Ok, makes sense. I hadn't followed that by 'data structure' you meant 'internal data structure'.
Arc's not just a userland racket program; ptr-set! and ptr-ref are low-level creatures. I hadn't focused on this fact, even though I'd seen the comments at set-ca/dr!
Exactly. Back in the MzScheme 3xx days Arc was, to MzScheme, just an ordinary (if rather large) MzScheme program. And, with my runtime project (if successful), Arc will once again to Racket be just a big Racket program, so any bugs with Racket will be legitimate Racket bugs that we can go ahead and file a Racket bug report on ^_^
This is actually a pretty big realization for me. Between the documentation thread 2 months ago (http://arclanguage.org/item?id=12860) and this realization that the queue bug is all the fault of our arc implementation, my opinion of the racket runtime is entirely rehabilitated. (http://arclanguage.org/item?id=12302 was the closest thing to a flame war I've been involved in here)
Now my only complaint with mzscheme in general is that it isn't dynamic enough, and forces us to use its module system :) But even that's just because we're using racket in this weird 'backwards compatibility' mode. I'm looking forward to ar because it'll let arc use all of racket's modern goodies (keyword args, extensible equality, ...)
Edit: I suppose we're still stuck with scheme's phase-separated macros.
