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From: barmar@think.com (Barry Margolin)
Newsgroups: comp.lang.clos
Subject: Re: clos and packages
Message-ID: <1990Dec11.044210.26810@Think.COM>
Date: 11 Dec 90 04:42:10 GMT
References: <183@anaxagoras.ils.nwu.edu>
Sender: news@Think.COM
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 59

In article <183@anaxagoras.ils.nwu.edu> brand@ils.nwu.edu (Matthew Brand) writes:
>	Now if I have to have a different name for the method of each
>	package, that rather defeats the point of using an object system.

If the two classes are implementing methods for the same generic function,
then they should both reference the same generic function name, not symbols
in different packages.

>	On the other hand, since CLOS doesn't allow me to make functions
>	private or public, I need to use packages.  Whoever wrote the intern
>	function forgot to check whether or not a symbol references a method;
>	in that case interning should proceed, provided that the argument
>	lists are congruent.  Hence my question:

The identity of symbols in Common Lisp is independent of what they happen
to be used to name at any particular time.  A symbol may simultaneously
name a global variable, a function (either generic or not), a property
list, a slot (of both a defstruct and a class), a property on a property
list, an entry in an alist, an entry in a hash table, etc.

>	> (use-package 'spline-package)
>	T
>	> (use-package 'fractal-package)
>	>>Error: If the USER package were to 'use' the FRACTAL-PACKAGE package,
>             there would be name conflicts with symbols found in the 
>             following packages ("SPLINE-PACKAGE" "FRACTAL-PACKAGE")
>             The particular symbols involved are:
>             (SHOW)

You complain above about INTERN, but your example uses EXPORT and
USE-PACKAGE, which are completely different.  Symbols have identity
independent of their names; they are first-class Lisp objects, and other
objects have references to them.  However, a package can only have one
symbol with a particular name.  The entry for "SHOW" in the using package
can either be refer to SPLINE-PACKAGE:SHOW or FRACTAL-PACKAGE:SHOW, but it
can't refer to both.  You can't just get rid of one of them, because there
are all these references to them.

>	Does anybody have a fix for this?

Create a new package, e.g. GEOMETRY-PACKAGE, which will be used for symbols
common to splines, fractals, etc.  Export these symbols from
GEOMETRY-PACKAGE, and have SPLINE-PACKAGE and FRACTAL-PACKAGE do
(use-package 'geometry-package).

>	Also, does this only happen in Lucid CommonLisp with PCL?

It's normal Common Lisp.  Symbols are just symbols.

There was a discussion in comp.lang.lisp a couple of months ago about
whether packages are appropriate for emulating the public/private
mechanisms of some other object systems, and whether one package per class
is reasonable.  The concensus is that the package system was designed for
a small number of large groups, not lots of tiny packages.
--
Barry Margolin, Thinking Machines Corp.

barmar@think.com
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