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From: Alf-Ivar Holm <alfh@ifi.uio.no>
Newsgroups: comp.lang.beta
Subject: Re: Is overwriting possible in BETA?
Date: 09 Jun 1997 15:03:00 +0200
Organization: The Department of Informatics, University of Oslo, Norway
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Arnd Poetzsch-Heffter <poetzsch@fernuni-hagen.de> writes:

> Is it possible in BETA to overwrite an attribute of a superpattern
> within a subpattern?

It is legal, but to access the overwriting attribute in the
subpattern, you have to have the right qualification.  Both the
overwriting and the overwritten attributes are part of the subpattern
and may be accessed directly.

The following program accesses the attributes through some
references qualified at different "levels":

	ORIGIN '~beta/basiclib/current/betaenv';
	--- PROGRAM : descriptor ---
	(#
	   a:  (# attr:< (# do 'a'->putline; inner #);#);
	   b: a(# attr:: (# do 'b'->putline; #);#);
	   c: a(# attr:  (# do 'c'->putline; #);#);
	   
	   ap: ^a;
	   bp: ^b;
	   cp: ^c;
	   
	do &b[]->bp[]->ap[];
	   '--ap.attr (b)--'->putline; ap.attr; 
	   '--bp.attr (b)--'->putline; bp.attr; 
				      
	   &c[]->cp[]->ap[];
	   '--ap.attr (c)--'->putline; ap.attr; 
	   '--cp.attr (c)--'->putline; cp.attr; 
	#)

This program outputs:

	--ap.attr (b)--
	a
	b
	--bp.attr (b)--
	a
	b
	--ap.attr (c)--
	a
	--cp.attr (c)--
	c

The last two lines of the program shows how the overwriting works.

	Affi