Why can't the type of a first-class function contain byrefs?

Question

For example, the following isn't allowed, and I'm not sure why:

> let f () = 
    let f2 (a : byref<int>) =
        ()
    let mutable a = 0
    f2 &a;;

My guess would be that the byref could be a mutable reference to a stack variable, which could go out of scope if f2 decides to store it somewhere. Or is it something else?

Answer 1

The .NET type system does not allow byref types to be used as generic type arguments (e.g. you can't create a List<byref<int>>). Since (first class) F# functions are actually instances of the FSharpFunc<_,_> type, this means that F# functions also can't use byrefs in their domain or range.

Answer 2

The error seems to stem from the fact that you are declaring f2 as a nested function. If you extract it from beneath f, it compiles:

let f2 (a : int byref) = ()

let f () =
    let mutable a = 1
    f2 &a