Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!ucsd!ucbvax!MITCH.ENG.SUN.COM!wmb
From: wmb@MITCH.ENG.SUN.COM
Newsgroups: comp.lang.forth
Subject: CATCH and THROW
Message-ID: <9009220055.AA13759@ucbvax.Berkeley.EDU>
Date: 21 Sep 90 20:17:58 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: wmb%MITCH.ENG.SUN.COM@SCFVM.GSFC.NASA.GOV
Organization: The Internet
Lines: 26

It is my belief that CATCH & THROW were the only possible way of getting
exception handling into ANS Forth.  The reason:

Control structures are a subject of much controversy.  They are difficult
to precisely specify (proof: the discussions about how to properly describe
them spanned many many meetings, and caused much heated debate).

In fact, there is pretty much a one-to-one correspondence between
non-postfix constructs and hard-to-specify constructs.

Any exception handling solution that involves a new form of syntactic
control structure was certain to fail, because it would have required
the rethinking and the rewriting of so much of the other control structure
text.  For instance, the interactions between CATCH1 and all other control
structures would have to be specified (i.e. you can't use CATCH1 inside
a loop), as well as its return stack restrictions, and also there would
have to be a decision about whether it is immediate or not and whether or
not it is defined for interpret state and what happens when you POSTPONE
it.

This is the can of worms that one opens when one considers the addition
of a new non-postfix control structure to a Forth standard.

CATCH and THROW, being "pure postfix", avoids all these problems.

Mitch Bradley, wmb@Eng.Sun.COM