group.hs

group :: (a -> a -> Bool) -> [a] -> [[a]]
group _ []   = []
group _ [el] = [[el]]
group predicate (a : b : list)
  | predicate a b = append a (group predicate (b : list))
  | otherwise     = [a] : group predicate (b : list)
  where append el (a : list) = (el : a) : list

nbr :: Int -> Int -> Bool
nbr num1 num2 = abs (num1 - num2) <= 1