precedence

[Alan Stewart]

~sdpm~
On 10 Mar 00, at 14:01, C. Abney wrote:

[. . .]

> but I'm so brain dead that knowing 'or' is evaled left to right
> isn't enough for me to be sure that its low precedence won't be
> trumped by that of the 'unless'.  Too, 'unless' isn't listed in
> the list of operators in which their precedence is scaled against
> each others (that's 'perldoc perlop'.)  I had to test it.  I used
> the following script:

unless isn't in the list of operator precedence because it isn't an operator, it is a statement 
modifier.

> 
> 	(2)[2136]$ cat or_unless.pl
> 	#! /usr/bin/perl
> 	$FOO = shift;
> 	$ERR = shift;
> 	$ERR or warn "gah!\n" unless $FOO;
> 
> Which, when executed, prints (or doesn't) the following:
> 
> 	(2)[2136]$ ./or_unless.pl 1 1
> 	(2)[2136]$ ./or_unless.pl 1 0
> 	(2)[2136]$ ./or_unless.pl 0 1
> 	(2)[2136]$ ./or_unless.pl 0 0
> 	gah!
> 	(2)[2137]$ 
> 
> Unless I interpreted it wrong, I would have gotten a warning in the
> second test if the 'unless' took precedence.  And I shouldn't have
> had /any/ warnings in the third and fourth tests.
> 

The unless doesn't bind left or right, it modifies the entire preceeding statement.

   $ERR or warn "gah!\n" unless $FOO;

is the same as

   unless ($FOO) {$ERR or warn "gah!\n"}

which is the same as

   if (!$FOO) {$ERR or warn "gah!\n"}

so only the fourth test should print a warning.
In your other example

	system ( "$cmd" ) == 0
		or warn "failed remote copy: $!" unless $QUIET;

I would expect $QUIET to control the whole statement, as in

     unless ($QUIET) {
	system ( "$cmd" ) == 0
		or warn "failed remote copy: $!";
     }

and Perl does what I expect :)

---------------------------------------------------------------
Alan Stewart          )-[]-(           Electronics Engineer
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~sdpm~

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