This resolved the .app issue.
A new minor issue I’ve spotted is that the compiled dependency is not detected until mix compile is rerun.
For example, if I remove _build/dev/lib/gleam_stdlib and run mix compile, I get:
==> gleam_stdlib
Generated gleam_stdlib app
** (Mix) Unknown dependency gleam_stdlib for environment dev
If I rerun mix compile, the dependency is then detected by Mix.
We can, of course, do mix deps.compile then mix compile. I’ve never needed to compile twice when working in Elixir though.
