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From: ma168a@sdcc3.UUCP (John Wavrik)
Newsgroups: net.lang.forth
Subject: Re: Information Hiding in FORTH
Message-ID: <3448@sdcc3.UUCP>
Date: Thu, 26-Jun-86 13:33:35 EDT
Article-I.D.: sdcc3.3448
Posted: Thu Jun 26 13:33:35 1986
Date-Received: Sat, 28-Jun-86 03:56:23 EDT
References: <12110@amdcad.UUCP>
Organization: U.C. San Diego, Academic Computer Center
Lines: 41
Summary: You can ZAP!

In article <12110@amdcad.UUCP>, tim@amdcad.UUCP (Tim Olson) writes:
> I have been recently thinking of a way to implement something like the
> 'C' static scoping type in forth.  This would allow a word or group of
> words to use a VARIABLE without letting the VARIABLE be visible outside
> the group;   
> 
> 	c)  Is there another (easier) way to do this that I am not
> 	    aware of?
  
   When a name is re-used in Forth, the earlier
   definition remains in the dictionary and is automatically used
   by all words compiled when it was active.  You can have many
   variables named X -- and all will be, in effect local.
      The problem comes if you would later like to refer to a variable
      (or other object) that has been "covered".  One way to do this
(absolutely against standards!) is to define a word ZAP
which locates the name field of a word and changes the
length byte to zero (make sure to preserve the higher
bits if necessary); or the first character of the name
to ascii 0; or in any other way destroys the name field
so that the word will not be found in a dictionary
search [the best change is system dependent].
    Vocabularies can also be used to equip sub-applications
with special names without destroying access to previous
words with the same name.
    Some Forth systems (e.g. MMS-Forth) provide for a
temporary dictionary area used only during compilation.
Words can have their name fields in the temporary area.
This creates local words -- with a saving of space in
the main dictionary (local words have no headers and
are not linked into the dictionary when the temporary
area is deleted - but their body is left in the dictionary).
   There have been some experiments in Forth syntax
in which stack manipulation is eliminated by making
all arguments to a word into named local variables
(whose scope is just the word being defined). See
Dr. Dobbs journal of a few years ago.

				     --John J Wavrik
				     Math Dept - UCSD
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