title: Tree size in haskell
body: <p>I have an implementation of a tree like this: </p>
  
  <p>data Tree a = Empty | Node a [Tree a] deriving Show</p>
  
  <p>and I need to get the size. </p>
  
  <p>I think this code can resolve this problem but I have an error: </p>
  
  <pre><code>size :: Tree a -&gt; Int
  size Empty       = 0
  size (Node a ts) = 1 + [size t | t&lt;-ts]
  </code></pre>