title: Usage of _ in scala lambda functions
body: <p>Can anyone please explain me why I can do:</p>
  
  <pre><code>a.mapValues(_.size)
  </code></pre>
  
  <p>instead of</p>
  
  <pre><code>a.mapValues(x =&gt; x.size)
  </code></pre>
  
  <p>but I can't do</p>
  
  <pre><code>a.groupBy(_)
  </code></pre>
  
  <p>instead of a </p>
  
  <pre><code>a.groupBy(x =&gt; x)
  </code></pre>