Add deferred template blocks

[bep]

• This is in ref to Add support for page backlinks (e.g. .GetBacklinks) #8077.
• This also related to resources.PostProcess

There are certain things we would want to do in Hugo but cannot because it's either 1) Very expensive (build time) to do or 2) Impossible (ref. resources.PostProcess and CSS purging which needs the build to complete before it can do its job).

Note that I'm not saying that would be easy to implement, I just say that it would be cool/useful.

What I'm suggesting would be a way to deffer template processing until after the build, somehow storing away the dot context passed to it, e.g.:

Some regular template blocks:
{{ range .Pages }}{{ .Title }}{{ end }}

{{ defer (dict "page" . "someotherParam" "foo") }}
{{ range .page.BackLinks }}
{{ .Title }}
{{ end }}
{{ end }}

The BackLinks above is just an example of a thing that we cannot effectively calculate upfront.

/cc @regisphilibert and gang.

[regisphilibert]

This seems promising but to this day I fail to find any other use case than Backlinks, except maybe info about the build on the site level. (Build time etc...)

In any case, there should be a system for singling out methods available only inside defer blocks. Maybe .Page.Defer.Backlinks, .Site.Defer.Build.Duration ?

While the word defer works great in a template context, it does not work so great as a page method, I'm just using it as an example.

[bep]

In any case, there should be a system for singling out methods available only inside defer blocks. Maybe .Page.Defer.Backlinks, .Site.Defer.Build.Duration ?

What you propose above is a variant of resources.PostProcess -- which is useful, but relatively complex behind the scenes, and with some shortcomings. This is, as one example, not possible:

{{ $css := $css | resources.PostProcess }}
{{ $upper := $css.Title | upper }}

Not the greatest example. But the defer keyword solves the above in a much cleaner and powerful way. If we implement defer I will probably eventually deprecate/remove resources.PostProcess.

The more I think about this, the more I like it.

[regisphilibert]

I see! Then I can see more present benefits and can't wait for the future ones!

[bep]

Thinking a little about this, we cannot "invent new syntax" (because of code formatters etc.), so what I now think makes most sense is to use the with keyword, e.g.:

{{ with defer (dict "page" . "someotherParam" "foo") }}
    {{ range .page.BackLinks }}
        {{ .Title }}
   {{ end }}
{{ end }}

Where defer wold be a template func that just echoes what it gets -- but that happens after the build.

[swamidass]

I like this. It would be great to have.

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