Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!zaphod.mps.ohio-state.edu!think.com!barmar
From: barmar@think.com (Barry Margolin)
Newsgroups: comp.lang.lisp
Subject: Re: structures
Keywords: structures
Message-ID: <1991Apr9.202146.18803@Think.COM>
Date: 9 Apr 91 20:21:46 GMT
References: <1991Apr09.091234.18122@kub.nl>
Sender: news@Think.COM
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 57

In article <1991Apr09.091234.18122@kub.nl> pberck@kub.nl writes:
>How can I 'choose slots' in a structure I have defined? Say I have the
>structure (user) with the slots (name) and (adres), and I want
>a function which accesses one of the two, supplied as an argument
>to the function. (I hope I am making myself clear.)
>For example:
>
>(defun prt (someone slot)
>    (print (user-slot someone)))
>
>I would like to supply 'name' or 'adres' as argument (slot) to the function.
>How do I do this? I could probably set things up differently, but I
>would like to do it this way :)

If we had a FAQ list, this would certainly be on it (hmm, I could probably
save myself lots of time by compiling a FAQ list, so maybe I will).

There's currently no portable way to do this automatically.  Eventually it
will be doable using the CLOS metaobject protocol, though.

Right now, the only thing you can do is write a function that manually
selects between the slots, e.g.:

(defun prt (someone slot)
  (print (ecase slot
    	   (name (user-name someone))
    	   (adres (user-adres someone))))

You can also abstract this out so that you don't have to repeat it in
different functions:

(defun user-accessor (slot)
  (ecase slot
    (name #'user-name)
    (adres #'user-adres)))

(defun user-setter (slot)
  (ecase slot
    (name #'(lambda (user new-value)
	      (setf (user-name user) new-value)))
    (adres #'(lambda (user new-value)
	       (setf (user-adres user) new-value)))))

You can then write

(defun prt (someone slot)
  (print (funcall (user-accessor slot) someone)))

and

(defun set-someone (someone slot value)
  (funcall (user-setter slot) someone value))
--
Barry Margolin, Thinking Machines Corp.

barmar@think.com
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