title: how to get nth element from List in f# using option
body: <p>Im new to f# and i'm trying to make this exercise:
  "Implement a function"</p>
  
  <pre><code>let rec nth(n : int) (l : List&lt;'a&gt;) : Option&lt;'a&gt; =
  </code></pre>
  
  <p>that returns the element in position n in l. The function must handle appropriately the case where the index is invalid</p>
  
  <p>this is my current code but I'm kinda stuck:</p>
  
  <pre><code>let rec nth (n : int) (l : List&lt;'a&gt;) : Option&lt;'a&gt; =
      if n &gt; l.Length then
          None
      else
          match l with
          | [] -&gt; None
  </code></pre>
  
  <p>Thanks for the help!</p>