Talking to other languages (with callbacks!) - possible solution

Daniel B. Faken faken at
Mon May 23 18:26:35 UTC 2005

Lex Spoon wrote:
> my working assumptions have been:
>	1. If you want to talk to the software world, speak C.  It's the 
> posixa and almost everything can speak it in some fashion.
> 	2. Speaking C without an accent means you have to support callbacks
> and using the Squeak VM as a library.
> 	3. Allowing callbacks means that the interpreter must be reentrant and
> that the VM is implemented with a call stack that alternates back and
> forth between Squeak and C.
> If I understand correctly, the DBus
> strategy might be an alternative that is easier than #3.

Exactly my point.. or, on an aside: easier for me, since I don't 
want to get involved in Squeak internals, which I'm guessing would take
a long time to understand well enough to modify nicely (and this assuming
they don't need a complete rearchitecting).

> Further, it
> seems plausible that DBus will become a widely used  "language" of its
> own -- perhaps even *the* lingua posixa of 2010 -- thus meaning #1 on
> the list above is unnecessary.

I'm more conservatively hoping it will at least be *supported* for quite a 
while :).  In any case I agree that it is a good portability strategy to 
converse easily with C.  Besides, Squeak's easy FFI to the DBus C library 
is what made the bindings realizeable in the first place..

> Some questions:
> - How hard is it to bind an *arbitrary* C program to DBus?  If it's
> easy, then that means we can talk to arbitrary C libraries even when
> they don't already have DBus support.  (It also means that DBus will be
> supported more widely, because people can say "why not" and offer DBus
> bindings.)

DBus is a C library, so in that sense its trivial..
If you mean 'how hard is it to bind an arbitrary object-oriented system',
then I would say it seems relatively simple given the work I've done for 
Squeak bindings.  It is *designed* to be easy to bind..

> - You mention some impressive high-level examples--TeapotMorphs
> and TFrames.  How about basic stuff like arithmetic?  That is, how hard
> would it be to write in C the following:
> 	obj = executeSmalltalkCode("Compile evaluate: '5 factorial'");
> How complicated is executeSmalltalkCode()?

Well, currently this is not very hard.. it would just be a matter of 
constructing a message to whatever object 'Compile' is, and reading the 
answer.. maybe 5 lines of C. 

> I ask about this example because I am wondering about what the exact
> overheads are of going via DBus.  Those overheads can easily disappear
> into the noise if you are doing a more complicated project.

If you mean performance overhead: I haven't done benchmarks, and things 
really aren't at that stage yet ("premature optimization..").
If you mean boilerplate-type overhead, this can be reduced...

> Please update the list with any further work you do on it -- this is useful stuff!

I'll announce something this week.. I appreciate the response - it lets me 
know other people are interested in the issue..


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