nixos-config/lib/hosts.nix

{ path, nixosModule }:
let

  inherit (builtins) mapAttrs attrValues attrNames readDir foldl' listToAttrs;

  # grab a list of systems, with associated hostnames
  # { x86_64-linux = [ "host1" "host2" ]; }
  sysToHosts = mapAttrs
    (system: _: attrNames (readDir (path + "/${system}")))
    (readDir path);

  # invert the attrset, from {sys=[name]} to {name=sys}
  # { host1 = "x86_64-linux"; host2 = "x86_64-linux"; }
  hostToSys = foldl' (a: b: a // b) { }
    (attrValues
      (mapAttrs
        (system: hosts: listToAttrs (map (name: { inherit name; value = system; }) hosts))
        sysToHosts));

in
# produce stub configurations
  # {
  #   host1 = { system = "x86_64-linux"; modules = [ ... ]; };
  #   host2 = { system = "x86_64-linux"; modules = [ ... ]; };
  # }
mapAttrs
  (hostName: system: {
    inherit system;
    modules = [
      (nixosModule)
      (path + "/${system}/${hostName}")
      { networking = { inherit hostName; }; }
    ];
  })
  hostToSys