clever's nixpkgs trick

{ pkgs, ... }:

let

  # This is a neat trick that clever taught me on #nixos. Here we
  # re-import nixpkgs (using our fixed nixpkgs via pkgs.path) and
  # force the system to be x86_64-linux, which in turn causes this
  # derivation always to be built on x86_64-linux, regardless of the
  # system it was intended to be built for (as long as we use
  # nixpkgs.x instead of pkgs.x in the derivation!).
  #
  # We do this because dhall-nix doesn't build on aarch64-linux due to
  # GHC currently being unsupported on that arch; and Nixpkgs will
  # attempt to build any config that's needed on an aarch64-linux on
  # an aarch64-linux... unless we use this trick.
  #
  # This is effectively equivalent to Debian's "all"
  # pseudo-architecture.

  nixpkgs = import pkgs.path { system = "x86_64-linux"; };

  state =
    let
      drv = nixpkgs.stdenv.mkDerivation {
        name = "foo.nix";
        src = ./.;

        buildCommand = ''
          dhall-to-nix <<< $src/foo.dhall > $out
        '';

        buildInputs = [ nixpkgs.dhall-nix ];
      };

    in
      import "${drv}";
in
  state