Re: [Caml-list] GADT memory representation

[Josh Berdine <josh@berdine.net>]

On Thu, Dec 01 2016, Alain Frisch 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.

FWIW, I would rather polymorphic comparison were removed than GADT's made heavier.