A way for a template (e.g. single.html) to signal a flow in the base template

The files in the PR in #11024 is a technical way of implementing this in the templates today (we could consider adding some built-ins for this if needed). Those file don't demonstrate "how useful" this is, but I will talk a little about that below.

-- hugo.toml --
-- content/_index.md --
-- content/p1.md --
-- layouts/_default/baseof.html --
{{ block "_baseof_start" . }}{{ end }}
<html>
<head>
{{/* Lots of imports of SEO partials, stylesheets, JS and whatnot. */}}
</head>
<body>
{{ $flow := .Store.Get "_baseof_flow" | default "default" }}
{{ if eq $flow "default" }}
    <div>
    <h1>Default flow</h1>
    {{ block "main" . }}{{ end }}
    </div>
{{ else if eq $flow "foo" }}
    <div>
        <h1>Foo flow</h1>
        {{ block "main" . }}{{ end }}
    </div>
{{ else if eq $flow "bar" }}
    <div>
        <h1>Bar flow</h1>
        {{ block "main" . }}{{ end }}
    </div>
{{ end }}
</body>
</html>
-- layouts/index.html --
{{ define "_baseof_start" }}{{ .Page.Store.Set "_baseof_flow" "foo" }}{{ end }}
{{ define "main" }}Main Home{{ end }}
-- layouts/_default/single.html --
{{ define "_baseof_start" }}{{ .Page.Store.Set "_baseof_flow" "bar" }}{{ end }}
{{ define "main" }}Main Single{{ end }}

In many sites, even bigger ones, you end up wanting to have only one baseof.html template. You can of course create several templates and avoid some DRYness by adding the common pieces via partials, but it can still be a hassle if only small parts of your base template changes.

The tooling you have for this today are:

block definitions in the base template that can be implemented with define in e.g. single.html.
partial or template invocations from the base template.

But there are often subtle structural difference in the different layouts (e.g. grid layouts, table of contents), which leads to either

Conditional on the form {{ if .IsPage }}Insert table of contents{{ end }}
A duplication of lots of markup in layouts.

The above construct may help simplify things, and it could also be a way to pass data up to the base template.

gohugoio / hugo

A way for a template (e.g. single.html) to signal a flow in the base template #11023