title: How do I replace space characters in a string with "%20"?
body: <p>I wanted to write a Haskell function that takes a string, and replaces any space characters with the special code %20. For example:</p>
  
  <pre><code>sanitize "http://cs.edu/my homepage/I love spaces.html"
  -- "http://cs.edu/my%20homepage/I%20love%20spaces.html"
  </code></pre>
  
  <p>I am thinking to use the concat function, so I can concatenates a list of lists into a plain list.</p>