title: Convert list of Integers into one Int (like concat) in haskell
body: <p>Pretty much what the title says. I have a list of Integers like so: [1,2,3]. I want to change this in to the Integer 123. My first thought was concat but that doesn't work because it's of the wrong type, I've tried various things but usually I just end up returning the same list. Any help greatly appreciated.</p>
  
  <p>Also I <i>have</i> found a way to print the right thing (putStr) except I want the type to be Integer and putStr doesn't do that.</p>