Implementing template fragments for EEx

cosmicrose

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.

6 comments

#macros #eex

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2024-03-08 03:21:37 UTC

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al2o3cr

If you’re using Phoenix, IMO the EEx sigil + functions would be more idiomatic:

defmodule SomeModuleWithFragments do
  use Phoenix.Html

  def first_fragment do
    ~E"""
    I'm the first fragment!
     """
  end

  def second_fragment do
    ~E"""
    I'm the second fragment!
     """
  end
end

Post #1

cosmicrose

I’d prefer not to involve Phoenix just for this, and I’m trying to solve for the proliferation of tiny templates like those in the first place. It can get really out-of-hand and I’m looking for an alternative.

Post #2

sodapopcan

Phoenix.HTML is its own package with a single optional dependency on Plug in case that makes any difference.

Post #3

al2o3cr

Oops, I typed my earlier response too fast without checking the version of the docs I was referencing - the EEx sigil used to be in Phoenix.HTML but was deprecated.

You could reuse the implementation, though. Apart from the Phoenix-specific engine, there’s no other coupling to that package:

github.com/phoenixframework/phoenix_html

lib/phoenix_html.ex

ef2df70d7


      
          @doc """
          Provides `~e` sigil with HTML safe EEx syntax inside source files.
          
          Raises on attempts to interpolate with `#{}`, so `~E` should be preferred.
          
              iex> ~e"\""
              ...> Hello <%= "world" %>
              ...> "\""
              {:safe, [[["" | "Hello "] | "world"] | "\\n"]}
          
          """
          defmacro sigil_e(expr, opts) do
            handle_sigil(expr, opts, __CALLER__.line)
          end
          
          @doc """
          Provides `~E` sigil with HTML safe EEx syntax inside source files.
          
          Does not raise on attempts to interpolate with `#{}`, but rather shows those
          characters literally, so it should be preferred over `~e`.