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:
- Adding and mutating shell environment variables (e.g.
$EDITOR)
- Installing command line applications (e.g.
/usr/bin/nvim)
- 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:
- 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.
- 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
