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From: rmc@teddy.UUCP
Newsgroups: net.lang.forth
Subject: Re: Virtues of Forth
Message-ID: <1894@teddy.UUCP>
Date: Tue, 7-Jan-86 14:42:47 EST
Article-I.D.: teddy.1894
Posted: Tue Jan  7 14:42:47 1986
Date-Received: Wed, 8-Jan-86 20:36:52 EST
References: <681@pucc-j> <187@aero.ARPA>
Reply-To: rmc@teddy.UUCP (R. Mark Chilenskas)
Organization: GenRad, Inc., Concord, Mass.
Lines: 51
Keywords: AI, Scoping, Garbage Collection, FORTH, POP-11
Summary: 


	I have also been interested in FORTH as an AI-ish language for
small computers.  In fact, the English have used a stack oriented language
called POP for some time now (see for example POPLOG, a system containing
an editor, several utilities, POP-11, Common Lisp and Prolog.  POP-11 is
used as the implementation language for the other parts of the environment,
and the stack architecture makes the whole system mildly portable.
University of Sussex is the originator, System Designers Software is the US
distributor, and more information is also available in Pop-11, a book by
Barrett, Ramsay and Sloman.  I don't know the publisher of the book but SDS
can probably give you order information).  

	There are two moderately large problems with FORTH per se as an AI
language.  The first is scoping, the second is garbage collection.
Together they make it rather hard to manage the large programs associated
with AI techniques.

	The two scoping techniques used in Lisp are dynamic and lexical
scoping.  Dynamic scoping means that the most recent value of a procedure
or variable gets used, regardless of who set the variable.  Lexical scoping
(Algol or C or Pascal technique) says that you can only set variables
within your own routine, or those containing you if you look at a printed
representation of the program.  It turns out that dynamic scoping is
simplest for implementing a naive interpreter, but that dynamic scoping has
a number of advantages for avoiding name space conflicts and object
oriented programming.

	FORTH scoping is sort of funny.  I guess the best way to
characterize it would be as dynamic scoping at compile / word definition
time rather than at execution time.  This means that you can't easily
change the definition of a word and have the new definition take effect in
existing words.  This makes many of the ideas of using programs to build
themselves (ie, putting knowledge in procedures) somewhat difficult to
handle.  

	A more important limitation is the absence of garbage collection.
One of the important techniques in AI is to try to solve a problem and, if
that solution fails, try again (ie, use backtracking).  The structures
describing the problem (schedule of actions in a robot plan, parse tree for
a sentence, what have you) are large, and some of the structure is re-used
in subsequent attemtpts to solve the problem.  In FORTH, all your data ends
up on the stack and in the dictionary, making recovering the space for
unneeded data extremely complicated.

	So while there are interesting parallels between FORTH and Lisp,
and attempts to use the stack architecture as a "portable engine" for
implementing AI languages, it seems that FORTH has some limitations that
make it awkward for supporting large, dynamic programs.

					R Mark Chilenskas
					decvax!genrad!panda!rmc