[Haskell-beginners] Why doesn't ghci load this?

[Jeff Lasslett]

Hi,

I've been making my way through the exercises in the book "Real World
Haskell".  Below is my solution to an exercise asking for an
implementation of groupBy in term sof a foldr.  I haven't tested it
extensively but is seems to work. 

GHCI rejects it though, if I uncomment the type signature for the 'step'
function.  Could someone please help me to understand why that is?

Thanks,
Jeff

myGroupBy :: (a -> a -> Bool) -> [a] -> [[a]]
myGroupBy p xs =
 foldr step [[]] xs
 where
  -- step :: a -> [ [ a ] ] -> [ [ a ] ]
  step x acc@( ys : yss ) 
   | null ys                      = [ x ] : []
   | p x ( head ys )              = ( x : ys ) : yss
   | ( p x ( head ys ) ) == False = [ x ] : acc