I wanted to contribute a bit to devenv. I consider myself a novice nix user. Anything beyond a couple of recipes I know fall out of my comfort zone. Yet the goals of devenv plus how supportive the maintainers seems to be were enough to do an attempt.

Build, share, and run your local development environments with a single command. Without containers.
https://devenv.sh/

I want to cover how you could contribute to add support for the language, or improve an existing one.

There are many options of course, but a workflow that worked for my was

1. Install Nix+Cachix+Devenv as explained in https://devenv.sh/getting-started/
2. (Optional) Install direnv, most likely you already use this :-)
3. Fork and Clone https://github.com/cachix/devenv

The devenv repo defines both:

• a cli tool that will set up your environment
• a repository of devenv modules

The modules are a description of which nix packages should be downloaded and how they should be setup when devenv options are declared in the devenv.nix file of your project.

We are interested in the second aspect: the modules repository.

For contributing to those we actually don't need to build a new version of the cli (as long as we are not changing some core functionality of course).

When you create a new project powered by devenv you will have at least:

• a devenv.nix file where you setup what languages/packages you want
• a devenv.yaml file that defines some pinning and source of truth to be used.

In particular devenv.yaml there is a input.devenv.url key that points to the modules repository. By default points to github:cachix/devenv?dir=src/modules (defined in flake.nix#L7), but we can override it! To a local path!

That's what happening in examples/supported-languages. The input.devenv.url key points to the working copy src/modules. We can use this example as a playground or create a new empty project devenv.

If you choose to work on a new empty project, run devenv init in an empty folder and edit devenv.yaml:

inputs:
  nixpkgs:
    url: github:NixOS/nixpkgs/nixpkgs-unstable
  devenv:
    url: path:/absolute/path/to/src/modules

After running direnv allow, or devsenv shell if you don't use direnv you should get the packages and languages defined in devenv.nix.

I added languages.elm.enable = true; to devenv.nix to get elm compiler and it works!

% devenv shell
Building shell ...
Entering shell ...

elm --version
0.19.1
elm2nix --version
/nix/store/3szqhljvvkynvfi5gymfnxsiljpd6hmg-elm2nix-0.3.0/bin/elm2nix
hello from devenv
git version 2.38.1

But this does not come with elm-format. We want the formatter, always. That's the contribution we want to make. We could make the formatter optional, but it's so widely used in the elm community that I think we can install it always. What could be nice is a pre-commit hook integration to run it. That could be configurable, but let's stick to get the elm-format installed for now.

The package already exists in nixpkgs as elmPackages.elm-format so this should be easy.

Go to src/modules/languages/elm.nix, and add elm-format to the package dependency. Additionally add some advertisement you think pertinent. Unfortunately elm-format does not returns it's version with --version, so we will just show that elm-format is indeed available.

--- a/src/modules/languages/elm.nix
+++ b/src/modules/languages/elm.nix
@@ -11,6 +11,7 @@ in
   config = lib.mkIf cfg.enable {
     packages = with pkgs; [
       elmPackages.elm
+      elmPackages.elm-format
       elm2nix
     ];

@@ -18,6 +19,8 @@ in
       echo elm --version
       elm --version

+      which elm-format
+
       echo elm2nix --version
       which elm2nix
     '';

Now, how we check if this changes are good?
If you followed along you should be on a devenv shell (via direnv or explicit devenv shell command).

Update the shell by either running direnv allow again, or exiting the devenv shell (ctrl+D) and entering again.

And that's it. You should have elm-format available.

Submit a PR!

Caveats for aarch64-darwin

• I needed to comment out pkgs.python3Packages.cairocffi in devenv.nix#L7 for devenv source to work on aarch64-darwin, thanks @domenkozar!