title: For returns Unit
body: <p>I want to zip the index of a month with its name:</p>
  
  <pre><code>  val res = for(i &lt;- 1 to 12) {
      val m = DateTime.parse(i.toString, DateTimeFormat.forPattern("mm").withLocale(Locale.US))
      yield (i, m.monthOfYear().getAsString)
    }
  </code></pre>
  
  <p>Why <code>res</code> is <code>Unit</code>?</p>