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:

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

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

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

Thanks in advance