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> From: Richard Kelsey <kelsey@xxxxxxx> > Date: Sun, 4 Jan 2004 17:21:41 -0800 (PST) > From: Tom Lord <lord@xxxxxxx> > The draft FFI says: > double SCHEME_EXTRACT_DOUBLE(scheme_value) > char * SCHEME_EXTRACT_STRING(scheme_value) > Neither says "(may GC)". > If I'm using some exotic number representation (constructive reals, > perhaps), then EXTRACT_DOUBLE may very well involve some pretty hairy, > hence possibly GC-causing, computation. > This doesn't worry me too much; there aren't a lot of such > implementations around. How about implementations that implicitly force promises? > If I'm using some exotic string representations (I'm working on a > functional-splay-tree string type for Pika) -- same deal: > extract-string may take some (possibly GC-causing) work. > This does worry me (it's listed in the 'issues' section of the SRFI). > I think we went overboard here. Something like > SCHEME_EXTRACT_STRING_CONTENTS(scheme_value, index, count, buffer) > which copies 'count' characters starting from 'index' into 'buffer' > would be better. Presumably this can be done without GCing. In the case of a splay string, sure. In the case of a lazilly instantiated string, not so obviously. (Mobile code, perhaps?) > Even something innocent like: > int SCHEME_CHAR_P(scheme_value) > can cause GC if my implementation let's me attach to a hook in its > implementation. > Again, I don't think this will be very common. Is there an > existing implementation for which this (or anything similar) is > an issue? That supports hooks in arbitrary Scheme primitives? I presume so. But you've lost me on why "not common" or "no implementation does that [quite reasonable thing] yet" arguments are relevent to this FFI. Regarded as a social engineering hack, I presume that the reason to make this an FFI is to (a) lead to lots of libraries being given FFI-using wrappers; (b) lead to lots of implementations supporting the FFI. Great goals -- but there's no reason to gratuitously force all present and future implementations to emulate the GC idiosyncrasies of S48. -t