Nix flakes with renv

I’ve been using Nix-based development environments for my projects for the last two years to declaratively install dependencies and define environment variables. Installing packages, from both nixpkgs and source, is reproducible. Making modifications like compiling something with an optional dependency or applying patches is easy. With direnv, you get a pseudo-container every time you enter the project directory with all software and variables available. Recently I’ve started using Nix flakes to set up and maintain the tools I need, all off one nixpkgs commit with versions locked by flake.lock. Non-nix systems don’t have such an easy and declarative way to lock dependencies, especially across languages or toolchains.

For this site, I use a flake to develop locally and an ubuntu runner on Github actions for deployment. I wanted to use the same R and package versions both locally and on the action runner to ensure that what works on my system works on Github. renv is the standard way to produce lockfiles in R but, since it’s normal operation requires initializing a project and installing the packages outside nix, I didn’t think I could use it. I found nix-based actions exist, but they are slower than installing R and the packages directly because they need to set up nix first.

Going back to renv’s documentation, I found lockfile_create(), a function for programmatic renv operations. Using .libPaths(), it create lockfiles from installed packages. Since R packages installed with nix are conveniently added to .libPaths() running lockfile_create() in a flake shell produces the required lockfile. However, since it doesn’t produce any output or write files, it took me a while to figure out that lockfile_write() takes lockfile_create() to produce renv.lock.

Adding this to a shell hook writes the renv.lock file on flake activation. To avoid running this every time I activate the environment, the shell hook only runs renv commands if renv.lock is older than either flake.nix (if adding or removing packages) or flake.lock (for updates). Every update of the flake will update the package versions in both the nix shell environment and the renv lockfile, controlling both the nix and the renv environments with just the flake.

...
devShells.default = pkgs.mkShell {
  buildInputs = [ inputs ];
  shellHook = ''
      if [[ flake.nix -nt renv.lock ]] || [[ flake.lock -nt renv.lock ]]; then
        R -q -e "renv::lockfile_write(renv::lockfile_create())"
      fi
  '';
}
...

Until Github gets a NixOS runner, this should keep my R-nix dependencies locked and declaratively managed.