Should flake-parts.modules set the `key` attribute?

[jfly]

In NixOS/nixpkgs#215496 (comment), @roberth said:

If you use flake-parts, this module sets a file and key for you.

But I think that is incorrect? See this TODO:

flake-parts/extras/modules.nix

Line 17 in 644e0fc

# TODO: set key?

.

[jfly]

Similar question about nixosModules, which appears to only set _file right now:

flake-parts/modules/nixosModules.nix

Line 18 in 644e0fc

apply = mapAttrs (k: v: { _file = "${toString moduleLocation}#nixosModules.${k}"; imports = [ v ]; });

[jfly]

@roberth, friendly ping here

[kalyanoliveira]

Should flake-parts set the key attribute?

I lk think that it should. When you write modules with the
flake.modules.class.name option, you can easily run into scenarios where
module deduplication does not work because key is not set.

This is a classic diamond-dependency scenario that was discussed
here by maintainer @roberth: a
module A depends on modules B and C, and the latter two each depend on some
module D.

What you would usually do in this scenario is make module A import both modules
B and C, and make modules B and C each import module D. Of course, this means
that we are importing module D twice, which is usually fine because it would
have a key. However, if it doesn't, you can easily run into issues (e.g.
setting the value of an option twice because it was set by module D).

Of course, the easy fix to this is to forgo any intentions of making your
configuration idiomatic in that way, and resort to importing module D just once
in module A (and never anywhere else, even in modules B and C).

But that sucks.

It especially sucks when we are hinted at the fact that we can do better:

# Excerpt of https://github.com/hercules-ci/flake-parts/blob/main/extras/modules.nix
      { ... }:
      {
        # TODO: set key?
        _class = class;
        _file = "${toString moduleLocation}#modules.${escapeNixIdentifier class}.${escapeNixIdentifier moduleName}";
        imports = [ module ];
      };

I've written a one-line patch in
"https://github.com/kalyanoliveira/flake-parts/tree/set-default-value-for-key"
for this:

[horyzon@laptop-nixos:~/flake-parts]$ git diff main
diff --git a/extras/modules.nix b/extras/modules.nix
index 7902a57..e01a8f6 100644
--- a/extras/modules.nix
+++ b/extras/modules.nix
@@ -22,6 +22,7 @@ let
       { ... }:
       {
         # TODO: set key?
+        key = "${class}.${escapeNixIdentifier moduleName}";
         _class = class;
         _file = "${toString moduleLocation}#modules.${escapeNixIdentifier class}.${escapeNixIdentifier moduleName}";
         imports = [ module ];

Feel free to try it with these changes to your flake.nix:

# Excerpt of `flake.nix`
{

  inputs = {

    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";

    flake-parts = {
      url = "github:kalyanoliveira/flake-parts/set-default-value-for-key";
      inputs.nixpkgs-lib.follows = "nixpkgs";
    };

The only reason I haven't created a PR is because I am not a maintainer of this
project, nor do I know the full extent of this (seemingly small) change.

I mean, the
CONTRIBUTING.md
makes it clear that I have to first look into similar issues for a solution to a
problem, THEN open issues if needed, and ONLY THEN make a PR.

But, uh... who knows? Maybe this one-line change is not that important, and can
just be merged.