title: How to use List.filter?
body: <p>I have this code to filter list of string that the first letter is capital:</p>
  <pre><code>fun f s = Char.isUpper(String.sub(s,0));
  fun only_capitals (xs : string list) =  List.filter(f , xs);
  </code></pre>
  <p>But when compile, I always receive error :</p>
  <pre><code>operator domain: 'Z -&gt; bool
  operand:         (string -&gt; bool) * string list
    in expression:
      List.filter (f,xs)
  </code></pre>
  <p>What does this error mean? How to fix it?</p>