Re: [Caml-list] GADT memory representation

[Dmitry Bely <dmitry.bely@gmail.com>]

On Thu, Dec 1, 2016 at 2:51 PM, Alain Frisch <alain.frisch@lexifi.com> wrote:
> On 01/12/2016 10:52, David Allsopp wrote:
>>
>> Dmitry Bely wrote:
>>>
>>> I need to access/modify GADT data from C glue code. What is their memory
>>> representation? Is there any difference from ordinary sum types?
>>
>>
>> It's the same - GADTs are "just" add a lot of clever typing stuff on top
>> of a normal sum type - they don't affect the runtime operation of the code.
>>
>>> Unfortunately OCaml manual doesn't even mention GADTs in section
>>> "Interfacing C with OCaml".
>>
>>
>> That's worth a GPR/Mantis issue.
>
>
> I'm not sure we want to document the memory layout of GADTs.  I don't think
> there are concrete plans to do so, but it might be considered to change the
> representation of GADTs so that they cannot be used to compare values of
> different types with the polymorphic comparison function.  Today you can
> write:
>
>   type t = E: 'a -> t
>
>   let () = assert(E 1 = E true)
>
> A similar "bad" behavior used to be available for exceptions and this was
> "fixed" by changing their representation (their "slot" now has object_tag
> and a (per process) unique id).  We might want to do the same for GADTs (not
> an easy decision since it would add a lot of overhead) and for first-class
> modules as well.

So if I need to return a GADT value for C code, what would be the best
(working) option? Oversimplifying the problem, is it possible to
implement the following safely w.r.t future runtime changes?

type _ gadt =
    | Int : int -> int gadt
    | Float : float -> float gadt

external get : unit -> 'a gadt = "get"

- Dmitry Bely