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From: srt@aerospace.aero.org (Scott "TCB" Turner)
Newsgroups: comp.lang.lisp
Subject: Re: Problems Copying Structures
Keywords: defstruct structures copying
Message-ID: <94565@aerospace.AERO.ORG>
Date: 14 Dec 90 22:25:29 GMT
References: <1990Dec8.180825.21702@Think.COM> <93985@aerospace.AERO.ORG> <4083@osc.COM>
Organization: The Aerospace Corporation, El Segundo, CA
Lines: 35

Joe Keane writes:
>In article <93985@aerospace.AERO.ORG> srt@aerospace.aero.org (Scott "TCB"
>Turner) writes (about structure copying):
>>One of the better arguments for object-oriented programming.
>
>I don't see what object-oriented programming has to do with this.  The same
>problems with deep vs. shallow copying come up, and there's no magic solution
>which always does what you want.

Imagine trying to write a "copy" function in generic Lisp that will take
any type of object and provide an unlimited depth copy of that object.  
You do something along these lines:

	(defun copy (foo)
	   (cond ((type-1 foo)
			(apply #'create-new-type-1-w-elements
			       (mapc #'copy (destructure-type-1 foo))))
		 ((type-2 foo) ...)))

For each type that you know about, you destructure it, copy all the 
elements and then put it back together into a new gestalt.

Problem is, this requires that you know how to destructure and create
each possible type that will ever exist in your system.  Difficult.

In object-oriented programming, each type simply has an operation
"copy" that knows how to destructure and create that type and "voila!"
the problem is solved.

The same information and problems exist in either case.  OOP is not a
panacea, no matter what certain C++ cultists seem to think.  But OOP
does make this problem more tractable (and expandable) but organizing
the information wisely.

					-- Scott Turner