title: Why does foldLeft failed to infer the type?
body: <p>Why does it give such an error?</p>
  
  <pre><code>scala&gt; 'a' - 'b'
  res0: Int = -1
  
  scala&gt; ('a' /: "abc") {_ - _ }
  &lt;console&gt;:8: error: type mismatch;
   found   : Int
   required: Char
                ('a' /: "abc") {_ - _ }
                                  ^
  </code></pre>