[Haskell-beginners] finding the cause of an error (here head: empty list)

Hey, I have written a program which, when executed produces: Main: Prelude.head: empty list Now, I can go through my program and replace all "head" with (\a -> trace ("<line-nr>: length=" ++ (show (length a))) a) Actually, I already did that, and by this I found the error. But I wonder if there would have been an easier way? Has anyone any debug advice how I could find out which call to "head" causes this without so much typing work? Thanks! Nathan