The point was that "debug" and "message" parts in the example were arbitrarily complex expressions which were computationally expensive to evaluate. Your wrapping of log debug would still require evaluating them to get the message string unconditionally, even when debug logging is not enabled.

Of course, with support for first-class functions, you could do something like:

  func debug(produceMessage ()->string) {
    if log.isDebugEnabled() {
      log.debug(produceMessage());
    }
  }