Constants and module state

Top-level let and const create compile-time constants and may be public. Top-level let mut creates private mutable module state for the running program.

Modules initialize in dependency order and top-to-bottom. Export functions around mutable state instead of exporting the cell. The concurrency chapter explains what happens when work moves to an isolate.

let NAME = "moon"
let mut count = 0

fn next() -> string {
  count += 1
  "$NAME $count"
}

fn main() {
  println(next())
  println(next())
}