Re: GNU Guile 1.9.6 released (alpha)

[Linas Vepstas <linasvepstas@gmail.com>]

2009/12/16 Thien-Thi Nguyen <ttn@gnuvola.org>:
> () Linas Vepstas <linasvepstas@gmail.com>
> () Tue, 15 Dec 2009 18:19:21 -0600
>
>   I have a new feature request -- it would be useful, in a
>   variety of situations, to be able to provide an opaque (void *)
>   pointer when calling make_gsubr, and then getting that pointer
>   back again whenever the primitive is called.
>
> Presuming i do not misunuderstand your situation, i find object
> properties work well.  Instead, of `void *procedure_private_data',
> you have:
>
>  (define private-data (make-object-property))
>  (define proc ...) ;; or C analog
>  (set! (private-data proc) foo)
>
> Am i missing something?

Err ...  When my C function is called, how do I go about
getting the void * pointer?

That's a rhetorical question -- I've had to answer it many times.

My solutions to this problem typically consisted of writing
a bunch of extra C code and scheme code that "magically"
wrapped the user's functions with some extra code, salted
away the void * pointer in some scheme structure, and
then pulled it out for the user, when needed. (None of them
used make-object-property, I've never heard of that one
before.)

Having to invent the various parts such as
"(define private-data (make-object-property))" etc. is painful
and cryptic and non-standard, and reinvented differently
for each different project on which I've used guile.  And so far,
I've needed this ability on *every* guile project I've worked on.

So I guess I'm saying "this is a common idiom, lets support it"
instead of leaving every user to reinvent it over and over.

--linas