Nix Guidelines

Guidelines for idiomatic Nix code with flakes and modules.

When to Use

• Writing flake.nix files
• Creating NixOS/Home Manager modules
• Writing Nix packages
• Reviewing Nix code

Critical Rule: No with Statements

NEVER use with statements. They:

• Break static analysis tools (nixd, nil)
• Create scope ambiguity
• Cripple IDE features

# ❌ WRONG
environment.systemPackages = with pkgs; [ git vim ];
meta = with lib; { license = licenses.mit; };

# ✅ CORRECT
environment.systemPackages = [ pkgs.git pkgs.vim ];
meta = { license = lib.licenses.mit; };

Core Principles

1. Explicit over Implicit - Always use explicit attribute paths
2. Declarative Design - Think in systems, not scripts
3. Pure Functions - Leverage Nix's functional nature
4. nixfmt-rfc-style - Standard formatter

Flake Structure

{
  description = "My project";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
    home-manager = {
      url = "github:nix-community/home-manager";
      inputs.nixpkgs.follows = "nixpkgs";  # Deduplicate
    };
  };

  outputs = { self, nixpkgs, home-manager, ... }: {
    nixosConfigurations.myhost = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      modules = [
        ./configuration.nix
        home-manager.nixosModules.home-manager
      ];
    };
  };
}

Module Structure

{
  lib,
  config,
  pkgs,
  ...
}:
let
  cfg = config.myNamespace.myModule;
in
{
  options.myNamespace.myModule = {
    enable = lib.mkEnableOption "my module";

    package = lib.mkOption {
      type = lib.types.package;
      default = pkgs.myPackage;
      description = "The package to use";
    };

    settings = lib.mkOption {
      type = lib.types.attrsOf lib.types.str;
      default = { };
      description = "Configuration settings";
    };
  };

  config = lib.mkIf cfg.enable {
    environment.systemPackages = [ cfg.package ];

    assertions = [
      {
        assertion = cfg.settings != { };
        message = "myModule.settings cannot be empty when enabled";
      }
    ];
  };
}

Best Practices Checklist

• No with statements
• Explicit destructuring in function signatures
• nixfmt-rfc-style formatting
• Options properly namespaced
• Input follows optimized (for flakes)
• Prefer let-in over rec
• lib.mkIf for conditional config
• Assertions for validation
• Comments for complex logic

Common Patterns

Conditional Configuration

config = lib.mkMerge [
  (lib.mkIf cfg.enable {
    services.myService.enable = true;
  })

  (lib.mkIf (cfg.enable && cfg.monitoring.enable) {
    services.prometheus.exporters.myService.enable = true;
  })

  {
    # Always applied with lower priority
    services.myService.port = lib.mkDefault 8080;
  }
];

Package Definition

{ lib, stdenv, fetchurl, openssl }:
stdenv.mkDerivation {
  pname = "mypackage";
  version = "1.0";

  src = fetchurl {
    url = "https://example.com/source.tar.gz";
    sha256 = lib.fakeSha256;
  };

  buildInputs = [ openssl ];

  meta = {
    description = "My package";
    license = lib.licenses.mit;
  };
}

Script Wrapper

pkgs.writeShellApplication {
  name = "my-script";
  runtimeInputs = [ pkgs.jq pkgs.curl ];
  text = ''
    curl -s https://api.example.com | jq .
  '';
}

Anti-Patterns to Avoid

Don't Use rec

# ❌ Avoid rec
rec {
  a = 1;
  b = a + 1;
}

# ✅ Use let-in
let
  a = 1;
in {
  inherit a;
  b = a + 1;
}

Explicit Destructuring

# ❌ Wrong
args: with args; stdenv.mkDerivation { ... }

# ✅ Correct
{ stdenv, fetchurl, lib }: stdenv.mkDerivation { ... }

Performance Tips

• Use inputs.nixpkgs.follows to deduplicate inputs
• Split outputs for granular dependencies
• Profile with NIX_SHOW_STATS=1 nix build
• Consider builtins.readFile for large strings

Validation Commands

# Check flake
nix flake check

# Format code
nixfmt-rfc-style .

# Evaluate without building
nix eval .#packages.x86_64-linux.default

# Show flake outputs
nix flake show

Module Option Types

lib.types.bool           # Boolean
lib.types.str            # String
lib.types.int            # Integer
lib.types.path           # Path
lib.types.package        # Nix package
lib.types.listOf type    # List of type
lib.types.attrsOf type   # Attribute set of type
lib.types.nullOr type    # Type or null
lib.types.enum ["a" "b"] # Enumeration
lib.types.submodule { }  # Nested options

Remember

Minor verbosity from explicit patterns is a feature - it makes code self-documenting and machine-readable.