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Format HEEx files quickly on CI

17 Apr 2022

Phoenix LiveView 0.17.8 provides a HTMLFormatter plugin to mix format your Phoenix templates and LiveView ~H snippets. Because the plugin module comes from a Hex dependency, Mix requires it to be compiled to do its magic. As per Mix module plugins documentation:

Remember that, when running the formatter with plugins, you must make sure that your dependencies and your application have been compiled, so the relevant plugin code can be loaded. Otherwise a warning is logged.

And indeed, that’s what happens when mix format runs immediately after checking out the repository:

$ mix format
Skipping formatter plugin Phoenix.LiveView.HTMLFormatter because module cannot be found 

I think it’s understandable, but also a bit unfortunate, because I like to get immediate feedback from CI when I forget to format my files. Compiling the whole project from scratch could take a few minutes!

Thankfully, I discovered that mix compile is not strictly necessary for the plugin to be available. At least currently, only the phoenix_live_view dependency needs to be compiled.

mix deps.compile works but I felt it was also a bit too slow for me. mix deps.compile phoenix_live_view doesn’t work because some of its dependencies are not compiled yet:

$ mix deps.compile phoenix_live_view
==> phoenix_live_view
Compiling 31 files (.ex)
warning: @behaviour Phoenix.Template.Engine does not exist (in module Phoenix.LiveView.HTMLEngine)
  lib/phoenix_live_view/html_engine.ex:1: Phoenix.LiveView.HTMLEngine (module)

== Compilation error in file lib/phoenix_live_view/js.ex ==
** (ArgumentError) could not load module Phoenix.HTML.Safe due to reason :unavailable
    (elixir 1.14.0-dev) lib/protocol.ex:323: Protocol.assert_protocol!/2
    lib/phoenix_live_view/js.ex:120: (module)
could not compile dependency :phoenix_live_view, "mix compile" failed. Errors may have been logged above. You can recompile this dependency with "mix deps.compile phoenix_live_view", update it with "mix deps.update phoenix_live_view" or clean it with "mix deps.clean phoenix_live_view"


Through trial and error I found this minimal set of packages to compile that works for me:

mix deps.compile phoenix_pubsub plug phoenix_html phoenix phoenix_live_view

This takes only a few seconds, and allows mix format (or mix format --check-formatted) to run, including the HEEx formatter plugin. I can have my cake and eat it too!