F# mutable function arguments

Use byref keyword which is equivalent to C# ref. See Passing by reference.

You can use ref as arguments

let v = ref 0
let mutate r = 
    r := 100
mutate v
printfn "%d" !v

Or byref keyword

let mutable v = 0
let mutate (r : byref<_>) = 
    r <- 100
mutate &v
printfn "%d" v