Tim Bradshaw <tfb@tfeb.org> wrote:
+---------------
| rpw3@rigden.engr.sgi.com (Rob Warnock) writes:
| > While it is certainly possible to create an editor that guarantees
| > that an initially valid SGML document remains valid after every
| > subsequent allowed editing step, Godel's Proof suggests that there
| > will always be some valid SGML documents which it is impossible
| > to create using such an editor.
|
| I think this is almost certainly false. Really the question is `given
| a finite number of allowed transformations between legal strings of
| some grammar G, can you start with some string A of that grammar and
| get to any string B'. I need to be more clear about what I mean by
| `allowed transformations', but I'll avoid that. I think that for SGML
| grammars that this is likely to be true.
+---------------
Unfortunately, `allowed transformations' *is* the issue, at least where
humans & editors are concerned. The point is *not* proving the validity
of an a priori given text, but starting with *nothing* (well, the SGML
grammar's "start" symbol) and writing something "useful" (whatever you
as a human consider a "useful" end-product to be) given only the `allowed
transformations' in a strict validating editor.
Think of it this way: Suppose you were asked to write a *huge* Lisp
program, but one of the rules was that you had to use an editor
that *forced* your program to be "legal Lisp" (including "validly"
compiling) after every single keystroke. E.g., imagine using Emacs
and putting a hook in the keyboard input processing that ran the
Lisp compiler after every keystroke[*] and if the compiler barfed
it would *undo* the keystroke! Do you actually think you could
do that? [I don't think *I* could!]
-Rob
[*] O.k., to make it easier we'll even allow "fat" keys and macros to
count as "one" keystroke. And maybe even allow a whole Lisp symbol...
-----
Rob Warnock, 8L-855 rpw3@sgi.com
Applied Networking http://reality.sgi.com/rpw3/
Silicon Graphics, Inc. Phone: 650-933-1673
1600 Amphitheatre Pkwy. FAX: 650-933-0511
Mountain View, CA 94043 PP-ASEL-IA