Trying to depend on a nix file in another repository

default.nix

{ pkgs ? import <nixpkgs> {} }:
let
  a = import pkgs.fetchgit {
    # This repo contains default.nix
    url = "file:///...";
    rev = "0657ac75791c8d4a0a3d68d8705151fa6c756f66";
  } {};
in rec {
 # What do I need to do here?!? It throws 'error: cannot coerce a function to a string'
 # Eventually I'm hoping to use this as a buildInput
 b = a;
}

/*
The problem - how do we build a graph of repositories using Nix where each repository contains it's own default.nix.
I believe this is necessary because each repository is very tied a specific set of dependencies at any given point in time,
and I would like Nix to resolve them as well (and so on).

Am I talking crazy?
*/