Review my F# QuickSort Program

Question

OK... last sort of the day and probably the most fun. F# is really awesome.

let rec quickSort list = 
    match list with
    | [] -> []
    | [x] -> [x]
    | hd :: _ -> 
        quickSort (List.filter (fun x -> x < hd) list) @ hd :: quickSort (List.filter (fun x -> x > hd) list)

[<EntryPoint>]
let main args = 
    printfn "%s" (List.fold (fun acc x -> acc + " " + x.ToString()) "" (quickSort [10; 7; 1; 0; -1; 9; 33; 12; 6; 2; 3; 33; 34;]))
    0

let me know How I can improve this code. I am trying to think functionally and want my program to be most efficient, concise and functional.

Answer 1

I'd:

• exclude unnecessary duplication (List.filter etc.)
• drop separate match rule for the list of one element
• rely on type inference
• use more succinct form of pattern matching

Something like this:

let rec qsort = function
    | [] -> []
    | x::xs -> let lt, ge = List.partition ((>) x) xs
               qsort lt @ x :: qsort ge