Implementing template fragments for EEx

I’m trying to implement template fragments in EEx. My extremely rough draft lets you create a template file that looks like this:

<%| first_fragment do %>
I'm the first fragment!
<% end %>
<%| second_fragment do %>
I'm the second fragment!
<% end %>

Evaluating this template with the arguments [fragment: :first_fragment] (and a custom EEx engine) would result in the string "I'm the first fragment!". Evaluating the template with no fragment argument would yield both sentences. Notice that this is using the reserved <%| marker for an EEx expression, I’m just using that because it gives my EEx.Engine implementation an easy flag to evaluate the upcoming AST differently. The Engine implementation just replaces this expression with an if statement that only returns the text inside the block if the fragment variable matches the given token.

I haven’t worked with macros or with EEx internals before, so I am looking for feedback and recommendations on how I can do this better, perhaps more idiomatically. Using EEx’s <%| expression marker as a flag to parse the next word as the template name feels extremely hacky but I wasn’t able to get this working any other way.

The code is available here: GitHub - Cantido/stencil: Moved to Codeberg · GitHub. I’m open to any and all feedback.

As a footnote, I would like to explain why I want this functionality for EEx, when I can just decompose one template into several smaller templates. It would certainly be easier to re-use templates that way. I read an explanation on the HTMX website about template fragments (here: </> htmx ~ Template Fragments) and I think they may be very handy to avoid a proliferation of small template files for HTML fragments that are only used in one place. The fragment stays surrounded by its context, but can be extracted and returned on its own. This is handy if you’re serving a lot of smaller HTML fragments like you do when you use HTMX. It’s a rare feature in templating languages, but I would love to see if it can be implemented in EEx or in a separate library.