Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!dino!ux1.cso.uiuc.edu!tank!keith@curry.uchicago.edu
From: keith@curry.uchicago.edu (Keith Waclena)
Newsgroups: comp.lang.forth
Subject: FORTH Modules: current confusion?
Summary: what's current @ @?
Keywords: modules; dictionary structure; compiler mods
Message-ID: <6708@tank.uchicago.edu>
Date: 13 Dec 89 21:24:33 GMT
References: <5596@sdcc6.ucsd.edu>
Sender: news@tank.uchicago.edu
Reply-To: keith@curry.uchicago.edu (Keith Waclena)
Organization: TIRA / UofC
Lines: 79

Some background: It's the end of the quarter and I'm about to slip a
little FORTH into a reading course I'm teaching on data encapsulation
in programming languages.  The course has followed a historical
progression from block structure through modules, abstract data types
and finally object-oriented programming.  We read a paper by Friedman
and Felleisen[1] detailing how one can add modules to Scheme with a
page or so of relatively portable code.  Now I thought I'd give my
students a one-page paper by Schorre[2] on how to add modules to FORTH
in 3 short lines of code.

My problem is this: Schorre's code is (I think) fig-FORTH and needs to
be updated to FORTH-83 for the systems I have around.  Here is
Schorre's complete code:

	: internal   current @ @ ;
	: external   here ;
	: module     pfa lfa ! ;

I think I understand how this is intended to work, and here's my
translation to FORTH-83 (using Harris's proposed words for definition
field address conversion):

	: internal   current @ @ ;
	: external   here ;
	: module     n>link ! ;

My problem is that, while Harris's words seem to be a de facto
standard (all my systems have them), this code only works on one
system: Laboratory Microsystems PC/FORTH.  It fails to work under F83
and F83S, and also under F-PC (version 2.something).

Here's an example of the use of these words, to create a global
variable that's private to three routines that use it for
communication:

	internal
	   variable foo
	external
	   : reset   0 foo ! ;
	   : inc     1 foo +! ;
	   : hmm     foo @ . ;
	module

The idea is that foo is only visible to the three words after
external, because module has adjusted the dictionary links to jump
over foo.  And sure enough, under PC/FORTH, the three words can find
foo, though it doesn't show up in the dictionary anymore.

In F83 and F83S, however, the three words can find foo, but it never
vanishes from the dictionary!  Exactly the same thing occurs with
F-PC.  Strangely enough, the system doesn't crash, even though I don't
know where I'm storing that address...

Can anyone tell me what the difference is between these systems that
explains this?  I realize that this sort of dictionary manipulation
isn't really in line with the 83 standard (which I think is too bad),
but I didn't think I'd have this much trouble with such a simple idea.
My two guesses as to where my problem lies are as follows: lack of
understanding of current (what exactly *does* it point to, anyway?
This is not at all clear from the standard document.), or perhaps a
problem with hashed dictionaries?

Thanks in advance,

						Keith

References:

Felleisen, M. and D. P. Friedman.  1986.  ``A Closer Look at Export
and Import Statements''.  *Computer Languages* (11), pp. 29-37.

Schorre, Dewey Val.  1980.  ``Adding Modules to Forth''.  *Proceedings
of the 1980 FORML Conference*, p. 71.


--
Keith WACLENA                             keith@curry.uchicago.edu
CILS / TIRA / U of Chicago                keith%curry@uchimvs1.bitnet
1100 E.57th.St Chi IL 60637 USA           ...!uunet!curry.uchicago.edu!keith