title: Scala: Value :: is not a member of Int
body: <p>I have following Scala code sample and I want to know why I get an Error on foldLeft but not with foldRight? </p>
  
  <pre><code>val xs = List(1,2,3) 
  val ys = List(4,5,6)
  (xs foldLeft ys) (_::_) // Error: Value :: is not a member of Int
  (xs foldRight ys) (_::_) // Res: List(1, 2, 3, 4, 5, 6)
  </code></pre>
  
  <p>I am new to Scala, so please reply as simple as you can. Thanks</p>