Implementing a tail recursive version of quicksort-like function in F#/OCaML

Question

Is it possible to implement a tail recursive version of the quick sort algorithm (via the continuation pattern)? And if it is, how would one implement it?

Normal (no

Answer 1

Quick attempt, seeems to work:

let rec quicksort list cont =
    match list with
    | [] -> cont []
    | element::[] -> cont [element]
    | pivot::rest ->
        let ``elements smaller than pivot``, ``elements larger or equal to pivot`` =
            rest |> List.partition (fun element -> element < pivot)
        quicksort ``elements smaller than pivot``
            (fun x -> quicksort ``elements larger or equal to pivot`` (fun y -> cont (x @ [pivot] @ y)))

> quicksort [2; 6; 3; 8; 5; 1; 9; 4] id;;
val it : int list = [1; 2; 3; 4; 5; 6; 8; 9]

Edit:

Of course, this code is highly inefficient. I hope nobody will use it in real code. The code was not difficult to write, but continuations might be difficult to read and can be error-prone (it's easy to forget a call to cont). If you want to play more, you can write a continuation monad (Brian wrote a blog post about it).