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From: knaut@schroeder (Claus-Peter Knaut)
Newsgroups: comp.lang.beta
Subject: Re: Global Variables
Date: 07 May 1996 15:10:37 +0200
Organization: University of Dortmund, Germany
Lines: 79
Sender: knaut@schroeder
Message-ID: <v7g20kw1vea.fsf@schroeder>
References: <v7gvii8n7jp.fsf@schroeder>
	<318F2E76.41C6@idefix.physik.tu-berlin.de>
Reply-To: Claus-Peter Knaut <knaut@ls6.informatik.uni-dortmund.de>
NNTP-Posting-Host: schroeder.informatik.uni-dortmund.de
In-reply-to: Kai Petzke's message of Tue, 07 May 1996 13:05:26 +0200
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>>>>> ">" == Kai Petzke <petz0433@idefix.physik.tu-berlin.de> writes:
In article <318F2E76.41C6@idefix.physik.tu-berlin.de> Kai Petzke <petz0433@idefix.physik.tu-berlin.de> writes:


    >> 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.
	
Maybe we did not explain the problem properly !


    >> 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 *) #);

We use the same method as in the code above !

We want to use the persistent store to save tuples of a
relation on a disk. But to use a global variable in tuple, tuple.bet
has to have its ORIGIN in a global database slot. To save the tuple,
the pattern has to be declared in betaenv.bet, best in lib:attributes
SLOT.

But then i can not access the global variable, because i can not
declare it in betaenv.bet. 

Hope that makes the point clear.
I want to diskuss this further, because i don't see the right way. 


    >> 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