Re: Simplified slot access in goops

[Panicz Maciej Godek <godek.maciek@gmail.com>]

2010/8/28 Andy Wingo <wingo@pobox.com>:
> Hi Maciek,
>
> I know its been a couple years, but you raised an interesting point that
> Guile wasn't really able to handle at that point. Your implementation of
> `in' (commonly known as `with-slots', I believe) follows the mail.
>
> On Thu 27 Nov 2008 12:36, "Maciek Godek" <pstrychuj@gmail.com> writes:
>
>> (use-modules (oop goops))
>> (use-syntax (ice-9 syncase))
>
> With Guile 2.0 (coming shortly!), there is no more need to use ice-9
> syncase.
>
>> Now suppose we have an instance of a class:
>> (define-class C ()
>>   a b c)
>> (define o (make C))
>>
>> Using the "in" macro, we can write:
>> (in o
>>   (set! a 5)
>>   (set! b (1+ a))
>>   (set! c (+ a b))
>>   (+ a b c))
>> => 22
>
> The problem with this macro is that `o' can be of any type at all, but
> Guile needs to know at least whether a given identifier is a macro or a
> local variable or what, at expansion time.
>
> So for that reason, with-slots usually takes a list of slots, explicitly:
>
>  (with-slots o (a b c)
>    (set! a 5)
>    (set! b (1+ a))
>    (set! c (+ a b))
>    (+ a b c))
>
> It would be possible to pass the class instead, but then the class would
> need to be known at compile-time, and you run into a number of issues
> there. Anyway, here's that with-slots definition:
>
>  (define-syntax with-slots
>    (syntax-rules ()
>      ((_ obj (slot ...) b b* ...)
>       (let-syntax
>           ((slot (identifier-syntax
>                    (id (slot-ref obj 'slot))
>                    ((set! id val) (slot-set! obj 'slot val))))
>            ...)
>         b b* ...))))
>
> Here we use the new settable identifier-syntax.
>
> Note that slot-ref / slot-set isn't the most efficient way to reference
> slots; the best thing is to use accessors, which JIT-compile (sorta) to
> struct-ref opcodes, but we don't know that accessors exist for these
> slots. You can define a with-accessors macro though, that does assume
> accessors are available.
>
> In fact with more assumptions, it's possible to compile to struct-ref /
> struct-set yourself -- see the examples in the "Syntax" chapter of "The
> Scheme Programming Language" 4th edition (available online) for an
> illuminating take on the issue.

It's good to know all that. Indeed, it's been some time and the
development of my project has lost some of its robustness,
but maybe I would continue my work soon, so this news can
turn out to be motivating :)

If it comes to the performance issues, I've been wondering,
what is the current state of guile-vm? I have a friend who's been
developing a variant of a SECD machine in order to research
the topics of partial evaluation. You can take a look at his project
here:
http://drczlang.sourceforge.net/
If you think that it would make sense, I could try to convince him
to contribute to guile-vm. (I have no idea how this machine is
implemented, but I've been thinking about implementing a SECD
atop of i386 assembler; I'm only not sure whether gcc isn't wiser
than me when it comes to low-level programming)

best regards
m