title: Parse error in pattern: xs - understanding
body: <p>i have the following two functions in haskell:</p>
  
  <pre><code>plusList :: [[Int]] -&gt; [Int]
  plusList [xs ys] = add xs + plusList [ys]
  plusList [[]] = 0
  
  
  add::[Int] -&gt; Int
  add (x:xs) = x + add xs
  add [] = 0
  </code></pre>
  
  <p>so, i think i have the error in plusList [xs ys] = add xs + plusList ys</p>
  
  <p>My idea was to go through the set of the sets, namely [[Int]], by taking the first List xs, apply "add" on it and after that, call the second list ys recursively with "plusList ys"</p>
  
  <p>I am new in haskell, can I do that? And if no, why?</p>