Support Build Time Config / Application Environment Config

[JonasGruenwald]

Gleam supports compiling Elixir dependencies and Elixir files.
In Mix projects, configuration can be applied at build time, through config/config.exs, which Mix reads into the application environment before compiling the project, see: https://hexdocs.pm/mix/Mix.html#module-build-time-configuration.

Obviously this is not 'the Gleam way', in a Gleam project configuration should be applied explicitly by passing it to functions at runtime, however some Elixir packages that one may want to use from a Gleam project must be configured this way.

For example, I am using the elixir package oban in my project, which uses postgrex (through ecto) to communicate with a Postgres DB.

postgrex by default wants to use the Elixir module jason for it's JSON encoding functions, but it can be configured to use the built-in JSON module of OTP27, this configuration however must be applied at build time, probably because it's used in macros, see: https://github.com/elixir-ecto/postgrex?tab=readme-ov-file#json-support

Once you change the value, you have to recompile Postgrex

It would be great if the Gleam compiler allowed configuring Elixir packages this way, so that they can be used more easily from Gleam.

One potential way to do this would be by supporting the same configuration convention as Mix, reading a config file at config/config.exs before building elixir modules.

[inoas]

This would mean 2 step compilation once for config and then another time with the result of whatever config produced?

What about one of those ways to go about it:

• Compile some elixir code via some gleam runner up front so that it is available once gleam compiles some elixir dep? Would that work?
• Wait for elixir 1.19 or 1.20 and embed gleam into an elixir project?
• Wait for gleam's 2-step compliation or macro system if it ever happens?

[JonasGruenwald]

I don't think 2-step compilation would be necessary though I'm not sure what that means, it's .exs, so the Elixir compiler should be able to compile it on the fly, there is a module in the Elixir standard library for doing that:
https://hexdocs.pm/elixir/Config.Reader.html
I'm assuming this could potentially be done in the erlang code gleam uses to compile elixir, before this line:
https://github.com/gleam-lang/gleam/blob/main/compiler-cli/templates/gleam%40%40compile.erl#L140
though I'm not sure.

Compile some elixir code via some gleam runner up front so that it is available once gleam compiles some elixir dep? Would that work?

I'm not sure what you mean by that exactly.

Wait for elixir 1.19 or 1.20 and embed gleam into an elixir project?

Would be possible yeah, I'd prefer a Gleam project but this could make do as well.

[lpil]

Elixir's config DSL is a way to write to OTP application configuration before code is compiled, so we want a way to do the same rather than anything specific to Elixir.

How do Erlang projects do this?

How would different environments work here? In Elixir's Mix there is the concept of dev, test, and prod.

Would this have implications for OTP releases built with Gleam? Or gleam export erlang-shipment.

[JonasGruenwald]

we want a way to do the same

Oh that's really exciting, I didn't know! I thought runtime config is the way in Gleam.
Would this be something that could potentially go in the gleam.toml, similar to how Erlang allows specifying application config under the env key of the application resource file? https://www.erlang.org/doc/apps/kernel/app.html#file-syntax

[lpil]

That only sets it for the current application, which wouldn't be helpful for this use case as you need to set it for the dependency application. I had a look but I couldn't work out how one is supposed to do it in a rebar3 project with Mix deps. Could you see how?

[JonasGruenwald]

I can't see any explicit rebar3 support for that, though I suppose it could be done via overrides?
https://rebar3.org/docs/configuration/configuration/#overrides