main.pony

actor Main
  new create(env: Env) =>
    var y: String ref = "ABCD".string()
    var x: String ref = "John".string()
    let f: {(String ref, String ref): None ? } ref =
      {(a: String ref, b: String ref)? => b(0)? = a(0)? = b(0)? } ref
    let f' = f~apply(y)?
    try f'(x)? end
    env.out.print(x.string())