title: Why not use %v to print int and string
body: <p>I know to print <code>int</code> we can use <code>%d</code>, and <code>string</code> we can use <code>%s</code>
  but we can still use <code>%v</code> to print them. So what if I always use <code>%v</code> to print them? What issue will happen if I do this?</p>