[Openmcl-devel] CLOS make-instance :around behaviour
hutch-lists at recursive.ca
Sun Oct 19 16:40:59 EDT 2008
Thanks. Comments and questions below...
On 19-Oct-08, at 3:33 PM, Pascal Costanza wrote:
> You're invoking undefined consequences.
> See Section 18.104.22.168.2 in the HyperSpec, bullet 19: You're defining
> a method on a predefined generic function (make-instance) which is
> applicable when its only required argument is a direct instance of a
> standardized class (here the class 'symbol, of which 'some-class is
> an instance: (typep 'some-class 'symbol) => T).
Never would have found that :-) Thanks.
> Common Lisp implementations are allowed to take that restriction
> into account and, for example, compile away the call to make-
> instance and replace it with something more efficient in case the
> first argument is a constant symbol (for example).
> If you really need :around methods on make-instance, consider
> defining your own metaclass using the CLOS MOP. But based on your
> example, I have the impression that an :after method on initialize-
> instance should be sufficient, which is generally preferable anyway...
That example is simplified. What I'm trying to do is implement a cache
like thing. If someone calls, say, (make-instance 'thing :id "abc")
then if there is already a thing with id "abc" return it rather than a
As far as I know there's no way to change the return value of the
primary method just using an :after method.
So... I suppose either I define my own metaclass (a new adventure for
me :-) or stop using make-instance.
> Lisp50: http://www.lisp50.org
> Pascal Costanza, mailto:pc at p-cos.net, http://p-cos.net
> Vrije Universiteit Brussel, Programming Technology Lab
> Pleinlaan 2, B-1050 Brussel, Belgium
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