Re: goops and equal?

[Jan Nieuwenhuizen <janneke@gnu.org>]

Mark H Weaver writes:

> 'equal?' is actually a "primitive generic", which means that it's a core
> C function that handles the cases it knows about, and only calls out to
> user-defined GOOPS methods for cases that aren't handled internally.
>
> One of the facts known by core 'equal?' is that objects of different
> types are not equal.

That makes sense; once you know it.

> It may be that we should relax this somewhat, but IMO it's probably a
> bad idea to allow instances of your user-defined type to be 'equal?' to
> symbols.  Out of curiosity, why do you want this?

Mainly because of surprise/symmetry.  The manual says equal? becomes
a generic once Goops is loaded.

I agree that it's unwise or at least bad taste to implement equal?
for different objects to be the same.

I have a situation where the compared objects could be the same,
or one could be a symbol which has the same semantics.

   (define-class <type> ()
    (name :accessor .name :init-value 'void :init-keyword :name))

   (define-method (type-equal? (a <type>) (b <symbol>))
     (eq? (.name a) b))

Now I must provide a type-equal? wrapper for the equal? method
I already have, ie

   (define-method (type-equal? (a <type>) (b <type>))
     (equal? a b))

that I could omit if the first type-symbol one could be called equal?

Thanks!
Greetings, Jan

-- 
Jan Nieuwenhuizen <janneke@gnu.org> | GNU LilyPond http://lilypond.org
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