Shared via Pony Playground (https://playground.ponylang.io/?gist=db32e71bb2552617f83450b28535481e)

main.pony

actor Main
  new create(env: Env) =>
    env.out.print(fib(5, env.out).string())

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