lib: init lib.fallback

[hsjobeki]

Reasoning behind this:

• if foo == [ ] then foo else something is verbose to type. I especially dont like to type foo twice since it can be very long in some cases (some.options.path.nested.attrs)

Some quick grep on gitub yielded this where it could be used:

• nixpkgs/nixos/modules/services/web-servers/nginx/default.nix

  Line 317 in 7a4c8e8

  if vhost.listen != [] then vhost.listen

• [SameFile]#L324
• https://github.com/NixOS/nixpkgs/blob/27f79a9daa9ca35b20f25a8effb5e5a75eebc164/lib/options.nix#L327C14-L327C123
• https://github.com/vinceliuice/grub2-themes/blob/f6ab2438e124f60a340a526543e498e5e33b3c53/flake.nix#L20
• https://github.com/eamsden/nixos-urbit/blob/b58ce4f19c3be7d5461ef36d022e5f7753e380b9/overlay.nix#L3

I also found that

lib.fallback a b (a: a) might make sense instead.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• 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/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 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
• Fits CONTRIBUTING.md.

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[infinisil]

Can you link to some example code that would benefit from this?

Not a fan of the weak typing, there's no precedent for "truthy/falsy" in lib, and I'm not missing it, because it can give confusing results.

[hsjobeki]

Can you link to some example code that would benefit from this?

Not a fan of the weak typing, there's no precedent for "truthy/falsy" in lib, and I'm not missing it, because it can give confusing results.

Hm yeah. Maybe it makes more sense to speak about the value beeing 'empty' or 'neutral'. Establishing a better mental model than thruthy/falsey.

I'll also add some links where this could been used to the description.