title: Using Map.update in OCaml
body: <p>I am attempting to change the value of a key in a map I made in OCaml:</p>
  
  <pre><code>module TestMap = Map.Make(String);;
  let m = TestMap.empty;;
  let m = TestMap.add "Chris" 1 m ;;
  let m = TestMap.add "Julie" 4 m;;
  </code></pre>
  
  <p>This compiles file, but when I try to update the value at key <code>Julie</code> with: </p>
  
  <pre><code>let m = TestMap.update "Julie" 10 m;;
  </code></pre>
  
  <p>I get an error from the compiler:</p>
  
  <pre><code>Error: This expression has type int but an expression was expected of type
           'a option -&gt; 'a option
  </code></pre>
  
  <p>I'm guessing that I'm maybe using the function incorrectly. I'm finding the documentation for Map.update pretty hard to understand:</p>
  
  <pre><code>val update : key -&gt; ('a option -&gt; 'a option) -&gt; 'a t -&gt; 'a t
  </code></pre>
  
  <p>Is my syntax or are my arguments incorrect? </p>