Re: calling scheme procedures by name from C

[Richard Shann]

On Sun, 2009-08-02 at 06:17 -0500, Linas Vepstas wrote:
> 2009/8/2 Richard Shann <address@hidden>:
> > Hi,
> > I want to call a scheme procedure with four arguments from C.
> > (The example scheme procedure here is just displaying its arguments.)
> >
> > (define (d-UploadRoutine a b c d)
> >  (display a)
> >  (display b)
> >  (display c)
> >  (display d))
> >
> > But I can't find what the correct parameters types are -
> 
> guile is not typed, or rather, is typed at runtime only, or dynamically typed.
> 
> > the headers
> > just say SCM, here are three things that give errors, two commented out
> >
> >
> > SCM proc =  gh_str2scm("d-UploadRoutine",  strlen("d-UploadRoutine"));
> 
> The gh_* routines are old and deprecated, and have been for about
> a decade now .. don't use them.
> 
> On the other hand, I notice that the tutorial at
> http://www.gnu.org/software/guile/docs/docs.html
> still uses gh_, so clearly, the gnu docs need cleanup :-(
> 
> > // proc = scm_string_to_symbol("d-UploadRoutine");
> > // proc = scm_string_to_symbol( gh_str2scm("d-UploadRoutine",  
> > strlen("d-UploadRoutine")));
> > SCM arg1 =  gh_str2scm("hello", strlen("hello"));
> > SCM arg2 =  gh_str2scm("he2lo", strlen("hello"));
> > SCM arg3 =  gh_str2scm("he3lo", strlen("hello"));
> > SCM arg4 =  gh_str2scm("he4lo", strlen("hello"));
> > scm_call_4(proc, arg1, arg2, arg3, arg4);
> >
> > The three definitions of proc lead to the following three error messages:
> >
> > ERROR: In procedure apply:
> > ERROR: Wrong type argument in position 1: "d-UploadRoutine"
> 
> google suggests that you read
> 
> http://www.lonelycactus.com/guilebook/x220.html
This seems to do the trick - I googled without finding anything so
useful, but I am a dreadful googler. The lines I needed were these:

        func_symbol = scm_c_lookup("do-hello");
        func = scm_variable_ref(func_symbol);


> and
> http://lonelycactus.com/guilebook/c1204.html
> 
> which I think is a start, and should get you going.
> 
> There's more stuff in
> http://www.gnu.org/software/guile/manual/html_node/Programming-in-C.html
> 
> but it lacks examples.
> 
> > It seems to me the docs are rather deficient here!
> 
> The examples are particularly so.
> 
> > They don't give the types of the
> > arguments, and the header only tells you they are SCM...
> 
> scheme is like perl, in that the types are determined during
> runtime.  This is considered to be a "strength", in that, in principle,
> you can define a routine that does the "right thing" for any type.

Unfortunately, the writer of scm_call_4 did not choose to define the
routine to do the right thing for any type I tried! And as the docs
didn't even give the C-prototype (which anyway only tells me it takes
SCMs) I cannot call the function without looking at the implementation.
Or asking the ever helpful band of schemers!
Thank you very much for such a speedy & informative reply.
Richard


> 
> --linas