F#: пример использования рекурсии

open System
open System.IO
open System.Diagnostics

let time f = 
    let watch = Stopwatch.StartNew()
    f()
    watch.Stop
    printfn "%A" watch.Elapsed

let rec sum xs = match xs with
    | [] -> 0
    | h::t -> h + sum(t)   

let random = new Random()
let data = List.init 1000000 (fun _ -> random.Next(Int32.MaxValue))
let rec qs xs = match xs with
    | [] -> []
    | [_] -> xs
    | h :: t -> qs([for i in t do if i<= h then yield i]) @ [h] @ qs([for i in t do if i>h then yield i])

let saveAs fn xs = 
    use stream = File.CreateText(fn)
    for v in xs do
        stream.WriteLine(sprintf "%A" v)

let saveData = 
    do saveAs "or.txt" data
    do saveAs "so.txt" (qs data)
    
time(fun _ -> printfn "sum = %A" (sum data))
time(fun _ -> printfn "sum = %A" (sum data))
time(fun _ -> qs data |> ignore)
time(fun _ -> saveData |> ignore)

printfn "done"