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From: pmy@uvacs.UUCP
Newsgroups: net.lang.forth
Subject: Re: Information Hiding in FORTH
Message-ID: <622@uvacs.UUCP>
Date: Mon, 7-Jul-86 16:11:36 EDT
Article-I.D.: uvacs.622
Posted: Mon Jul  7 16:11:36 1986
Date-Received: Sat, 12-Jul-86 06:14:30 EDT
References: <12110@amdcad.UUCP>
Organization: U.Va. CS dept.  Charlottesville, VA
Lines: 58

> 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; something like:
> 
> 	: random-package ;
> 	
> 	VARIABLE randval
> 	
> 	: init-rand ( seed -- )
> 		randval ! ;
> 	
> 	: rand ( range -- rand# )
> 		randval @ 21709 + 15273 * randval !
> 		mod ;
> 	
> 	HIDE randval
> 
> (No flames on the "randomness" of this random package, please :-)
> 
> HIDE is a word which, like tic ('), searches the dictionary for the
> target word.  HIDE also keeps a trailing pointer to the previous
> word in the dictionary.  When it finds the target word in the dictionary,
> it takes it out of the dictionary search path by changing the previous
> word's next pointer to point to the word following the target word.
> This allows a VARIABLE or procedure word to be used normally in a
> "package" and then be hidden from subsequent definitions.
> 
> 	a)  Is this a useful construct, or is it the attempt of an
> 	    admittedly neophyte forth programmer (me) to force
> 	    other language ideas onto forth?

Yes, i think this is a useful construct. It'd be nice too, if it
would "collapse" the now-useless word header to regain some dictionary
space. Of course, it would also have to relocate everything defined after
the target word. Could be tricky!

> 	b)  Can HIDE be written in forth, or must it be written in
> 	    assembly language?

I haven't played with FORTH in a long time, but i'm fairly certain that
it could be used to implement HIDE.

> 	c)  Is there another (easier) way to do this that I am not
> 	    aware of?

You could also corrupt the name field (e.g. blank fill it) to make the
word lexically unrecognizable. I think the pointer-shifting scheme is better
though, as it removes the word from the search stream altogether.

-- 

Pete Yadlowsky      University of Virginia, Academic Computing Center
                    UUCP: decvax!mcnc!ncsu!uvacs!pmy 
                    CSNET: pmy@virginia

"If ya can't stand the heat ... it's too hot!"