title: Why underline not work in some scenario of monad?
body: <p>Underline <code>_</code> cannot work for some scenario in following code:</p>
  
  <pre><code>case class Item
  (
    para: Int
  )
  
  val a = List(1, 2)
  val b = List(Item(1), Item(2))
  
  a foreach {
    perA =&gt; println(perA * 2) // Line A: ok
  }
  
  a foreach {
    println(_) // Line B: ok
  }
  
  a foreach {
    println(_ * 2) // Line C: not ok
  }
  
  b foreach {
    perB =&gt; println(perB.para) // line D: ok
  }
  
  b foreach {
    println(_.para) // line E: not ok
  }
  </code></pre>
  
  <p>Could someone explain for me about line C &amp; line E, thanks.</p>