From: Till Kranz
Subject: methods & types
Date: Fri, 10 Oct 2003 17:06:05 +0000
Message-ID: <3F86E6FD.B2D43727@stud.uni-goettingen.de>

Hi,

this might be obvious but I'm pretty new to CL so I do not know how to
do it

For easy maintenance I wanted to do the following

defconstant Vec (class-name Vec2D)    ;Vec2D is a class defined
elsewhere

(defmethod bla ((v Vec) b)
	t)

As I understand it Vec does not get evaluated in the method definition
because defmethod is a macro. Thus I get an error becaus a class with
the name of Vec does not exist. But how can I get around this
limitation, i.e.: How can I have methods that depend on class names in
variables?

Thx Till

• Re: methods & types Barry Margolin
• Re: methods & types Christophe Rhodes

From: Barry Margolin
Subject: Re: methods & types
Date: Fri, 10 Oct 2003 17:27:24 +0000
Message-ID: <0_Bhb.284$pd.151@news.level3.com>

In article <·················@stud.uni-goettingen.de>,
Till Kranz  <·······@gmx.de> wrote:
>Hi,
>
>this might be obvious but I'm pretty new to CL so I do not know how to
>do it
>
>For easy maintenance I wanted to do the following
>
>defconstant Vec (class-name Vec2D)    ;Vec2D is a class defined
>elsewhere
>
>(defmethod bla ((v Vec) b)
>	t)
>
>As I understand it Vec does not get evaluated in the method definition
>because defmethod is a macro. Thus I get an error becaus a class with
>the name of Vec does not exist. But how can I get around this
>limitation, i.e.: How can I have methods that depend on class names in
>variables?

The "right" way is to use the Meta-Object Protocol to create the method.
But a crude solution is:

(eval `(defmethod ((v ,Vec) b) t))

-- 
Barry Margolin, ··············@level3.com
Level(3), Woburn, MA
*** DON'T SEND TECHNICAL QUESTIONS DIRECTLY TO ME, post them to newsgroups.
Please DON'T copy followups to me -- I'll assume it wasn't posted to the group.

From: Christophe Rhodes
Subject: Re: methods & types
Date: Fri, 10 Oct 2003 18:21:59 +0000
Message-ID: <sq4qyhx8ag.fsf@lambda.jcn.srcf.net>

Till Kranz <··········@stud.uni-goettingen.de> writes:

> defconstant Vec (class-name Vec2D)    ;Vec2D is a class defined
> elsewhere
>
> (defmethod bla ((v Vec) b)
> 	t)
>
> As I understand it Vec does not get evaluated in the method definition
> because defmethod is a macro. Thus I get an error becaus a class with
> the name of Vec does not exist. But how can I get around this
> limitation, i.e.: How can I have methods that depend on class names in
> variables?

Barry's answered your exact question.  But maybe consider
  (setf (find-class 'vec) (find-class 'vec2d))
  (defmethod bla ((v vec) b)
    t)
which is portable Common Lisp (under the assumption that the VEC2D
class already exists, of course.

Christophe
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