Forbidden Patterns

These patterns are explicitly prohibited in Weyl Standard Nix.

with Statements

FORBIDDEN (except in short list contexts).

# NON-CONFORMANT
with lib;
with pkgs;
{
  options.foo = mkOption { type = types.str; };
}

# CONFORMANT
let
  inherit (lib) mkOption types;
in
{
  options.foo = mkOption { type = types.str; };
}

Rationale

• Obscures where names come from
• Breaks tooling (no go-to-definition, no accurate autocomplete)
• Creates shadowing hazards when lib adds new attributes
• Makes code review require mental scope tracking

Exception

Short lists where the source is obvious:

environment.systemPackages = with pkgs; [
  vim
  git
  curl
];

rec in Derivations

FORBIDDEN.

# NON-CONFORMANT
stdenv.mkDerivation rec {
  pname = "my-tool";
  version = "1.0.0";
  src = fetchFromGitHub {
    rev = "v${version}";
  };
}

# CONFORMANT
stdenv.mkDerivation (finalAttrs: {
  pname = "my-tool";
  version = "1.0.0";
  src = fetchFromGitHub {
    rev = "v${finalAttrs.version}";
  };
})

Rationale

With rec, overrideAttrs doesn’t work correctly. The version reference is baked in at definition time, not resolved after overrides.

if/then/else in Module Config

FORBIDDEN.

# NON-CONFORMANT
config = if cfg.enable then {
  services.foo.enable = true;
} else {};

# CONFORMANT
config = mkIf cfg.enable {
  services.foo.enable = true;
};

Rationale

Eager evaluation causes infinite recursion when modules reference each other’s options.

Import From Derivation (IFD)

FORBIDDEN.

# NON-CONFORMANT
let
  generated = pkgs.runCommand "gen.nix" {} ''echo "{ x = 1; }" > $out'';
in
import generated

Rationale

IFD forces builds during evaluation. This breaks:

• Flake evaluation without network access
• Lazy evaluation
• Caching
• Error messages

default.nix in packages/

FORBIDDEN (except in nix/overlays/).

# NON-CONFORMANT
nix/packages/
├── my-tool/
│   └── default.nix

# CONFORMANT
nix/packages/
├── my-tool.nix

Rationale

The filename becomes the attribute name. default.nix discards this information.

Configuring nixpkgs Outside weyl-std

FORBIDDEN.

# NON-CONFORMANT
perSystem = { system, ... }: {
  _module.args.pkgs = import inputs.nixpkgs {
    inherit system;
    config.cudaSupport = true;  # NO
  };
};

# CONFORMANT
perSystem = { system, ... }: {
  _module.args.pkgs = inputs.weyl-std.nixpkgs.${system};
};

Rationale

Central nixpkgs configuration ensures CUDA versions, capabilities, and overlays are consistent across the entire stack. Per-flake configuration creates version mismatches.

Overriding nixpkgs Config in NixOS

FORBIDDEN.

# NON-CONFORMANT
{ ... }:
{
  nixpkgs.config.cudaSupport = false;  # NO
}

Rationale

NixOS configs receive their pkgs from the flake via nixpkgs.pkgs. Overriding config at the NixOS level breaks consistency.

Inline Foreign Code Over 10 Lines

FORBIDDEN.

# NON-CONFORMANT
buildPhase = ''
  #!/bin/bash
  # ... 50 lines of bash ...
'';

# CONFORMANT
buildPhase = builtins.readFile ./build.sh;

Rationale

Inline strings:

• Have no syntax highlighting
• Cannot be linted or tested independently
• Encourage growing complexity without factoring

Summary Table