resources.Get $url }}
        {{ $result = . }}
    {{ else }}
        {{ with resources.GetMatch $url }}
            {{ $result = . }}
        {{ else }}
            {{ with resources.Get $url }}
                {{ $result = . }}
            {{ else }}
                {{ with resources.GetRemote $url }}
                    {{ $result = . }}
                {{ end }}
            {{ end }}
        {{ end }}
    {{ end }}
{{ end }}

Compare that to:

{{ with $page.Resources.GetMatch $url }}
    {{ $result = . }}
{{ else with $page.Resources.Get $url }}
    {{ $result = . }}
{{ else with resources.GetMatch $url }}
    {{ $result = . }}
{{ else with resources.Get $url }}
    {{ $result = . }}
{{ else with resources.GetRemote $url }}
    {{ $result = . }}
{{ end }}

The difference might seem trivial if you're used to only touching templates occasionally or superficially, but it's a big quality of life improvement for those who spend hours writing non-trivial programs in templates, like authors of Hugo themes/modules.

[seankhliao]

or you could format your existing code like this

{{ with $page.Resources.GetMatch $url }}
    {{ $result = . }}
{{ else }}{{ with $page.Resources.Get $url }}
    {{ $result = . }}
{{ else }}{{ with resources.GetMatch $url }}
    {{ $result = . }}
{{ else }}{{ with resources.Get $url }}
    {{ $result = . }}
{{ else }}{{ with resources.GetRemote $url }}
    {{ $result = . }}
{{ end }}{{ end }}{{ end }}{{ end }}{{ end }}

[willfaught]

It's not the shape of the code that's important, it's the complexity. {{ else }}{{ with is harder to read and reason about. How can you even tell if {{ end }}{{ end }}{{ end }}{{ end }}{{ end }} has enough end's without counting them?

[bep]

@seankhliao there's no sane template formatter (e.g.: https://github.com/NiklasPor/prettier-plugin-go-template) that would format the template source the way you suggest, and throwing a way automatic template formatting is not something anyone would want to do.

I agree with proposal. It should be of tremendous value (save lots of end user time) compared to the time spent on implementing it. I suspect that the people who say "thumbs down" to this are people who really don't spend any amount of time writing Go templates.

[rsc]

Given the use cases presented here, this seems OK to do.

[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 accept.
— rsc for the proposal review group