F#: How to Call a function with Argument Byref Int

Question

I have this code:

let sumfunc(n: int byref) =
  let mutable s = 0 
  while n >= 1 do
    s <- n + (n-1)
    n <- n-1
  printfn "%i" s

sumfunc 6

I get the error:

(8,10): error FS0001: This expression was expected to have type
    'byref<int>'
but here has type
    'int'

So from that I can tell what the problem is but I just dont know how to solve it. I guess I need to specify the number 6 to be a byref<int> somehow. I just dont know how. My main goal here is to make n or the function argument mutable so I can change and use its value inside the function.

Answer 1

This is such a bad idea:

let  mutable n = 7
let sumfunc2 (n: int byref)  =
    let mutable s = 0
    while n >=  1 do
        s <- n + (n - 1)
        n <- n-1
        printfn "%i" s

sumfunc2 (&n)

Totally agree with munn's comments, here's another way to implode:

let sumfunc3 (n: int)  =
    let mutable s = n
    while s >=  1 do
        let n =  s + (s - 1)
        s  <- (s-1)
        printfn "%i" n

sumfunc3 7