Re: [Caml-list] GADT memory representation

[octachron <octa@polychoron.fr>]

I would tend to think that it would be nicer to document this memory 
layout − with all
necessary caveats embedded in the documentation itself − rather than 
hide away the
documentation due to the potentiality of a change in an unknown future.

− octachron.

On 01/12/16 à 12:51, 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.
>
>
> Alain
>