Re: continuation barriers

[Linas Vepstas]

Hi Neil.

Wow!

2009/8/26 Neil Jerram <address@hidden>:
> Linas Vepstas <address@hidden> writes:
>
>> i.e. I'd like something like this to work:
>>
>> scm_c_eval_string(" ... (call/cc (lambda (k) (set! *myk* k))) ...");
>> ... some_c_code(...);
>> scm_c_eval_string(" ... (*myk* 42) ...");
>
> I think there are a couple of problems here.
>
> The first is as you've noted, that scm_c_eval_string() has a
> scm_c_with_continuation_barrier() hiding inside it.  You can avoid
> that by using some other method for calling from C into Scheme, for
> example:
>
> (define (entry-point code)
>  ... set up whatever catches you want around the
>      code that is going to be evaluated, e.g. for
>      debugging ...
>  (eval (with-input-from-string code read) (current-module))
>  ...)
>
> SCM entry_point_proc = SCM_VARIABLE_REF (scm_c_lookup ("entry-point"));
>
> scm_call_1 (entry_point_proc, code_string);
>
> The second is that you almost certainly don't want the continuation
> call to make C think it is returning again from the first scm_call_1
> ().  That kind of thing tends to confuse C code :-)
>
> I solved that problem (when I wanted to do something very like what
> you're doing) with an approach like this:
>
> (define current-c-code-continuation #f)
>
> (define (entry-point code)
>  (call/cc (lambda (k)
>             (set! current-c-code-continuation k)
>
>             ... set up ... (as above) ...
>             (eval (with- ...) ...)
>             ... (as above)
>
>             (current-c-code-continuation))))

>From what I can tell, I think the above will work for me.  The
"wow" comes from the realization that I'm still not thinking in
a functional kind of way.  Presuming that I can get this working,
I think that I can spend a few hours re-phrasing your email into
an example suitable for the formal guile docs.  If I do, should
I send a patch against guile-docs (wherever these may be?)

>> I think (I haven't yet tried) that the above can work if I wrap
>> the whole darn thing with scm_with_guile() .. but is there
>> some way of getting the above to run without a big wrap
>> of this kind?
>
> I don't think the above qualifies as "without a big wrap".

Yeah, I've discovered its more or less impossible to try to
wrap my main program with an "scm_with_guile()"  Too
 too many dynamically loaded wacko libs opening too many
threads calling to/from god-knows what langauge (java, lua, python...)

--linas