Mustache has built-in spec for parents

[someodd]

https://jgonggrijp.gitlab.io/wontache/playground.html

You can use any single JavaScript expression as input, including nested objects and methods. Naming templates is optional. Doing so enables you to interpolate the named template inside other templates using {{>partial}} or {{<parent}}{{/parent}} tags. (You can also interpolate a template inside itself, but you must be careful to prevent infinite recursion in that case.)

See also: https://mustache.github.io/mustache.5.html

This should drastically simplify code if implemented correctly in the Haskell Mustache library. The implementation for using frontmatter to define could be simplly adding the parent tags around the content, too.