Hi :) On Thu 05 May 2011 18:35, l...@gnu.org (Ludovic Courtès) writes:
> Andy Wingo <wi...@pobox.com> writes: > >> If you know GOOPS, then you know that we have classes, rooted at >> <class>. And indeed <class> shows up a lot in documentation and in >> code. But that's not how it is in CLOS: our <class> corresponds to >> their `standard-class'. They have a superclass, called `class', which >> is the real root, and from which e.g. structure classes are derived. >> >> We need to do this. Currently, class-of on a struct/record data type >> gives a useless class that can't instantiate instances, doesn't know its >> slots, and does not reflect the vtable hierarchy. > > Here’s an illustration: > > scheme@(guile-user)> (use-modules (oop goops) (srfi srfi-9)) > scheme@(guile-user)> (define-record-type <foo> (make-foo x) foo? (x foo-x)) > scheme@(guile-user)> (make-foo 2) > $3 = #<<foo> x: 2> > scheme@(guile-user)> (class-of $3) > $4 = #<<class> <> 148a4b0> Here it probably should have a name, even now; probably a bug, that srfi-9 should call set-struct-vtable-name! on the rtd. > scheme@(guile-user)> (class-slots $4) > $5 = () > scheme@(guile-user)> (class-of $4) > $6 = #<<class> <class> 8e7a50> Here's the problem, for me: scheme@(guile-user)> (define-record-type <foo> (make-foo x) foo? (x foo-x)) scheme@(guile-user)> (make-foo 10) $1 = #<<foo> x: 10> scheme@(guile-user)> (struct-vtable $1) $2 = #<vtable:2356fa0 pruhsruhpwphuhuh 6a55640> scheme@(guile-user)> (struct-vtable $2) $3 = #<vtable:2356fa0 pruhsruhpwphuhuh 2356fa0> scheme@(guile-user)> (struct-vtable $3) $4 = #<vtable:2356fa0 pruhsruhpwphuhuh 2356fa0> See? The struct's vtable is actually an instance of another vtable -- of another class -- and that metaclass is a vtable-vtable (because $3 and $4 are equal). So class-of $2 should yield the class-of $3, which is actually some other bug: scheme@(guile-user)> (class-of $2) $5 = #<<class> <> 368e870> scheme@(guile-user)> (class-of $3) $6 = #<<class> <> 368e870> >> So we need a <basic-class>, interposed between <class> and <object>, >> which will be the real root of our class meta-object hierarchy. > > Why? > > Couldn’t ‘scm_i_define_class_for_vtable’ build a full-blown class, > populating its CPL, its ‘slots’ slot, etc.? Yes, it could. I didn't mention that. I think that such a duplicate hierarchy is unnecessary, because we can define methods for some of the class protocol (class-name at least!). Vtables *are* classes, on a fundamental level. Bare vtables are not as nice as <class>, but they do describe instances. SCM_CLASS_OF() is SCM_STRUCT_VTABLE(). WDYT? Am I off the deep end again? :) Andy -- http://wingolog.org/