lib: add forSystems

[MatthewCroughan]

Description of changes

This adds a function lib.forSystems which is similar to forAllSystems from the c-hello flake template

Co-authored-by: Justin Restivo justin@restivo.me

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.05 Release Notes (or backporting 21.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[MatthewCroughan]

@alyssais Currently all of our https://github.com/NixOS/templates contain the following boiler plate:

      # System types to support.
      supportedSystems = [ "x86_64-linux" "x86_64-darwin" "aarch64-linux" "aarch64-darwin" ];

      # Helper function to generate an attrset '{ x86_64-linux = f "x86_64-linux"; ... }'.
      forAllSystems = nixpkgs.lib.genAttrs supportedSystems;

      # Nixpkgs instantiated for supported system types.
      nixpkgsFor = forAllSystems (system: import nixpkgs { inherit system; });

But if we merge this, we would be able to reduce it to this:

    forAllSystems = 
      nixpkgs.lib.forSystems
      [ "x86_64-linux" "x86_64-darwin" "aarch64-linux" "aarch64-darwin" ]
      nixpkgs;

[jtojnar]

As mentioned on Matrix, I do not think it add value either.

In your examples, you can just use nixpkgs.legacyPackages directly. And I would say most of the time, you will actually want to pass extra arguments to nixpkgs, and in that case you would be instantiating Nixpkgs twice (first in legacyPackages just to get path, and then in your function).

Also see #123117, #124866, #155709, #171235. We probably want to stabilize an API first.

[adisbladis]

With the same motivation as #173093 (comment) I'm closing this.
The lib namespace is not a place for experimental features.