# counter.ludic — 0.S: no function writes a global. The changing data is a `state`; it reaches a
# function only as a parameter - `c: mut Tally` to change it, `c: Tally` to read it - and the
# runtime supplies it where the program is entered: `entry (c: mut Tally)`, an `@On` listener's
# `(c: mut Tally)`, and any function value (`fn tick` is tick with its Tally supplied).
#
# Running it prints:  11 3 heard 5
program Counter {
  state Tally {
    n: int = 0
    heard: int = 0
    runs: []int = null
  }
  event Ping { amount: int }
  @On(Ping) handler Heard(c: mut Tally) { c.heard += amount }
  function bump(c: mut Tally, by: int) -> void {
    c.n = c.n + by
    if c.runs == null { c.runs = new []int }
    push(c.runs, by)
  }
  function total(c: Tally) -> int { return c.n }
  function tick(c: mut Tally, by: int) -> void { bump(c, by) }
  function run_later(f: fn(int) -> void, by: int) -> void { f(by) }
  entry (c: mut Tally) {
    bump(c, 3)
    run_later(fn tick, 8)
    emit Ping(amount: 5)
    print(`{total(c)} {len(c.runs) + 1} heard {c.heard}`)
  }
}