Path: news.daimi.aau.dk!jlk
From: jlk@daimi.aau.dk (Jorgen Lindskov Knudsen)
Newsgroups: comp.lang.beta
Subject: Re: Metaclasses in Beta
Date: 16 Nov 1996 15:25:52 GMT
Organization: DAIMI, Computer Science Dept. at Aarhus University
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References: <328C37F7.41C67EA6@cs.kuleuven.ac.be>
Reply-To: jlknudsen@daimi.aau.dk (Jorgen Lindskov Knudsen)
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In <328C37F7.41C67EA6@cs.kuleuven.ac.be> Walid Al-Ahmad <walid@cs.kuleuven.ac.be> writes:

>Beta considers patterns as objects in the sense that they are
>first-class values (pattern variables). However, it seems that Beta does
>not support the notion of metaclass as in Smalltalk. 

Right.  However a few proposals have been made to support various aspects 
of metaclasses in BETA, but nothing have made it into the public version
(yet).

>Some languages like
>Java, C++, Eiffel do not treat classes as objects, but they do offer
>facilities to define class data; i.e., data common to all objects of the
>class (static members in C++, static variables and methods in Java, and
>once routines in Eiffel).
>My question is whether or not Beta offers some support to achieve the
>same effect ? 

BETA does, but blockstructure can be used to simulate the 'class variables'.
Assume, you want a class X with class variables x1, x2 (let's say they i
are text variables).  In BETA, you can simulate this by:

	Xenv: @
          (# x1, x2: @text;
	     X: (# ...x1...x2... #);
	  #)

You create instances of X by:
	aX: @Xenv.X;
and you access the x1, and x2 class variables by e.g.:
	Xenv.x1[]->putline;

It shoud be noted, that in typical BETA programs, the design is not 
based on the model of class variables, as in Smalltalk, but on 
composite objects, nesting and inheritance.

>Please let me know where to find information in literature
>on Beta regarding this topic.
>Thank you very much beforehand,
>Best regards,
>Walid Al-Ahmad

Regards

Jorgen Lindskov Knudsen