Reproducible Development Environment for Elixir with Nix

Recently, I started learning about Elixir and at first, I was somewhat unsure about how to setup my development environment. After reading the various recommendations from the Elixir community, my questions remained. This is when I asked on Twitter1 to gather more thoughts on the matter. It led me to research further how I could setup a reproducible development environment for Elixir. I will share with you how I achieved this with Nix.

What Is Nix? Link to heading

Did a colleague or a friend ever tell you “But it works on my computer!” ? No…? Well consider yourself lucky then! For the unlucky ones, you know how this sucks… you simply want to run some application on your computer and it doesn’t work. It’s perhaps a missing undocumented dependency or some dependency version mismatch.

You don’t have to go through this. Do not rely on system packages, this is asking for trouble. Leave your runtime versions manager like asdf behind. They aren’t much better. Those approaches aren’t reliable and reproducible. This is where Nix comes in. It’s a purely functional and cross-platform package manager. It can be installed in various ways, this will depend on your operating system.

Once you have Nix on your computer, you can create reproducible development environment with nix-shell.

The nix-shell Command Link to heading

One of the commands provided by Nix is nix-shell.

Whenever executing nix-shell --pure in your terminal, the command looks for the file shell.nix in the current working directory. This file contains Nix code to create a shell or if you prefer, your development environment. As for the --pure flag, it ensures your system configuration and its packages do not interfere with the shell you created just now. Everything else stays the same in this shell, you can freely navigate your filesystem, run a Phoenix application or start IEx, the world is yours!

Show Me the Money Link to heading

Let’s have a look at a real-world shell.nix example. The comments will walk you through what happens. You can horizontally scroll in the code block below if it’s not fully visible on your device.

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# To ensure this nix-shell is reproducible, we pin the packages index
# to a commit SHA taken from a channel on https://status.nixos.org/.
# This commit is from the nixos-22.05 channel, the current stable channel.
with (import (fetchTarball https://github.com/NixOS/nixpkgs/archive/8de8b98839d1.tar.gz) {});

let
  # Set the version of Erlang/OTP you want to use, this is Erlang/OTP 25.
  # `erlang` is a variable referring to a Nix package
  erlang = beam.packages.erlangR25;
in
  mkShell {
    # Install Nix packages you want to use in your development environment
    buildInputs = [
      # Elixir with Erlang/OTP specified in the `erlang` variable defined above.
      # Relying on the package `elixir` alone isn't enough, as the version
      # of Erlang cannot be specified.
      erlang.elixir_1_14
      # The package manager for Erlang
      erlang.hex
      # The build tool for Erlang.
      # If changing this package, do not forget to replicate the change
      # below in `shellHook`.
      erlang.rebar3
      # For the Live Reloading feature in Phoenix
      inotify-tools
      # Locales
      glibcLocales
    ];

    # Prepare your development environment here
    shellHook = ''
      # Set LANG for locales
      # Otherwise it is unset when running "nix-shell --pure"
      export LANG="C.UTF-8"

      # Keep Mix and Hex data in the project
      # Be sure to ignore those directories in your `.gitignore`
      export MIX_HOME="$PWD/.nix-mix"
      export HEX_HOME="$PWD/.nix-hex"
      mkdir -p "$MIX_HOME" "$HEX_HOME"
      # Put executables from Mix and Hex directories in $PATH
      export PATH="$MIX_HOME/bin:$MIX_HOME/escripts:$HEX_HOME/bin:$PATH"

      # Set development environment for Mix
      export MIX_ENV=dev

      # Persist history of the IEx (Elixir) and erl (Erlang) shells
      export ERL_AFLAGS="-kernel shell_history enabled"

      # Set the path to the rebar3 package from Nix
      mix local.rebar --if-missing rebar3 ${erlang.rebar3}/bin/rebar3
    '';

    # Without this, there are warnings about LANG, LC_ALL and locales.
    # Many tests fail due those warnings showing up in test outputs too...
    # This is from https://gist.github.com/aabs/fba5cd1a8038fb84a46909250d34a5c1
    LOCALE_ARCHIVE = "${glibcLocales}/lib/locale/locale-archive";
  }

Give this nix-shell a try by copying it in one of your Elixir projects, then running nix-shell --pure. Adapt it to your needs, perhaps you want to change the Elixir or Erlang version. There is a lot of information on the Nix packages available for Elixir and Erlang on GitHub. Regarding other Nix packages, you can find them with the Nix Search.

Will You Reproduce This? Link to heading

Let me know how it went for you by sending me a message on the Contact Me page. Constructive feedback is always welcomed!