If we want to deprecate this, there needs to be some deprecation warning for users. We can't just flip the switch and possibly break every second person's config. A good way to do that is to change the loaOf type to issue a warning when the list case is used, something like

Warning: You're assigning the list [ { name = "foo"; bar = <CODE>; ... } ... ] to
<option name>, which won't be supported in the future, use an attribute set like
{ foo = { bar = <CODE>; ... }; ... } instead. See
https://github.com/NixOS/nixpkgs/pull/63103 for more info.

Ok, I removed the attempts to replace loaOf with attrsOf and added a deprecation warning.
I don't know how to do something like that though

This will break the programs.ssh.matchBlocks option in Home Manager. That option allows use of list when the order matters and an attribute set as a convenience for users whose setup does not require a certain order.

I believe I originally had it as a listOf but got complaints that it was cumbersome for simple setups.

I don't particularly mind switching back to listOf since I, in principle, agree about the rationale for deprecating loaOf but thought I'd make a note of this use-case.

@rycee I don't know how home-manager works: are you somehow bound to nixos option types? Couldn't you simply define an equivalent loaOf for home-manager?

Thank you everyone, especially @rnhmjoj for your hard work!! 🎉

If docbook inconveniences you, I can translate your markups if that would be convenient for you.

No, it's not a problem: I have written documentation for NixOS before.

If you could make the note similar to how our warning works in types.nix, explains how and why loaOf is deprecated and how code should be changed to not use it. And what phase of deprecation we're in, I believe it's not removed and code that uses it should work as it has.

Ok, I'll do something like it. Thank you.

I think there should be a pinned issue for this, since it will throw warnings and errors for many people and the efforts to fix those should be coordinated somehow.

I think there should be a pinned issue for this, since it will throw warnings and errors for many people and the efforts to fix those should be coordinated somehow.

Sounds good to me, will open.

Made a little pop up issue #77189. Should have relevant strings within the body to pop up in relevancy searches.

The deprecation should probably be documented in nixos/doc/manual/development/option-types.xml.

I just want to say I got the most wonderful error message from a Nix evaluation today:

trace: warning: In file [...]/packet-spot-buildkite-agent/network.nix
a list is being assigned to the option config.programs.ssh.knownHosts.
This will soon be an error as type loaOf is deprecated.
See https://git.io/fj2zm for more information.
Do
  programs.ssh.knownHosts =
    { flexo.gsc.io = {...}; }
instead of
  programs.ssh.knownHosts =
    [ { hostNames = [ "flexo.gsc.io" "r13y.com" "147.75.105.137" ]; ...} ]

This error message:

✔️ is specific: "a list is being assigned to the option config.programs.ssh.knownHosts"

✔️ tells me where to go for more information: https://git.io/fj2zm

✔️ tells me where the violation is: .../packet-spot-buildkite-agent/network.nix

✔️ SHOWS me the code which is wrong:

      programs.ssh.knownHosts =
        [ { hostNames = [ "flexo.gsc.io" "r13y.com" "147.75.105.137" ]; ...} ]

✔️ AND tells me the solution:

      programs.ssh.knownHosts =
        { flexo.gsc.io = {...}; }

This is incredible, and ticks the main three boxes of good errors:

1. What is the problem?
2. Why did it happen?
3. How do I fix it?

Way to go everyone, this is a massive step forward for our community and I am completely floored and delighted. Thank you.