Question

I’m using out argument : [<Out>] message : string

But when I’m trying to set it : message <- “there is no figure on start position”

I’m getting error because message isn’t mutable. How to set out argument ?

Answer 1

Expose method parameter with type byref<string> and attribute [<Out>] and use as argument a mutable value with address-of operator &:

open System.Runtime.InteropServices

let mutable msg = "abc"

let outmsg ([<Out>]message : byref<string>) =
    message <- "xyz"

msg <- "test"
outmsg(&msg)
msg;;

val mutable msg : string = "xyz"
val outmsg : byref<string> -> unit