Compose with modules — Nix, by design

08.1 — DIVIDE BY CONCERN

Give each file a clear job.

A module may be a plain attribute set or a function receiving arguments such as config, lib, and pkgs. The imports list brings other modules into the same evaluation; it is not a sequence of shell operations.

Move a small set of related options into tools.nix, then import that file from your existing configuration. The paths are relative to the file that contains them.

two module excerpts

1# tools.nix
2{ pkgs, ... }: {
3  environment.systemPackages = [ pkgs.git pkgs.hello ];
4}
5 
6# In configuration.nix
7imports = [
8  ./hardware-configuration.nix
9  ./tools.nix
10];

Next section: Definitions combine by option type.

08.2 — MERGING HAS RULES

Definitions combine by option type.

List-valued options often concatenate. Incompatible definitions for a single-value option usually cause an evaluation error rather than silently choosing whichever file came last.

lib.mkDefault supplies a low-priority default; lib.mkForce overrides ordinary priorities. Use them intentionally. If a conflict is surprising, first look for duplicate definitions instead of immediately forcing a value.

TAKE THIS WITH YOU

Modules merge declarations. Their import order is not an imperative setup script.

Go a little deeperThe Nix module system

Smaller files.
One system.

Give each file a clear job.

Definitions combine by option type.

Build, test, switch

Smaller files.One system.

Give each file a clear job.

Definitions combine by option type.

Build, test, switch

Smaller files.
One system.