learning swift: expressions are not allowed at the top level
<p>I'm learning Swift. I met a problem cannot be solved.</p>

<pre><code>import UIKit

func helloword(str:String) {
    print(str)
}

helloword("say")
</code></pre>

<p>I use <code>helloword("say")</code> but Xcode tell me the error:</p>

<blockquote>
  <p>expressions are not allowed at the top level</p>
</blockquote>