Shared via Pony Playground (https://playground.ponylang.io/?gist=791b6824c3475c71d14502ccdb94715e)

main.pony

actor Main
  let env: Env
  
  new create(env': Env) =>
    env = env'
    env.out.print(fib(5).string())
    
  fun fib(n: U64): U64 =>
    match n
    | 0 => 0
    | 1 => 1
    else
      fib(n - 1) + fib(n - 2)
      env.out.print(n.string())
    end