Why is settings.x.custom expected to be unique?

[bglgwyng]

I have a haskell-flake module whose output is imported from multiple haksell-flake module, and they form a diamond.

It includes settings.hmatrix-gsl.custom config like,

{
  settings = {
    hmatrix-gsl = ({ super, ... }: {
      custom = _: super.hmatrix-gsl.override {
        gsl = pkgs.symlinkJoin {
          name = pkgs.gsl.name;
          paths = [ pkgs.gsl.out pkgs.gsl.dev ];
          buildInputs = [ pkgs.patchelf ];
          postBuild = ''
            cd $out/lib
            mv libgsl.so .libgsl.so
            patchelf --add-rpath ${pkgs.gsl}/lib \
                     --add-needed libgslcblas.so \
                     --output libgsl.so \
                     .libgsl.so

          '';
        };
      };
    });
  };
}

When the configuration of the module placed at the bottom of the diamond is evaluated, the following error occurs.

error: The option `custom.<function body>' is defined multiple times while it's expected to be unique.

Definition values:
  - In `/nix/store/sqa8r0w39lrzmcbpffzg1zniiy5ir06g-source/flake.nix, via option perSystem, via option perSystem.x86_64-linux.haskellProjects.default.settings.hmatrix-gsl': <derivation hmatrix-gsl-0.19.0.1>
  - In `/nix/store/sqa8r0w39lrzmcbpffzg1zniiy5ir06g-source/flake.nix, via option perSystem, via option perSystem.x86_64-linux.haskellProjects.default.settings.hmatrix-gsl': <derivation hmatrix-gsl-0.19.0.1>

I don't understand why this error occurs.
custom option's type is types.functionTo types.package and it allows multiple configurations and can merge them when the result of the function is equal, as far as I know. Do we have an extra unique constraint in the other part of the source code? Or am I missing something?

[srid]

Hmm, I'm not sure either. Looks like you can merge functionTo:

https://github.com/NixOS/nixpkgs/blob/268bd33d0b302beba1fdc67fe511008fc974f6a6/lib/types.nix#L1062

That said merging doesn't make any sense here, does it?

[khazaddum]

I'm getting this same error when a haskell-flake shows up more than once in the dependency tree. I tried using mkDefault as the error message suggests but I end up in the same spot because it's comparing the custom field to itself.

[khazaddum]

Would this be trying to enforce uniqueness?

haskell-flake/nix/modules/project/settings/default.nix

Line 91 in af2ba40

lib.concatMap

[khazaddum]

@srid any ideas on how this could be addressed?

[srid]

@khazaddum I haven't had the time to look into this, but if you wish to explore and come up with a solution, I'd be happy to review and merge your PR.

[khazaddum]

@srid I'd love to help if you can point me in the right direction. From the reading I've done, it's probably occurring from two (or more) calls to mkOption with the same name but I'm not sure where in the haskell-flake code to begin.