Switching from nil to nixd Language Server for Better Nix Development Experience

I was using nil language server for a while, and it was working fine.

However, after a great video from vimjoyer’s new video nixd seems to have more features, as it is in very active development state right now.

Among the killer features for me was options auto-completion.

While vimjoyer did a great job, setting up nixd auto-completion wasn’t a straightforward process for me. It this blog post, I’ll share my experience in setting it up.

Installing nixd

If you’re using flakes, installing is as easy as following code and rebuinding your system:

{ pkgs, ... }: {
  environment.systemPackages = with pkgs; [
    nixd
  ];
}

For other options, check out nixd installation guide

Figure out correct paths and system name in Nix repl

nix-repl> builtins.getFlake "/Users/sab/dotfiles/nix"
{
  _type = "flake";
  checks = { ... };
  darwinConfigurations = { ... };
  darwinModules = { ... };
  devShells = { ... };
  homeConfigurations = { ... };
  homeModules = { ... };
  ...
}

I'm using flakes, and my Nix flake is not 'pure', i.e. not in the git root, following
configuration worked:

```nix
nix-repl> import (builtins.getFlake(toString ./.)).inputs.nixpkgs { }
{
  AAAAAASomeThingsFailToEvaluate = «error: Please be informed that this pseudo-package is not the only part
of Nixpkgs that fails to evaluate. You should not evaluate
entire Nixpkgs without some special measures to handle failing
packages, like using pkgs/top-level/release-attrpaths-superset.nix.
»;
...

Finally, figure out correct system name and username for nixosConfigurations, darwinConfigurations(if you’re using one) and homeConfigurations:

sab@mbp16 ~> cd ~/dotfiles/nix/

# Starting nix repl
sab@mbp16 ~/d/nix (dev|✔)> nix repl
Nix 2.24.8
Type :? for help.
Loading installable 'git+file:///Users/sab/dotfiles?dir=nix#'...
Added 15 variables.

# Loading flake
nix-repl> :lf .
Added 27 variables.

# Checking outputs for correct name

nix-repl> outputs.darwinConfigurations.mbp16.options
{
  _module = { ... };
  assertions = { ... };
  ...
}

nix-repl> outputs.homeConfigurations."sab@nz".options
  _module = { ... };
  accounts = { ... };
  assertions = { ... };
  custom = { ... };
  ...
}

Use auto-completion to figure out required parameters.

Configuring nixd in Neovim

I’m using following configuration to set up to enable and configure language server via nvim-lspconfig:

require("lspconfig").nixd.setup {
    cmd = { "nixd" },
    settings = {
        nixd = {
            nixpkgs = {
                -- For flake.
                -- This expression will be interpreted as "nixpkgs" toplevel
                -- Nixd provides package, lib completion/information from it.
                -- Resource Usage: Entries are lazily evaluated, entire nixpkgs takes 200~300MB for just "names".
                -- Package documentation, versions, are evaluated by-need.
                expr = "import (builtins.getFlake(toString ./.)).inputs.nixpkgs { }",
            },
            formatting = {
                command = { "alejandra" }, -- or nixfmt or nixpkgs-fmt
            },
            options = {
                nixos = {
                    expr = "let flake = builtins.getFlake(toString ./.); in flake.nixosConfigurations.nz.options",
                },
                home_manager = {
                    expr = 'let flake = builtins.getFlake(toString ./.); in flake.homeConfigurations."sab@mbp16".options',
                },
                darwin = {
                    expr = "let flake = builtins.getFlake(toString ./.); in flake.darwinConfigurations.mbp16.options",
                },
            },
        },
    },
},

Check out for configuration-overview for more details.

Using this config, I’m:

• using alejandra for formatting
• using options from “nz” host for nixos options completion
• using options from “mbp16” host for darwin options completion
• using options from “sab@mbp16” user/host for homeManager options completion

Example completion: