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Message-ID: <1996520.IoEltsQCmd@PointedEars.de>
From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Reply-To: Thomas 'PointedEars' Lahn <php@PointedEars.de>
Organization: PointedEars Software (PES)
Date: Tue, 25 Feb 2014 01:37:26 +0100
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Subject: Re: Operator precedence
Newsgroups: comp.lang.php
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Christoph Michael Becker wrote:

> Thomas 'PointedEars' Lahn wrote:
>> Ben Bacarisse wrote: 
>>> Thomas 'PointedEars' Lahn <PointedEars@web.de> writes:
>>>> Ben Bacarisse wrote:
>>>>> […] Expressions are built from operators, but is ',' an operator in
>>>>> [PHP?
>>>>> It appears in the operator precedence table but it isn't an operator
>>>>> in an sense that would normally be understood by someone familiar with
>>>>> these terms.
>>>>
>>>> ECMAScript has a “Comma Operator”, too.
>>>
>>> But my point was the PHP doesn't have one --
>> 
>> But it does:
>> 
>>   for ($i = 0, $j = 42; $i < $j; ++$i);
>>              ^
> 
> The comma is used in sevaral places in the PHP grammar[1].  However,
> IMHO your example (inside a for_expr) is the only case where the comma
> is used as an operator.  The other uses are for declaration and
> parameter lists (spoken in a broad sense).

I am aware of that.
 
>>> not in any normal sense of the word.
>> 
>> Please define what you consider to be an operator in a “normal sense of
>> the word”.
> 
> It might be defined as an operator that can be used in arbitrary
> expressions.

A speculation, one that is almost self-evidently false.  Just consider the 
operators (including, but not just in, programming languages) that require 
their operands to be of a certain type.

> Consider the following ECMAScript expression:
> 
>   a = 1, b = 2;
> 
> The "equivalent" expression in PHP is a syntax error:
> 
>   $a = 1, $b = 2;

I am aware of that.
 
>>> ECMAScript really does have one (like C and C++ do).
>> 
>> Why do you think C and C++ have a comma operator (like ECMAScript)?
> 
> According to the C grammar[2]:
> 
>   exp  : assignment_exp
>        | exp ',' assignment_exp
> 
> [1] <http://lxr.php.net/xref/PHP_5_5/Zend/zend_language_parser.y>
> [2] <http://www.cs.man.ac.uk/~pjj/bnf/c_syntax.bnf>
 
ACK; I stand corrected.  For reasons I cannot explain (probably it is just 
late), I had assumed that the comma operator in ECMAScript also worked with 
assignments, and this is what I tested with C and C++:

$ cat foo.c
#include <stdio.h>

int main() {
  int x = 42, 23;
  printf("%i\n", x);
  return 0;
}

$ gcc -Wall -o foo.out foo.c
foo.c: In function ‘main’:
foo.c:4:15: error: expected identifier or ‘(’ before numeric constant
   int x = 42, 23;
               ^

$ cat foo.cpp
#include <iostream>
using namespace std;

int main() {
  int x = 42, 23;
  cout << x << endl;
  return 0;
}

$ g++ -Wall -o foo.out foo.cpp  2>&1| tee /tmp/v6wTKDF/0
foo.cpp: In function ‘int main()’:
foo.cpp:5:15: error: expected unqualified-id before numeric constant
   int x = 42, 23;
               ^

However, in ECMAScript:

| >>> var x = 42, 23;
| SyntaxError: Unexpected number

What I meant instead was

| >>> 42, 23
| 23

which does work like that in certain contexts in C and C++, but not in PHP:

$ cat foo.c
#include <stdio.h>

int main() {
  printf("%i\n", (42, 23));
  return 0;
}

$ gcc -Wall -o foo.out foo.c  2>&1| tee /tmp/vJ0RqVR/0
foo.c: In function ‘main’:
foo.c:4:21: warning: left-hand operand of comma expression has no effect
[-Wunused-value]
   printf("%i\n", (42, 23));
                     ^

$ ./foo.out 
23

$ cat foo.cpp
#include <iostream>
using namespace std;

int main() {
  cout << (42, 23) << endl;
  return 0;
}

$ g++ -Wall -o foo.out foo.cpp
foo.cpp: In function ‘int main()’:
foo.cpp:5:16: warning: left operand of comma operator has no effect
[-Wunused-value]
   cout << (42, 23) << endl;
                ^

$ ./foo.out 
23

$ php -r 'echo (42, 23) . "\n";'
PHP Parse error:  syntax error, unexpected ',' in Command line code on line 
1

[Interesting that gcc says “operand” and “comma *expression*”, while g++ 
says “operand” and “comma *operator*”.]


PointedEars
-- 
Sometimes, what you learn is wrong. If those wrong ideas are close to the 
root of the knowledge tree you build on a particular subject, pruning the 
bad branches can sometimes cause the whole tree to collapse.
  -- Mike Duffy in cljs, <news:Xns9FB6521286DB8invalidcom@94.75.214.39>