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Re: goops proposal: proper struct classes
From: |
Andy Wingo |
Subject: |
Re: goops proposal: proper struct classes |
Date: |
Thu, 05 May 2011 20:25:20 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/23.2 (gnu/linux) |
Hi :)
On Thu 05 May 2011 18:35, address@hidden (Ludovic Courtès) writes:
> Andy Wingo <address@hidden> 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
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