Path: utzoo!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 && (was: precedence of ?:))
Message-ID: <11082@smoke.BRL.MIL>
Date: 16 Sep 89 23:41:14 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> <11054@smoke.BRL.MIL> <3260@solo5.cs.vu.nl> <11069@smoke.BRL.MIL> <3269@solo5.cs.vu.nl>
Reply-To: gwyn@brl.arpa (Doug Gwyn)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 25

In article <3269@solo5.cs.vu.nl> maart@cs.vu.nl (Maarten Litmaath) writes:
>\	conditional-expression:
>\		logical-OR-expression
>\		logical-OR-expression ? expression : conditional-expression
>\So the interesting question becomes, why is that "expression" for the
>\second operand of ?: and not "logical-OR-expression".  (I think that
>\"conditional-expression" might result in ambiguity.)  [...]
>I don't think so: a ? b ? c : d : e has only one parse.

I had thought perhaps if you got enough interlaced ?: pairs that they
might be parsed ambiguously.  However (see below), if that were the
case then the Standard would have the same problem.  After doing some
"yacc" experiments with ?: grammars for a while, I found that it's not
possible to parse them in the wrong order even under
logical-OR-expression ? conditional-expression : conditional-expression
However,
conditional-expression ? logical-OR-expression : conditional-expression
or
conditional-expression ? conditional-expression : conditional-expression
would lead to an ambiguous grammar.

>(If "expression" is OK, "conditional-expression" surely is too!)

Yes, "expression" can be expanded to produce "conditional-expression".
The more general form in the Standard appears to not introduce ambiguity.