foo.nix

{ pkgs ? import <nixpkgs> {} }:

let
  callPackage = pkgs.lib.callPackageWith { foo = 1; };
  bar = 2;
  baz = 3;
in

{
  x = callPackage ({ foo }: foo) {};
  y = callPackage ({ bar }: bar) { bar = bar; };
  z = callPackage ({ baz }: baz) {};
}

# nix-instantiate --eval foo.nix -A x                                                                
# 1
# nix-instantiate --eval foo.nix -A y
# 2
# nix-instantiate --eval foo.nix -A z
# error: anonymous function at foo.nix:12:20 called without required argument 'baz'