title: Terraform - why can't I assign a value from 1 variable to another variable
body: <p>I am trying to assign an output variable from a module to a local variable so that I can conveniently use a local variable. Is there another way ? </p>
  
  <pre><code>variable "vpc_id" {
    default = "${module.vpc.vpc_id}"
  }
  </code></pre>
  
  <p>Error I am getting is : </p>
  
  <pre><code>Error: Unsupported argument
  
    on main.tf line 22, in variable "subnetid_private":
    22:   default = "${module.vpc.subnet_private}"
  
  Variables may not be used here..
  </code></pre>
  
  <p>I spent good amount of time to google this but could not see any example. Am I missing something here. This is a pretty standard convenience feature of any language.</p>