Compile time configuration of Elixir libraries

[calm swallow]

I have achieved a horrible kludge. I have compile time configured a setting into the Elixir MIME library.

src/config/config.ex

defmodule MyConfig do
  # Path to package's directory
  project_dir = Path.expand("../../../../../packages/mime", __DIR__)
  # Path to Elixir's ebin directory
  ebins_dir = Path.expand("../../../elixir/ebin", __DIR__)
  # Path to custom configuration file
  custom_config = Path.expand("../../priv/config.erl", __DIR__)

  System.cmd(
    System.find_executable("elixir"),
    [
      # I copied most of these from https://github.com/gleam-lang/gleam/blob/abdf034c4b694127402b5340e149006bb414b99e/compiler-core/src/build/project_compiler.rs#L483
      "-pa",
      ebins_dir,
      "-S",
      "mix",
      "compile",
      "--force",
      "--no-deps-check",
      "--no-load-deps",
      "--no-protocol-consolidation",

      # Handy flag: pass custom configuration to compilation
      "--erl-config",
      custom_config
    ],
    env: [{"MIX_ENV", "prod"}],
    cd: project_dir
  )
end

priv/config.erl

{mime, [{types, #{<<"a">> => [<<"b">>]}}]}.

% Corresponds to:
% config :mime, :types, %{"a" => ["b"]}

Now we can test it:

➜  elixir_config_test git:(trunk) ✗ gleam build
Downloading packages
 Downloaded 3 packages in 0.01s
  Compiling gleam_stdlib
  Compiling gleeunit
  Compiling mime
  Compiling elixir_config_test
   Compiled in 2.19s
➜  elixir_config_test git:(trunk) ✗ gleam shell
   Compiled in 0.01s
    Running Erlang shell
Erlang/OTP 27 [erts-15.0] [source] [64-bit] [smp:10:10] [ds:10:10:10] [async-threads:1] [jit]

Eshell V15.0 (press Ctrl+G to abort, type help(). for help)
1> 'Elixir.MIME':compiled_custom_types().
#{<<97>> => [<<98>>]}

Hooray!

Downside: MIME is compiled twice, first by Gleam and then by us.

CC @sick sequoia, thanks for the nerdsnipe. 🙂

This kludge is too good to not be shared, gotta write about it on my blog

[calm swallow]

looking at the code, I guess this part

  # Path to Elixir's ebin directory
  ebins_dir = Path.expand("../../../elixir/ebin", __DIR__)

should actually be

  # Path to all ebin directories
  ebins_dir = Path.expand("../../../*/ebin", __DIR__)

[calm swallow]

I think this --erl-config could be used by Gleam to compile the config into deps directly 🙂

Also a module is not needed in the .ex file, can just put everything on top level. I'll write a blog post with the updated details

[sick sequoia]

good lord

am I understanding right that it's a macro that shells out to the elixir compiler and recompiles the project with mix, including the custom config?

ah not the whole project, just the MIME dependency, as each package has it's own ebins dir I suppose?

[calm swallow]

Yeah you'd have to make it into a function to run for all the dependencies you have

It's not a macro really, just Elixir code. Compiling Elixir = running Elixir