Why is my flake overlay failing?

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1

Hello all,

I have a fairly simple situation where I’m defining an overlay in one flake and using it in another.

In the dependency flake I’ve defined a package called zlib - nb, there is already a package with the same name in nixpkgs, and put it into that flake’s overlay:

        zlib = with pkgs; callPackage ./zlib.nix {};
      in {
        packages.default = zlib;
        overlays = final: prev: { inherit zlib; };

That all builds fine and no errors there. nix eval --raw .# gives me the store path for this, /nix/store/vn36lf7ilbcnwl2bd0qpsixi7gfxkvmi-zlib. So far so good.

I then have a second flake, which imports the dependency flake, and uses its overlay to define pkgs:

  inputs = {
    nixpkgs.url = github:NixOS/nixpkgs/nixos-21.11;
    flake-utils.url = "github:numtide/flake-utils";
    zlib.url = <path to zlib flake here>;
  }
  ...
          pkgs = import nixpkgs {
            inherit system;
            overlays = [ zlib.overlays.${system} ];
          };
          packages = {
            mainPackage = with pkgs; callPackage ./main_package.nix { };
          };

where the derivation in main_package.nix lists zlib as one of its parameters.
Unfortunately it’s taking the pre-existing nixpkgs zlib instead of my new one, as I can see by looking at the store paths in nix show-derivation .#mainPackage. Am I making some elementary mistake here? I can’t see how.

Thanks!

2

Overlays are not system-spaced, so I’m not sure what you’re doing zlib.overlays.${system} for, nor why it’s not yelling at you when you try.

Also, when defining the overlay, you should probably be using zlib = final.callPackage ./zlib.nix {};, rather than pulling in a separate nixpkgs invocation just for zlib.

However, for the main question, of why it’s not picking up the new package, the only thing I can think of is that you defined callPackage with a let or an argument binding somehow, and it’s overriding the with pkgs;.

3

OK, thank you, I’m at least reassured by the fact that I’ve not misunderstood the expected behaviour.

Are you sure overlays are not system-spaced? If I do nix flake show it will show me:

< flake base >
├───overlays
│   ├───aarch64-darwin: Nixpkgs overlay
│   ├───aarch64-linux: Nixpkgs overlay
│   ├───x86_64-darwin: Nixpkgs overlay
│   └───x86_64-linux: Nixpkgs overlay
└───packages
    ├───aarch64-darwin
...
4

Overlays are named. Somehow you’re getting system strings as the names.

5

I’m using flake-utils.lib.eachDefaultSystem from "github:numtide/flake-utils";, that’s probably the reason.

6

Yeah, a quick check of the code says flake-utils.lib.eachDefaultSystem doesn’t know about overlays not being system-spaced. I think you might be expected to know that yourself and put them in with // outside of the call.

7

Is there any way you know of to examine the contents of an overlay? They don’t get expanded out in eg nix flake show.

8

Load it up in the repl, and examine overlay pkgs pkgs (technically wrong, but wouldn’t matter for this case).

9

OK, thanks, I’ll figure it out from there.