Is it actually possible to implement quicksort in a pure functional language like Haskell (with immutable data structures)? Sorting in place would seem to involve mutating the list every time two values are swapped.
http://hackage.haskell.org/package/vector-0.7.0.1
Mutability is attained by using the ST monad. The ST monad uses mutable memory, but since it does not allow other interactions with the outside world, its value can be extracted (unlike the IO monad). When you are done with the modification of the vector, it can be frozen to obtain a pure vector.
The IO monad can also be used, but not if you want to return a pure value.
A good tutorial can be found at:
http://www.haskell.org/haskellwiki/Numeric_Haskell:_A_Vector...
I used mutable vectors in the ST monad in maximum entropy training software, and they are really performant.
I was like, "Wait. Wait. Waaaiiiitt. How does it do that?"