That's not how it works. The Nix build system is effectively a pure function, in that you specify all of the inputs up-front, which includes all dependencies, and then the build system performs the build (which is locally effectful, analagous to say, using the ST monad in Haskell, or Transients in clojure, etc). However, those local effects are discarded when we no longer need the chroot, and the result of the build process, which we want - should be a package that is exactly the same for all inputs. (We're not quite there yet, but approaching this.)