Re: In how many ways should this fail?

From	"BartC" <bc@freeuk.com>
Newsgroups	comp.lang.c
Subject	Re: In how many ways should this fail?
Date	2012-01-31 12:43 +0000
Organization	A noiseless patient Spider
Message-ID	<jg8no4$9de$1@dont-email.me> (permalink)
References	<87hazdcikg.fsf@huddi.jernurt.dk><chine.bleu-61D2CA.08302730012012@news.eternal-september.org><jg6hcq$3mc$1@dont-email.me> <4F26D3FE.4010606@verizon.net><jg6mrf$6it$1@dont-email.me> <4F26E9F6.3020101@verizon.net><jg6r4h$1fh$1@dont-email.me> <0.d2a24acbe4d003562f6e.20120130214854GMT.8739awsvy1.fsf@bsb.me.uk>

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"Ben Bacarisse" <ben.usenet@bsb.me.uk> wrote in message 
news:0.d2a24acbe4d003562f6e.20120130214854GMT.8739awsvy1.fsf@bsb.me.uk...
> "BartC" <bc@freeuk.com> writes:

>> That doesn't make sense at all. The conditional-operator syntax is more
>> like:
>>
>>  expression1 ? expression2 : expression3
>
> That's not how syntax is expressed.  You'd need productions for all of
> expression1, expression2, and expression3.  The C grammar correctly
> encapsulates what is valid (syntactically) and what it not.

OK, so the C standard uses a far more pedantic and unwieldy way of 
expressing syntax than I would, if it has to enforce each of the 17 
precedence levels in the grammar!

(I would write the formal syntax as just:

 expr ? expr : expr

worrying about any restrictions on each expr later, while the production of 
'expr' doesn't itself worry about precedence (which would just be an 
attribute of a binary operator).

I've implemented such a conditional operator for real, using this informal 
approach, and it works fine. Although I do insist in enclosing the thing in 
parentheses.)

>> Anyway, being just above assignment, your would expect a?b:c=d to 
>> evaluate
>> a?b:c just before being assigned d.
>
> Forget about "before" and "after" -- precedence is about parsing, not
> evaluation order.

You can evaluate the left and right sides of the assignment in any order, 
but the assignment itself can only take place after both have been 
evaluated.

> assignment-expression:
>   conditional-expression
>   unary-expression assignment-operator assignment-expression
>
> just says that an assignment expression can be a conditional expression
> on its own.

You have to look further into the Standard to discover that 
'assignment-expression' just means 'any expression'. Whatever the merits of 
defining the grammar in this way, it's not very intuitive; the above 
suggests that any conditional expression is a form of assignment!

-- 
Bartc

Thread

In how many ways should this fail? Anders Wegge Keller <wegge@wegge.dk> - 2012-01-30 16:32 +0100
  Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-30 11:07 -0500
    Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-30 11:11 -0500
    Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-30 17:42 +0000
  Re: In how many ways should this fail? gwowen <gwowen@gmail.com> - 2012-01-30 08:16 -0800
    Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-30 17:35 +0000
      Re: In how many ways should this fail? gwowen <gwowen@gmail.com> - 2012-01-30 23:47 -0800
      Re: In how many ways should this fail? Tim Rentsch <txr@alumni.caltech.edu> - 2012-02-01 00:00 -0800
        Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-02-01 12:14 +0000
          Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-02-01 08:04 -0500
          Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-01 13:31 +0000
        Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-01 12:59 +0000
          Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-02-01 08:37 -0500
            Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-01 16:40 +0000
              Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-02-01 12:13 -0500
                Re: In how many ways should this fail? Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-08 12:49 -0800
    Re: In how many ways should this fail? Anders Wegge Keller <wegge@wegge.dk> - 2012-01-30 19:20 +0100
  Re: In how many ways should this fail? Devil with the China Blue Dress <chine.bleu@yahoo.com> - 2012-01-30 08:30 -0800
    Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-30 16:43 +0000
      Re: In how many ways should this fail? Devil with the China Blue Dress <chine.bleu@yahoo.com> - 2012-01-30 09:17 -0800
      Re: In how many ways should this fail? Kaz Kylheku <kaz@kylheku.com> - 2012-01-30 17:31 +0000
      Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-30 12:31 -0500
        Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-30 18:16 +0000
          Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-30 14:05 -0500
            Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-30 19:30 +0000
              Re: In how many ways should this fail? Keith Thompson <kst-u@mib.org> - 2012-01-30 12:50 -0800
              Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-30 16:17 -0500
              Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-30 21:48 +0000
                Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-31 12:43 +0000
                Re: In how many ways should this fail? Zoltan Kocsi <zoltan@bendor.com.au> - 2012-02-01 00:46 +1100
                Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-31 13:57 +0000
                Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-31 15:05 +0000
                Re: In how many ways should this fail? Devil with the China Blue Dress <chine.bleu@yahoo.com> - 2012-01-31 07:16 -0800
                Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-31 17:37 +0000
                Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-31 11:34 -0500
                Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-31 17:39 +0000
                Re: In how many ways should this fail? Keith Thompson <kst-u@mib.org> - 2012-01-31 19:42 -0800
                Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-02-01 11:57 +0000
                Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-02-01 08:14 -0500
                Re: In how many ways should this fail? Philip Lantz <prl@canterey.us> - 2012-02-01 00:04 -0800
                Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-31 09:05 -0500
                Re: In how many ways should this fail? Devil with the China Blue Dress <chine.bleu@yahoo.com> - 2012-01-31 06:45 -0800
                Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-31 17:34 +0000
                Re: In how many ways should this fail? Keith Thompson <kst-u@mib.org> - 2012-01-31 19:26 -0800
                Re: In how many ways should this fail? Keith Thompson <kst-u@mib.org> - 2012-01-31 19:29 -0800
        Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-30 21:28 +0000
  Re: In how many ways should this fail? timprince <tprince@computer.org> - 2012-01-30 11:44 -0500
    Re: In how many ways should this fail? Kaz Kylheku <kaz@kylheku.com> - 2012-01-30 17:35 +0000
  Re: In how many ways should this fail? Kaz Kylheku <kaz@kylheku.com> - 2012-01-30 17:21 +0000
    Re: In how many ways should this fail? Anders Wegge Keller <wegge@wegge.dk> - 2012-01-30 19:22 +0100
      Re: In how many ways should this fail? 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-30 11:09 -0800
  Re: In how many ways should this fail? jacob navia <jacob@spamsink.net> - 2012-01-30 19:04 +0100
    Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-30 18:21 +0000
      Re: In how many ways should this fail? Jens Gustedt <jens.gustedt@loria.fr> - 2012-01-30 20:15 +0100
        Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-30 19:32 +0000
          Re: In how many ways should this fail? Jens Gustedt <jens.gustedt@loria.fr> - 2012-01-30 21:01 +0100
            Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-30 20:22 +0000
              Re: In how many ways should this fail? Keith Thompson <kst-u@mib.org> - 2012-01-30 12:56 -0800
  Re: In how many ways should this fail? blp@cs.stanford.edu (Ben Pfaff) - 2012-01-30 10:07 -0800

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