Re: Gurus? Care to re-explain the absense of gh_set_x() for me?

[Matthias Koeppe]

Sam Tregar <address@hidden> writes:

> On Fri, 13 Jul 2001, Matthias Koeppe wrote:
> 
>> You should not attempt to code this as a C function.  += is not simply
>> a new procedure but needs to be a *special form* (i.e., a new syntax).
>> So I suggest you make a macro that expands (+= A B) into (SET! A (PLUS
>> A B)) and bind it to `+='.  In Scheme, this is easily done with
>> DEFINE-MACRO; if you insist in doing this from the C level, try your
>> luck with the functions in `macro.h'.  Then implement PLUS as a C
>> function that simply computes the desired value.
> 
> Let me explain more about what I'm doing.  I'm writing a Perl module
> called Inline::Guile.  One part of this project is an overloaded Perl
> object that will represent a Guile SCM.  To build an overloaded object in
> Perl you need to define functions for each of Perl's operators, +=
> included.  These operator functions can be written in Perl or in C but
> obviously not in Guile.
> 
> Does that make it clear why a macro won't suffice?

Have I understood you correctly: You have Perl variables, $foo and
$bar say, each of which hold one SCM object.  For instance, $foo holds
an SCM representing the integer 1, and $bar holds an SCM representing
2.  Now you do $foo += $bar, and you expect $foo to hold a SCM
representing the integer 3?

Again, there is no Scheme function (+= FOO BAR) which could accomplish
that because you pass the *value* FOO, and integer (and other
immediate) values cannot be mutated.  You need to pass the *location*
of FOO in some way.  I seem to have understood that FOO and BAR aren't
actually Scheme variables (whether local or global) but "boxes" with
SCM content that reside on the Perl side; so my original macro
suggestion doesn't help.  You should simply reify your boxes on the
Scheme side and provide functions like (BOX-REF BOX) and (BOX-SET! BOX
VALUE) in order to access them.

-- 
Matthias Köppe -- http://www.math.uni-magdeburg.de/~mkoeppe