[Openmcl-devel] CLOS make-instance :around behaviour

Bob Hutchison hutch-lists at recursive.ca
Sun Oct 19 13:40:59 PDT 2008

Hi Pascal,

Thanks. Comments and questions below...

On 19-Oct-08, at 3:33 PM, Pascal Costanza wrote:

> You're invoking undefined consequences.
> See Section 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  
new instance.

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.

Better ideas?


> Pascal
> -- 
> 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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