Expand support for byref to match C# 7
C# is adding support for byref locals and returns (see https://github.com/dotnet/roslyn/issues/118, slated for milestone 1.3). This will result in many libraries that expose these features (which the CLR already supports), but methods with such return types aren't currently usable from F#. F# already supports byref locals, but doesn't support implementing byref-returning methods nor does it support calling byref-returning methods.
At a minimum, F# should support calling byref-returning-methods (e.g. SomeRefReturningMethod(x,y,z) <- w), since C# users will be creating methods like these and being unable to call them will limit F#'s reach.
It would be nice if on top of that base level of support F# also supported declaring such methods, using the same safety rules that C# is using (e.g. the only refs that are safe to return are those that point to values stored on the heap or existing refs that are passed into the method).
Yes, this should be done, thanks
Approved in primciple subject to a detailed design, resolution of any remaining issues, and an implementation, with testing. Also subject to the feature actually appearing in C# 7 :)
Don Syme, F# Language Evolution
exercitus vir commented
Would it be possible to simply return a `byref` type to implement this feature?
Keith Battocchi commented
Concretely, this code works today:
let array = [| 1 .. 10 |]
let r = &array.
r <- -1
But if you create a method using ildasm that wraps int's Address method (with signature int * int -> int&), then this doesn't
let arr = [| 1 .. 10 |]
let r = ArrayHelper.GetAddress(arr, 0) // error FS0412: A type instantiation involves a byref type. This is not permitted by the rules of Common IL.
r <- -1