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Path: utzoo!dciem!aquila!chris
From: chris@aquila.UUCP (chris)
Newsgroups: net.lang.lisp
Subject: Re: A BNF for Lisp
Message-ID: <63@aquila.UUCP>
Date: Mon, 18-Nov-85 11:28:49 EST
Article-I.D.: aquila.63
Posted: Mon Nov 18 11:28:49 1985
Date-Received: Mon, 18-Nov-85 16:10:30 EST
References: <60@aquila.UUCP> <68@nbs-amrf.UUCP>
Reply-To: chris@aquila.UUCP ()
Lines: 36
Summary: 

I never though a BNF for Lisp would generate so much Net traffic!
As some people have commented, the grammar I posted cannot handle
S-expressions of the form ( a . b . c ) -- changes were posted to do this.

Remember that what I posted was a BNF description of a grammar! A BNF
deals with tokens and productions; it is not concerned with lexical analysis,
or even Lisp read macros! An ATOM is any non-list Lisp object; it can
be an integer, a string, even an escaped symbol name (which may contain
suitably quoted parantheses, like |this (is an) atom|. Non-lisp programmers
would certainly find this last example confusing (imagine a C variable
named 'if (x > 0) exit(1)'), but the intent is clear to Lisp programmers.
hopp@nbs-amrf.UUCP (Ted Hopp) has been chasing a red herring when he
expects a grammar description to deal with issues of scanning as well as
parsing; he further confuses himself with premature concern for evaluation
of the read form. The BNF is not concerned with EVALUATION -- if you
don't understand this, then go away and read a compiler book.

Read macros can safely be ignored by the BNF because they ARE macros;
the Lisp reader still gets (after macro processing) a valid sequence of
tokens that is handled by the BNF. That is, 'A ==> (quote A). (Q.E.D.)
(And don't tell me about back-quote; `(a b ,c ,@d ...) is an EVALUATION
mechanism. The Reader produces something like (back-quote (a b (comma a)
(comma-at d) ...)), which is a valid form for later EVALUATION).

Note that just because Lisp places the read macro
handling in the reader does not mean it has to be there; as C programmers
know, macro scanning can be done in another program entirely so long
as valid code is produced. Of course, the Lisp macros are more powerful
because they ARE in the language; however, this greater power is rarely used.
An example would be a read macro that generates different code depending
on the value of some other Lisp symbol; such a macro would be confusing
to debug and dangerous to use in most cases. You can write one if you want
to try convince me of its utility.

		Chris Retterath ( ..!dciem!aquila!chris )
		Consensys Corporation.