Nix Language Antipatterns Skill

MUST CHECK after every .nix file edit

1. No top-level `with` statements

nix

# BAD
with lib;
with pkgs;

# GOOD
let inherit (lib) mkIf mkDefault mkOption types; in

Why: Breaks static analysis, obscures name origins, causes shadowing bugs.

2. No `rec` — use `let ... in`

nix

# BAD
rec { x = 1; y = x + 1; }

# GOOD
let x = 1; y = x + 1; in { inherit x y; }

Why: rec risks infinite recursion when names are shadowed.

3. No lookup paths `<...>`

nix

# BAD
import <nixpkgs> {}

# GOOD (flake)
inputs.nixpkgs.legacyPackages.${system}

Why: Depends on $NIX_PATH, breaks reproducibility.

4. No unquoted URLs

nix

# BAD
url = http://example.com/file.tar.gz;

# GOOD
url = "https://example.com/file.tar.gz";

5. Use `lib.recursiveUpdate` for nested merges

nix

# DANGEROUS — nested keys lost
config = old // { nested = { newKey = 1; }; };

# SAFE
config = lib.recursiveUpdate old { nested = { newKey = 1; }; };

6. Always include `lib` when using lib functions

nix

# BAD — runtime error
{ config, pkgs, ... }:
{ services.foo.enable = mkDefault true; }

# GOOD
{ config, lib, pkgs, ... }:
{ services.foo.enable = lib.mkDefault true; }

7. Use `builtins.path` for derivation sources

nix

# BAD — unreproducible
src = ./.;

# GOOD
src = builtins.path { name = "my-project"; path = ./.; };

Detection Commands

bash

grep -rHn "^with " /etc/nixos --include="*.nix"
grep -rHn "rec {" /etc/nixos --include="*.nix"
grep -rHn "<nixpkgs>" /etc/nixos --include="*.nix"
grep -rHn "= http://" /etc/nixos --include="*.nix"

Nix Language Antipatterns Skill

MUST CHECK after every .nix file edit

1. No top-level with statements

2. No rec — use let ... in

3. No lookup paths <...>