title: No instance for (Num a) arising from a use of ‘+’ Haskell
body: <p>I can't figure out why this won't work:</p>
  
  <pre><code>final' :: [a] -&gt; a
  final' lst = foldl(\accum x -&gt; accum - accum + x) 0 lst
  </code></pre>
  
  <p>I always get the error No instance for (Num a) arising from a use of ‘+’</p>