Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!uflorida!haven!adm!smoke!gwyn
From: gwyn@smoke.BRL.MIL (Doug Gwyn)
Newsgroups: comp.lang.c
Subject: Re: precedence of && (was: precedence of ?:)
Message-ID: <11054@smoke.BRL.MIL>
Date: 13 Sep 89 18:38:19 GMT
References: <1265@gmdzi.UUCP> <11030@smoke.BRL.MIL> <11039@smoke.BRL.MIL> <3236@solo10.cs.vu.nl> <11045@smoke.BRL.MIL> <3242@solo12.cs.vu.nl>
Reply-To: gwyn@brl.arpa (Doug Gwyn)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 34

In article <3242@solo12.cs.vu.nl> maart@cs.vu.nl (Maarten Litmaath) writes:
>gwyn@smoke.BRL.MIL (Doug Gwyn) writes:
>\... there is no legal way to parse
>\	0 && i = 0
>\as
>\	(0 && i) = 0
>\but there is a legal parse as
>\	0 && (i = 0)

Actually, now that I've gotten back to my desk where I keep a copy of
the proposed Standard, I find that you must supply the parentheses to
get a legal parse.  Without them there is no derivation from the
grammar production rules.  (I think.  I keep getting this wrong.)

>gcc (ANSI or what?) does accept
>	0 ? 0 : i = 0
>but it does NOT accept
>	0 && i = 0
>In fact, I've never used a C compiler that accepted the latter construct.
>Of course I fully agree it should be accepted.

Maybe the intention is to avoid confusion.  For example,
	i = 0 && i = 0
would either have to have an ambiguous parse, or else its interesting
subexpression would be parsed differently depending on context, which
is confusing.

I seem to recall Dave Prosser telling me that the Standard's grammar
for C (apart from the preprocessor) constituted a "phrase structure"
grammar.  Perhaps if I knew what that meant I'd understand the
rationale behind these particular expression parsing rules.

It does appear that the current rules avoid parsing ambiguity.  That's
probably a worthwhile constraint.