title: Finding types in the standard library
body: <p>Using this function (constructor):</p>
  
  <pre><code>func NewDecoder(r io.Reader) *json.Decoder
  </code></pre>
  
  <p>How can you identify who is a <code>io.Reader</code> in the standard library?
  I know that a <code>os.Stdin</code> is one, but how do you find other readers in the standard lib?</p>