Re: FFI and disjoint types

[Andy Wingo]

Hi,

On Mon 01 Mar 2010 15:18, address@hidden (Ludovic Courtès) writes:

> With ‘(system foreign)’, C pointers are mapped to Scheme foreign
> objects, regardless of the type of object pointed to.  Thus, to get
> disjoint types in Scheme, foreign objects would need to be boxed in
> structs (since structs are the only way to create disjoint types from
> Scheme).
>
> If this analysis is correct

It is

> the resulting code may be somewhat
> inefficient since we end up boxing C pointers twice.

Do you think this is an issue? We could add an extra word to foreign
pointers to make them have a type tag; or we could change the foreign
pointer representation to have a vtable. The downside with that is that
you can't allocate static foreign pointers that way.

I have a feeling that "double-wrapping" will perform fine. I figure a
high-level FFI will wrap these primitive foreign pointers in such a way
that they can preserve typing information, and hide the primitive
foreign pointer (if appropriate). But I could be wrong about that!

Cheers,

Andy
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