let rec qsort = function
| [] -> []
| x :: xs ->
    let smaller = List.filter (fun a -> a <= x) xs
    let larger  = [for a in xs do if a > x then yield a]
    qsort smaller @ [x] @ qsort larger   
    
let unsorted = [5; 2; 1; 4; 3]     
let sorted = qsort unsorted
printfn "%A" unsorted   
printfn "%A" sorted