QSort in Haskell

qsort [] = []
qsort (lst) = ((qsort lesser) ++ mid ++ (qsort greater))
                  where p = head lst
                        lesser  = [z | z<-lst, z<p]
                        greater = [z | z<-lst, z>p]
                        mid = [z | z<-lst, z==p]

main :: IO()
main = do
    putStrLn $ show (qsort [2,7,3,5,1])