(You made a small mistake qsort [1, 2] ++ [3] ++ [5, 4] should be qsort [1, 2] ++ [3] ++ qsort [5, 4])
Also, quicksort is a pretty bad example of an pure algorithm, since it loses a lot of its performance benefits when not done in place.
Also, quicksort is a pretty bad example of an pure algorithm, since it loses a lot of its performance benefits when not done in place.
I went over the issues with this version of quicksort in the original comment, but I still think it was a good example because most people are familiar with quicksort, and I was trying to address the original posters claim that algorithms in Haskell could only be used in Monads.
Also, quicksort is one of the few I can do from memory.