{
  unstableSource,
  attributeName ? "unstable",
  # callPackage :: function | boolean
  callPackage ? false,
}:

self: super:

let
  useUnstable = self.config.useUnstable or true;
  sanitizePlatform = platformConfig: self.lib.removeAttrs platformConfig [
    "emulator"
    "emulatorAvailable"
    "darwinSdkVersion"
    "darwinMinVersion"
    "parsed"
  ];

  # if overlay is found in previous layer, then it was provided in "overlays" argument
  # otherwise, it is provided in "crossOverlays" argument.
  overlays = self.lib.groupBy (overlay:
    if self.lib.elem overlay self.buildPackages.overlays then "allLayers"
    else "finalLayer"
  ) self.overlays;

  unstablePkgs = import unstableSource {
    # localSystem -> pkgs.stdenv.buildPlatform
    localSystem = sanitizePlatform self.stdenv.buildPlatform;
    # crossSystem -> pkgs.stdenv.hostPlatform or pkgs.stdenv.targetPlatform ??
    # passing below
    # config -> pkgs.config
    config = self.config;
    # overlays -> pkgs.buildPackages.overlays
    overlays = overlays.allLayers;
    # crossOverlays -> pkgs.overlays without pkgs.buildPackages.overlays
    crossOverlays = overlays.finalLayer;
  } // self.lib.optionalAttrs (
    self.lib.systems.equals self.stdenv.buildPlatform self.stdenv.hostPlatform
  ) {
    # workaround for some odd structured packages that changes behaviour
    # when crossSystem is passed.
    crossSystem = sanitizePlatform self.stdenv.hostPlatform;
  };

  callPackage' = if builtins.isFunction callPackage then callPackage
    else if builtins.isBool callPackage && callPackage then self.callPackage
    else if builtins.isBool callPackage && !callPackage then throw "this should never be evaluated"
    else throw ''
      callPackage argument should be a function or a boolean.
      If you want to use the callPackage from self, set it to true.
      If you want to use a custom callPackage, set it to a function (pkgs.callPackage).
    '';

  callPackagesUnstablePkgs = self.lib.mapAttrsRecursiveCond (
    attrset: !(self.lib.hasAttr "override" attrset) && attrset.recurseForDerivations or false
  ) (
    _: unstablePackage: callPackage' {
      # For some reason, override functor of the package has its argument set as required,
      # which is totally false! the override functor can take all of those arguemnts optionally.
      __functionArgs = self.lib.mapAttrs (_: _: true) (self.lib.functionArgs unstablePackage.override);
      __functor =
        if builtins.isFunction unstablePackage.override then unstablePackage.override
        else unstablePackage.override.__functor;
    } {}
  ) unstablePkgs;

in
{
  "${attributeName}" = if !useUnstable then self
    # if callPackage is not false
    else if !(builtins.isBool callPackage && !callPackage) then callPackagesUnstablePkgs
    else unstablePkgs;
}