Re: [Caml-list] GADT memory representation

[Alain Frisch <alain.frisch@lexifi.com>]

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