As a follow up to this ... assume that this dynamically rewriting the code is how tracing works ...

does this essentially mean that trying to get the entire stack frame / call stack would be rather difficult (or atleast the current approach would not easily work)

sorry for many spammy emails; the internals are fascinating, i'm trying to get the gurus to answser my questions :-)

On Mon, Feb 23, 2009 at 12:44 AM, lowly coder <lowlycoder@huoyanjinjing.com> wrote:
Join me for episode 201 of "gambit detective stories", today, we're going to try to solve the case of "how the f*ck does trace insert hooks"

So looking at lib/_repl.scm , we see in ##trace that we have:

(##interp-procedure-entry-hook-set! proc new-hook)

which looks something like:

(define-prim (##interp-procedure-entry-hook-set! proc hook)                              
 (let (($code (##interp-procedure-code proc)))                                          
   (macro-code-set! $code (##fixnum.- (macro-code-length $code) 2) hook)))  

this "2" here looks like a magic constant, let's ignore it for now; looking for macro-code-set! in lib/_eval#.scm , we see:

(##define-macro (macro-code-set! c n x)                                                  
 `(##vector-set! ,c (##fixnum.+ ,n 5) ,x))    

now, this 5 here is anothe rmagical constant -- but this looks interesting; for the interpreter, does gambit basically store the procedure in a vector ... and for inserting traces, we just _DYNAMICALLY REWRITE THE CODE_ to insert stuff before & after? If so, this is really really cool.

Any insights / clarifications / tips / deatils appreciated.

Thanks!