Groups > comp.lang.c > #162020

Re: How to disambiguate macro?

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c
Subject	Re: How to disambiguate macro?
Date	2021-07-21 16:28 +0200
Organization	A noiseless patient Spider
Message-ID	<sd9au6$m5$1@dont-email.me> (permalink)
References	(5 earlier) <sd8r8p$fph$1@dont-email.me> <sd8rra$f4o$1@reader1.panix.com> <87k0ljlvsu.fsf@bsb.me.uk> <8735s7ls5z.fsf@bsb.me.uk> <sd9a5q$qs0$1@dont-email.me>

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On 21/07/2021 16:15, Bart wrote:
> On 21/07/2021 14:20, Ben Bacarisse wrote:
>> Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
>>
>>> Neither B nor BCPL had short-circuit logical operators, relying instead
>>> on bitwise & and |, as did early C.
>>
>> Minor correction.  B didn't have short-circuit & and | as such, but it
>> had a weird rule that in conditional contexts like
>>
>>    if (a & b) ...
>>
>> the operator short-circuited!  C rightly ditched this contextual
>> interpretation in favour of explicitly short-circuiting operators.
>>
> 
> That's not so weird. I used to have something similar (not in C) where
> 'A and B' (ie. logical and, also 'or') short-circuited in conditional
> expressions as used in 'if' and 'while' control expressions, which
> tended to be implemented with branching.
> 
> But in ordinary expressions, they didn't.

It is perhaps not /weird/, but it is highly confusing and risky in the
face of side-effects.  It is a seriously bad idea.

Borland Pascal (and Delphi) let you choose to make "and" (and "or")
short-circuiting as an optimisation option, controllable locally with
the Pascal equivalent of pragmas.  Again, it is a dangerous idea.

If a language allows side-effects in expressions and sub-expressions,
then the programmer needs to know /exactly/ when they will be executed.
 It's fine to say "a && b" is /always/ short-circuiting, and fine to say
it is /never/ short-circuiting - you know either way.  It is a terrible
idea to say it is /sometimes/ short-circuiting, depending on how the
expression is used.

Of course, if "b" has no side-effects then it might be good for the
compiler to short-circuit it regardless of the language rules - but
that's just optimisation and efficiency, not a matter of language
semantics and program correctness.

> 
> I now have and/or operators that always short-circuit, but I did briefly
> have versions that never short-circuited. I called these 'andb' and
> 'orb' (the 'b' meaning evaluate both). However I couldn't think of
> enough use-cases for them.
> 
> Evaluation of the second term can usually be forced when needed.

Thread

How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-20 11:15 +0000
  Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-20 13:49 +0100
    Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-20 12:56 +0000
      Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-20 15:36 +0100
        Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-21 01:48 +0000
          Re: How to disambiguate macro? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-07-20 19:18 -0700
          Re: How to disambiguate macro? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-07-20 20:27 -0700
            Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-21 04:15 +0000
              Re: How to disambiguate macro? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-07-21 02:00 -0700
              Re: How to disambiguate macro? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-07-21 06:02 -0400
          Re: How to disambiguate macro? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-07-21 06:00 -0400
            Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-21 10:10 +0000
              Re: How to disambiguate macro? Bart <bc@freeuk.com> - 2021-07-21 11:30 +0100
              Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-21 13:31 +0200
              Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-21 13:02 +0100
                Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-21 14:20 +0100
                Re: How to disambiguate macro? Bart <bc@freeuk.com> - 2021-07-21 15:15 +0100
                Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-21 16:28 +0200
                Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-21 16:20 +0100
                Re: How to disambiguate macro? Bart <bc@freeuk.com> - 2021-07-21 17:34 +0100
                Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-21 19:46 +0200
                Re: How to disambiguate macro? Manfred <noname@add.invalid> - 2021-07-21 20:42 +0200
                Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-22 08:37 +0200
                Re: How to disambiguate macro? Bart <bc@freeuk.com> - 2021-07-21 20:47 +0100
                Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-22 08:47 +0200
                Re: How to disambiguate macro? Bart <bc@freeuk.com> - 2021-07-22 11:31 +0100
                Re: How to disambiguate macro? antispam@math.uni.wroc.pl - 2021-07-22 14:37 +0000
                Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-22 18:15 +0200
                Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-21 21:19 +0100
                Re: How to disambiguate macro? Manfred <noname@add.invalid> - 2021-07-21 15:42 +0200
                Re: How to disambiguate macro? scott@slp53.sl.home (Scott Lurndal) - 2021-07-21 15:39 +0000
              Re: How to disambiguate macro? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-07-21 09:22 -0700
              Re: How to disambiguate macro? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-07-21 17:24 -0400
                Re: How to disambiguate macro? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-07-21 16:42 -0700
                Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-22 05:49 +0000
                Re: How to disambiguate macro? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-07-22 19:11 -0400
                Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-22 05:30 +0000
                Re: How to disambiguate macro? scott@slp53.sl.home (Scott Lurndal) - 2021-07-22 14:14 +0000
                Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-22 17:06 +0100
                OT: possessive adjectives (Was: How to disambiguate macro?) Manfred <noname@add.invalid> - 2021-07-22 15:04 +0200
                Re: OT: possessive adjectives Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-14 13:37 -0700
          Re: How to disambiguate macro? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-14 13:58 -0700
  Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-20 14:50 +0200
    Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-20 13:19 +0000
      Re: How to disambiguate macro? Bart <bc@freeuk.com> - 2021-07-20 14:40 +0100
        Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-20 21:02 +0200
      Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-20 21:00 +0200
      Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-21 08:15 +0200
      Re: How to disambiguate macro? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-14 23:17 -0700
  Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-20 12:51 +0000
  Re: How to disambiguate macro? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-07-20 08:47 -0700

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