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deps_nix

A Mix task that converts Mix dependencies to Nix derivations. Inspired by mix2nix.

While mix2nix is a function of a mix.lock, this project instead uses Mix's internals to allow you to choose packages from certain environments. It also supports git dependencies.

Why?

You want this if you plan to release your Elixir project using nixpkgs' mixRelease, or have other uses for wrapping each of your Mix dependencies in derivations.

Using separate Nix derivations for each dependency:

  • Avoids downloading and compiling all of your dependencies for each release, which is what happens when you use a Fixed-Output Derivation (mixFodDeps in mixRelease).
  • Lets you cache compiled dependencies and reuse them when they don't change, making your release faster. This is especially important when your dependencies take a while to compile.
  • Gives you loads of geek points.

Installation

def deps do
  [
    {:deps_nix, "~> 0.0", only: :dev}
  ]
end

Optional: add aliases for deps.get and deps.update. This helps to keep your Nix dependencies in sync with what's declared in mix.exs:

def project do
  [
    ...
    aliases: [
      "deps.get": ["deps.get", "deps.nix"],
      "deps.update": ["deps.update", "deps.nix"]
    ]
  ]
end

Docs can be found at https://hexdocs.pm/deps_nix.

Usage

mix deps.nix

By default, this will generate a deps.nix file in the current directory, using only the :prod dependencies for your project.

See mix help deps.nix for more options.

Rustler-precompiled

deps_nix will work with some dependencies that use rustler-precompiled. See the default overrides for a list of them. For better compatibility with the Nix ecosystem, deps_nix eschews precompiled artifacts altogether: it compiles the dependency from scratch to let your project leverage Nix caching. This will mean that you'll have to wait for an initial compilation in your CI system.

You must disable Rustler's compilation in your release environment as well. This is done at compile time in the Nix derivation, but your runtime configuration must match this.

e.g. for explorer, in config/prod.exs:

config :explorer, Explorer.PolarsBackend.Native, skip_compilation?: true

The advantage of this approach is that you don't have to update hashes when dependencies change. The disadvantage is that you must wait for the compilation for each Nix machine that isn't configured to use a cache, and for each new version.

If you'd prefer to use the precompiled Rust libraries, this is still possible by using a Nix fetcher and providing the library to the dependency inside a directory that is set as RUSTLER_PRECOMPILED_GLOBAL_CACHE_PATH in a custom override.