overlay-test.nix

let
  nixpkgs = import <nixpkgs> {};
  overlays = [
    (self: super: { aa-bb = "${self.aa} ${self.bb}"; })
    (self: super: { aa = "a"; })
    (self: super: { bb = "b"; })
    (self: super: { aa = super.aa + "a"; })
    (self: super: { bb = "bbb"; })
  ];
  inherit (builtins) foldl';
  combineOverlays =
    overlays: self: super:
    foldl' (
      lastOutput: overlay:
      let
        lastSuper = super // lastOutput;
        output = overlay self lastSuper;
        newOutput = lastOutput // output;
      in newOutput
    ) {} overlays;

  all-overlays = combineOverlays overlays;
  pkgs = nixpkgs.extend all-overlays;
in {
  aa = pkgs.aa;
  bb = pkgs.bb;
  aa-bb = pkgs.aa-bb;
}