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From: toma@tekgvs.UUCP (Thomas Almy)
Newsgroups: net.lang.forth
Subject: Re: Information Hiding in FORTH
Message-ID: <1626@tekgvs.UUCP>
Date: Wed, 16-Jul-86 10:03:41 EDT
Article-I.D.: tekgvs.1626
Posted: Wed Jul 16 10:03:41 1986
Date-Received: Fri, 18-Jul-86 04:29:06 EDT
References: <12110@amdcad.UUCP> <622@uvacs.UUCP> <242@iscuva.UUCP>
Reply-To: toma@tekgvs.UUCP (Thomas Almy)
Organization: Tektronix, Inc., Beaverton, OR.
Lines: 46

In article <242@iscuva.UUCP> jimc@iscuva.UUCP (Jim Cathey) 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; something like:
>
>I believe map-Forth (Michael A Perry's version for my old CompuPro 68000)
>included the words:
>
>INTERNAL
> <<boring and/or dangerous words to be hidden here>>
>EXTERNAL
> <<the visible stuff>>
>MODULE     (actually does the work)
>
>I recall that both INTERNAL and EXTERNAL merely left addresses on the 
>data stack, and that MODULE patched the vocabulary search chain to
>miss the internals.  Woe be unto him whose internal or external definitions
>which (on purpose or not) swallowed any of the data stack or left any extra!

Which caused me to define these as so:

: INTERNAL  LATEST  12543 ;

: EXTERNAL  12543 <> ABORT" Where is INTERNAL?"
            LATEST N>LINK 
            25314 ;

: MODULE    25314 <> ABORT" Where is EXTERNAL?"
            ! ;


(This code will not work for Laxen & Perry F83 because of the hashed 
 vocabularies)

My current favorite technique (because it allows referencing the hidden words)
is a function which moves a word into a different vocabulary.

FooVoc DEFINITIONS BarVoc  REVOC FooBar

moves word FooBar from vocabulary BarVoc to vocabulary FooVoc.

The implementation is straightforward, but is very system dependent.

Tom Almy
Tektronix