Using Nix on Flakes on OSX

I use Nix Flakes on OSX to setup my development environment. I’ve not seen anyone else document this approach. Maybe you will find it useful.

What’s in a development environment?

By “development environment,” I mean three things:

1. Adding and mutating shell environment variables (e.g. $EDITOR)
2. Installing command line applications (e.g. /usr/bin/nvim)
3. Adding config files (e.g. $HOME/.config/nvim/init.lua)

Unfortunately, 2 and 3 are “impure” according to Nix because they require access to mutable paths. But there are simple workarounds:

• Instead of installing binaries to /usr/bin/, I can install it to the store and add it to the $PATH. For example, instead of installing /usr/bin/nvim, I would install /nix/store/abc123-nvim/bin/nvim.
• Instead of adding config files, I can wrap a binary to point to the store. For example, instead of generating $HOME/.config/nvim/init.lua, I’d:
  • Generate /nix/store/abc123-init.lua/init.lua.
  • Generate /nix/store/abc123-nvim-wrapped/bin/nvim, which just does nvim -u /nix/store/abc123-init.lua/init.lua $@. The -u flag lets you pass a path to the config, and $@ forwards arguments.
  • Add /nix/store/abc123-nvim-wrapped/bin to $PATH.

So if I can mutate environment variables— including $PATH— then I can do everything!

But first, I need to explain Flakes a little bit.

A Nix Flakes primer

Sorry, I feel like every Nix article that touches on Flakes has to explain Flakes from scratch. I’ll try and stick with what’s relevant to what I’m doing. If you’re interested in a deep dive, I recommend Xe Iaso’s Nix Flakes: an Introduction.

Flakes, at their core, are a configuration format for the Nix toolchain. This format accepts inputs, which are dependencies that live in the Nix store, and produces outputs, which are read by various tools. For example, the nix CLI tool’s nix build subcommand builds the packages.default output for the flake.

See? That wasn’t so bad, was it? If this still seems a bit abstract, read on for an example.

Note: In versions of nix prior to 2.7, packages.default was known as defaultPackage. If you care about compatibility with old versions, you may want to alias it to point to packages.default.

Designing a development environment

Using Flakes, I need to mutate environment variables. To do this, I’ll use a little-known command called nix print-dev-env:

nix print-dev-env - print shell code that can be sourced by bash to reproduce the build environment of a derivation

If you run nix print-dev-env, it will build the packages.default output of your current flake.nix.

This approach has two steps:

1. Make a packages.default output that mutates shell environment variables as desired. For example, it should add /nix/store/abc123-nvim-wrapped/bin to the $PATH.
2. Source the output of nix print-dev-env in my development shell.

Putting the pieces together

To construct the packages.default output, you can use pkgs.mkShell:

# In flake.nix
let
  neovim-with-config = neovim.override {
    customRC = ''
      lua << EOF
        -- init.lua goes here
      EOF
    '';
  };
in 
  {
    outputs = flake-utils.lib.eachDefaultSystem (_system: {
        packages.default = pkgs.mkShell {
          packages = with pkgs; [
            neovim-with-config
            # anything else
          ];

          shellHook = ''
            # Optionally, inject other stuff into your shell
            # environment.
          '';
        };
      });
  }

Since the shell requires neovim-with-config, its ‘build environment’ will append /nix/store/abc123-neovim-with-config/bin/ to $PATH. That’s exactly what we want.

And finally, source the output of nix print-dev-env:

# `print-dev-env` assumes bash. It mutates env variables such as
# `LINENO` that # are immutable in zsh, so I need to exclude them.
# This is annoying, but in practice works fine.
$ nix print-dev-env \
  | grep -v LINENO \
  | grep -v EPOCHSECONDS \
  | grep -v EPOCHREALTIME \
  > $HOME/development-configuration.zsh 

$ echo 'source $HOME/development-configuration.zsh' >> $HOME/.zshrc

If you inspect development-configuration.zsh, you’ll see a giant RC file that includes:

PATH='...:/nix/store/abc123-neovim-with-config/bin:...'

Indeed, running nvim works as expected. We have set up a development environment using Nix Flakes!

Full dotfiles

If you want to see my full dotfiles, it lives on sourcehut. Here’s a link to where I define packages.default and here’s where I run print-dev-env