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Re: Macro Expansion

From Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups comp.lang.c
Subject Re: Macro Expansion
Date 2017-01-08 01:37 +0000
Organization A noiseless patient Spider
Message-ID <8737gunxgz.fsf@bsb.me.uk> (permalink)
References <o4rcbl$4cp$1@dont-email.me> <o4ri5t$pqj$1@dont-email.me> <874m1appnx.fsf@bsb.me.uk> <o4rr80$pj7$1@dont-email.me>

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BartC <bc@freeuk.com> writes:

> On 07/01/2017 20:42, Ben Bacarisse wrote:
>> BartC <bc@freeuk.com> writes:
>
>>> I wish they would just state all this stuff instead of leaving people
>>> to either deduce it or eventually discover it for themselves.
>>
>> "They" (presumably the C committee) have an obligation to be precise
>> rather than explanatory.  It's the various books and tutorials about C
>> that should explain this in simpler terms.  If no such thing exists -- a
>> lot of C tutorials are either overly simplistic or wrong -- then it's
>> not the fault of the C committee.
>
> Who was responsible for standardising such a complicated macro
> language? I can't believe most C programmers are aware of all arcane
> details.

That's a rhetorical question, I assume.

Macro languages are notoriously hard to understand unless they are
hopelessly under-powered.  Some may even be very simple to specify (TRAC
for example) but the consequences of the definition can be
extraordinarily complex.

<snip>
>> ...  A test:
>>
>> #define X Y
>> #define Y T
>> #define T X Y
>>
>> <T> => <X><Y>
>>        <X> => <Y>
>>               <Y> => <T>
>>           <Y> => <T>
>
> This introduces new problems. Just take this:
>
>  #define X T
>  #define T X
>  T
>
> How does it manage to not keep expanding indefinitely? The simplest
> case is taken care of:
>
>  #define T T
>
> by being explicitly mentioned (somewhere or other). For the T=>X, X=>T
> example, I can remember a 'topmost' macro name, and avoid expanding
> that even deep inside an unrelated macro expansion. But with this
> tweak of your example:
>
>  #define X Y
>  #define Y X
>  #define T X Y

BTW, simpler and easier to see what happens would be

#define X (Y)
#define Y [X]
#define T {X}
T

> Now it's X=>Y, Y=>X that threatens to go on forever. I can't add these
> names to the list of names that have been expanded, as that is local
> to each level of macro. I might need to propagate a whole sequence of
> 'topmost' macro names to each level.

Expansions are nested rather than iterated, so you just need to be able
to find what expansion you are in and what expansion that expansion is
in, and so on up.  Passing some sort of parent pointer down into each
sub-expansion will automatically give you a chain of "active" macros.

<snip>
> Yikes. I guess whoever devised this scheme didn't think too much about
> efficiency in implementing it.

I'm not sure you can say that unless you are sure you've got the right
interpretation of the rules and a good design for implementing them.
Again, you may be right and it's hell to implement, but I would not be
confident in saying that.

<snip>
-- 
Ben.

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Thread

Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 18:35 +0000
  Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-01-07 11:23 -0800
    Re: Macro Expansion Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-07 21:28 +0000
  Re: Macro Expansion Siri Cruise <chine.bleu@yahoo.com> - 2017-01-07 11:45 -0800
    Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 20:16 +0000
      Re: Macro Expansion Siri Cruise <chine.bleu@yahoo.com> - 2017-01-07 12:23 -0800
        Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 20:31 +0000
          Re: Macro Expansion Siri Cruise <chine.bleu@yahoo.com> - 2017-01-07 12:42 -0800
          Re: Macro Expansion Ian Collins <ian-news@hotmail.com> - 2017-01-08 14:42 +1300
            Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 13:18 +0000
  Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 20:15 +0000
    Re: Macro Expansion Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-07 20:42 +0000
      Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 22:50 +0000
        Re: Macro Expansion Ian Collins <ian-news@hotmail.com> - 2017-01-08 11:59 +1300
          Re: Macro Expansion Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-01-07 15:10 -0800
        Re: Macro Expansion supercat@casperkitty.com - 2017-01-07 17:24 -0800
          Re: Macro Expansion Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-15 21:55 -0800
            Re: Macro Expansion supercat@casperkitty.com - 2017-01-16 09:06 -0800
              Re: Macro Expansion Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-31 08:51 -0800
                Re: Macro Expansion supercat@casperkitty.com - 2017-01-31 13:06 -0800
                Re: Macro Expansion Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-03 07:49 -0800
                Re: Macro Expansion supercat@casperkitty.com - 2017-02-03 12:03 -0800
                Re: Macro Expansion Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-09 09:00 -0800
        Re: Macro Expansion Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-08 01:37 +0000
          Re: Macro Expansion Jean-Marc Bourguet <jm@bourguet.org> - 2017-01-08 10:22 +0100
  Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-01-07 12:18 -0800
    Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 20:28 +0000
      Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-01-07 12:36 -0800
        Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 22:59 +0000
  Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-01-08 00:20 -0800
    Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 13:30 +0000
      Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-10 14:00 +0000
        Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-01-10 07:43 -0800
          Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-10 17:47 +0000
            Re: Macro Expansion Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-10 22:49 +0000
              Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-10 23:28 +0000
              Re: Macro Expansion "James R. Kuyper" <jameskuyper@verizon.net> - 2017-01-10 18:51 -0500
                Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-11 00:06 +0000
          Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-10 20:30 +0000
            Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-04-26 10:04 -0700
              Re: Macro Expansion Keith Thompson <kst-u@mib.org> - 2017-04-26 12:00 -0700
                Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-04-26 12:24 -0700
              Re: Macro Expansion bartc <bc@freeuk.com> - 2017-04-26 20:12 +0100
                Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-04-26 12:34 -0700
                Re: Macro Expansion jameskuyper@verizon.net - 2017-04-26 13:46 -0700
  Re: Macro Expansion Jean-Marc Bourguet <jm@bourguet.org> - 2017-01-08 10:13 +0100
    Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 13:10 +0000
      Re: Macro Expansion Jean-Marc Bourguet <jm@bourguet.org> - 2017-01-08 15:51 +0100
        Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 17:17 +0000
          Re: Macro Expansion supercat@casperkitty.com - 2017-01-08 11:04 -0800
            Re: Macro Expansion Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-01-08 11:10 -0800
              Re: Macro Expansion supercat@casperkitty.com - 2017-01-08 14:08 -0800
            Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 19:49 +0000
              Re: Macro Expansion supercat@casperkitty.com - 2017-01-08 14:17 -0800
          Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 19:19 +0000
          Re: Macro Expansion Jean-Marc Bourguet <jm@bourguet.org> - 2017-01-08 21:43 +0100
      Re: Macro Expansion supercat@casperkitty.com - 2017-01-08 10:43 -0800
      Re: Macro Expansion Keith Thompson <kst-u@mib.org> - 2017-01-08 12:39 -0800
  Re: Macro Expansion antispam@math.uni.wroc.pl - 2017-01-08 22:54 +0000
    Re: Macro Expansion "James R. Kuyper" <jameskuyper@verizon.net> - 2017-01-08 18:36 -0500
      Re: Macro Expansion antispam@math.uni.wroc.pl - 2017-01-09 01:15 +0000
        Re: Macro Expansion "James R. Kuyper" <jameskuyper@verizon.net> - 2017-01-08 20:49 -0500
    Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 23:53 +0000
      Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-09 01:11 +0000
      Re: Macro Expansion antispam@math.uni.wroc.pl - 2017-01-09 01:31 +0000
        Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-10 11:28 +0000

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