Talk:Higher order function

I'm a beginner in Haskell and needs help I need a resourse of haskell from scratch and also need someone to explain in details the quicksort that is published:

quicksort :: Ord a => [a] -> [a] quicksort [] = [] quicksort (p:xs) = (quicksort lesser) ++ [p] ++ (quicksort greater)

   where
       lesser  = filter (< p) xs
       greater = filter (>= p) xs

without line no 1, it works fine what about line three ? and how we use ++ as concatenate ???

Thanks in advance