title: error using List.filter OCaml
body: <p>Hy, i'm trying to create a histogram but i keep getting an error.</p>
  
  <pre><code>Histogram example:
  input :[2;1;2;3;2;1;2;2;5]
  output :[(2,5);(1,2);(3,1);(5,1)] 
  </code></pre>
  
  <p>My code:</p>
  
  <pre><code> let rec count a ls = match ls with
    |[]              -&gt; 0
    |x::xs  when x=a -&gt; 1 + count a xs
    |_::xs           -&gt; count a xs
  let rec histo l = match l with
  |[] -&gt; []
  |x :: xs -&gt;  [(x, count x l)] @ List.filter(fun x -&gt; xs != x)histo xs;;
  </code></pre>
  
  <p>Error:
  This function has type ('a -> bool) -> 'a list -> 'a list                       It is applied to too many arguments; maybe you forgot a `;'. </p>