default.nix

{ nixpkgsFun ? <nixpkgs>, compiler ? "default", cross ? "" }:

let
  pkgs = import nixpkgsFun {};

  rpi = pkgs.lib.systems.examples.raspberryPi;
  localSystem =  pkgs.lib.systems.elaborate { system = builtins.currentSystem; };
  crossSystem = if cross == "pi1" then rpi
                else if cross == "" then null
                else throw "Unknown cross target: " ++ cross;

  pkgsCross = import nixpkgsFun { inherit localSystem crossSystem;  };

in
  pkgsCross.hello