title: Concatenating strings with foldr in SML
body: <p>I'm trying to declare a function, string list -> string, that with the input for instance 
  <code>["Chicago","city","USA"]</code> should return <code>"Chicago city USA"</code>. What I did so far was this:</p>
  
  <pre><code>fun gather ts = foldr op ^ "" ts;
  </code></pre>
  
  <p>This seems to be somewhat along the lines, however the problem is, I would like to include the spaces between the words, as this function would return <code>"ChigagocityUSA"</code>.</p>