Shared via Pony Playground (https://playground.ponylang.io/?gist=b9bc8e53ff243eef90525c6d5688b0f3)

main.pony

actor Main
  let pa: PrintingActor

  new create(env: Env) =>
    pa = PrintingActor(env.out)
    env.out.print(fib(5).string())

  fun fib(n: U64): U64 =>
    match n
      | 0 => 0
      | 1 => 1
    else
      let n1 = n - 1
      let n2 = n - 2
      pa.debugger(n.string()) // -I need to print right here
      fib(n - 1) + fib(n - 2)
    end

actor PrintingActor
  let _out: OutStream

  new create(out: OutStream) =>
    _out = out

  be debugger(n: String) =>
    _out.print(n)