Add a channel: flakeref

[WeetHet]

Is your feature request related to a problem?

Nix infra now supports lockable taballs protocol for nixpkgs tarballs. This means that one can use https://channels.nixos.org/nixpkgs-unstable/nixexprs.tar.xz as opposed to github:nixos/nixpkgs/nixpkgs-unstable to save 15MB on download and get additional benefits in case of nixos- tarballs, such as getting a program-not-found database bundled together with the tarball

It's however more wordy than the github: flakeref

Proposed solution

I propose adding a channel:name flakeref pointing to https://channels.nixos.org/name/nixexprs.tar.xz to shorten this

Alternative solutions

Using a nixos: or similar flakeref or leaving things as is

Additional context

Checklist

• checked latest Nix manual (source)
• checked open feature issues and pull requests for possible duplicates

---

Add 👍 to issues you find important.

[roberth]

Another alternative is to use the global flake registry to provide such aliases.
Some benefits

• Easier and quicker to achieve
• Works for all Nix versions that support versioned tarballs
• The scheme: part remains about the protocol as opposed to intruding on the registry's responsibilities

[WeetHet]

Another alternative is to use the global flake registry to provide such aliases

The issue is naming. Sadly, channel/nixpkgs-<something> is not a valid flake id and just nixpkgs-<something> tends to be overridden by configurations

[WeetHet]

We also have a precedent set that channel: links resolve to something like that, see

$ nix-instantiate channel:nixpkgs-unstable --eval -A hello.outPath
"/nix/store/a9l6gcmcq3mll6d15pc3nfcpbsmd2zqg-hello-2.12.1"

[lf-]

I would not call the umm. as we put it. cursed file syntax to be a good inspiration: https://gerrit.lix.systems/c/lix/+/1190

Part of the problem with trying to extend the flakeref format is that that is effectively extending the contract of libfetchers which then becomes observable as derivation API/ABI changes. I don't know if it is prudent to add more to that contract when it is currently underdocumented and full of problems. Another alternate solution might be doing the aliasing in the infrastructure: https://c.nixos.org/c/nixos-24.05.tar.xz or so.

Secondarily I think it's troublesome to create a mechanism that is only usable for nixpkgs; there are other things that one might want to release other than nixpkgs that also would benefit from short names. This does lead to thinking about the flake registry.

From a Lix perspective I think it would be acceptable to add these as aliases nixos-unstable nixpkgs-unstable, nixos-24.05-small, etc, to the flake registry, for interactive use, and we could in principle vendor the updated flake registry. This does have an unfortunate side effect of requiring flake registry changes for every NixOS release, which I think points to the flake registry being a poor mechanism as well. As ever, the flake registry is inadvisable inside of flake.nix, and the full URLs should be used there.