Question about "1000 instances of nixpkgs"?

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1

The article “1000 instances of nixpkgs” suggests the following to avoid multiple independent instantiations of nixpkgs:

Instead of instantiating a new nixpkgs, access nixpkgs.legacyPackages.${system} and then make sure that all dependencies use the same instance of nixpkgs.

{
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
  inputs.other-dep.url = "github:other/dep";
  # Use the same version of nixpkgs as us
  inputs.other-dep.inputs.nixpkgs.follows = "nixpkgs";

  outputs = { self, nixpkgs, other-dep }@inputs: {
      packages = nixpkgs.lib.genAttrs [ "x86_64-linux" ] (system:
        let
          p = {
            nixpkgs = inputs.nixpkgs.legacyPackages.${system};
            # other-dep would also access `inputs.nixpkgs.legacyPackages.${system}`
            # thus only using a single instance of it.
            other-dep = inputs.other-dep.packages.${system};
          };
        in
        # your code here accessing `p.nixpkgs` and `p.other-dep`
     );
  };
}

That way, there will only be a single instance of nixpkgs being evaluated, and consumers of your project can again follow the same practice.

In this approach, is one effectively overriding the “nixpkgs” input of the “other-dep” flake, as well as ignoring entries to that input in that flake’s “flake.lock” file?

2

Yes.

But it won’t apply your overlays/config to that nixpkgs instance, you’re just syncing revs.

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