title: What's the difference between the single quote 'a and a' in OCaml?
body: <p>The single quote <code>'a</code> in OCaml represents a general type. What does <code>a'</code> represent? It looks like it's not a grammar error, like in the following code:</p>
  
  <pre><code>type nat =
    | O
    | S of nat
  
  let rec plus n m =
    match n with
    | O -&gt; m
    | S n' -&gt; S (plus n' m)
  </code></pre>