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From: Kai Petzke <petz0433@idefix.physik.tu-berlin.de>
Newsgroups: comp.lang.beta
Subject: Re: Global Variables
Date: Tue, 07 May 1996 13:05:26 +0200
Organization: Technical University Berlin, Germany
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To: Claus-Peter Knaut <knaut@ls6.informatik.uni-dortmund.de>

Claus-Peter Knaut wrote:
> 
> Hello,
> 
> We are developing an Probabilistik Database in BETA.
> We need global variables. How do you manage to use them. You can not
> place them in lib: attributes because there you can define no static
> objects.

Yeah, this is something, that puzzles many newcomers to BETA. Why are
there no easily useable global variables?

Well, I believe, it is because the Mjolner people want you to write code
with BETA, that can easily be re-used later in different types of
applications.


Think of your probabilistik database as a big machine.  The first thing,
that you want to do, is to define the machine itself, say in a file
"prob.bet":

	(* probabilistik database *)
	ORIGIN '~beta/basiclib/v1.4/betaenv';
	--- lib: attributes ---
	(* the main source file *)
	probDatabase:
	  (#
	     <<SLOT probDatabase: Attributes>>;

	     (* private "global" data *)
	     DatabaseGlobals: @<<SLOT DatabaseGlobals: Descriptor>>;
	     InstanceGlobals: @<<SLOT InstanceGlobals: Descriptor>>;
	     (* add more slots here, if required *)

	     (* main methods *)
	     init: (# <<SLOT DatabaseInit: DoPart>> #);
	     query:
		(# querystring: ^text; result: ^text
		enter querystring
		<<SLOT DatabaseQuery: DoParT>>
		exit result
		#);

	      (* add more methods, if required *)
	   #);


The result of that scheme is, that all methods of the probDatabase
"machine" have access to those variables, that are "global" to the
database, like those defined in DatabaseGlobals and InstanceGlobals.
You can bind the DatabaseGlobals and InstanceGlobals patterns in some
other files, say "database.bet" and "instance.bet".  All of the modules
of tha probabilisic database, that need the database globals should then
INCLUDE the "database.bet" file, and all those, that need the instance
globals should INCLUDE "instance.bet".


Hope, that this is enough explanation.  The advantage is, that in the
end, all of your application is contained in a single BETA pattern.
When you later include your appliation into some other BETA code, you
have to include just one pattern, and need not care about name conflicts
between global variables and other nice problems ...



Kai