Well, here's what the perlop manpage says:

As more readable alternatives to && and || when used for control flow, Perl provides and and or operators (see below). The short-circuit behavior is identical. The precedence of ``and'' and ``or'' is much lower, however, so that you can safely use them after a list operator without the need for parentheses:

    unlink "alpha", "beta", "gamma"
            or gripe(), next LINE;
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With the C-style operators that would have been written like this:

    unlink("alpha", "beta", "gamma")
            || (gripe(), next LINE);
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So in other words, if you said this:

unlink "alpha", "beta", "gamma" || gripe();
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the third argument to unlink would be "gamma" || gripe(); that is, always "gamma" and gripe() would never be evaluated.

'or' is smarter than that. Look at C-style Logical Or and Logical or and Exclusive or on the perlop manpage. It talks a bit about why this happens.

local $_ = "0A72656B636148206C72655020726568746F6E41207473754A";
while(s/..$//) { print chr(hex($&)) }
[download]

$a=$b || $c;
$a=$b or $c;
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$a=($b || $c);
($a=$b) or $c;
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@info = stat($file) || die;     # oops, scalar sense of stat!
@info = stat($file) or die;     # better, now @info gets its due
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$a=$b || $c
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$a=$b or $c
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If you still don't believe me, try this:

$a=0 or 1;
$b=0 || 1;
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Andrew.



>or is almost always what you mean in these cases.

This is largely a matter of programming style, and the nature of the project, not something inherent to Perl. I find myself using || about ten times as often as or.

Because most of my program data is non-zero, I find myself using a lot of things like:

$param = $value || $default;
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OTOH, maybe I am using too many if/else blocks...

 

Paris Sinclair    |    4a75737420416e6f74686572
pariss@efn.org    |    205065726c204861636b6572
I wear my Geek Code on my finger.

I'm not sure what you mean here - you provide a classic example of where or is most certainly not appropriate (as if you use it, $param will never get the value of $default).

That is,

$param = $value or $default;
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Will result in $param always getting the value of $value, never $default. or has lower precedence than = (in fact it has the absolute lowest precedence of all the operators). Maybe it was just me not being clear, though. You should always use 'or' where what you really want to say is:

($param=$value) or die "darn"
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That is, you don't want the value of die to get assigned to $param (yeah, die doesn't return very often. But it forces scalar context, which is why the alternative is a Bad Thing). Using or, your example would have to be:

$param=$value or $param=$default
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So it's not always a matter of style.

Andrew.

open(FH, "filename.txt") || die "$!";
 # OR
open(FH, "filename.txt") or die "$!";
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$a = $b || $c;
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You are right, the functionality is identical in the cases you cite. But || comes with a *lot* of caveats when you use it in circumstances like this. The RHS of or will always be executed last (unless parentheses are used, or there is more than one or). This means if you want to say (psuedo-code):

  if (open fails) { die }
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or gives you a way of writing literally that. || means something a bit different, and can cause considerable confusion. It's a bad habit and a maintainance problem to use the wrong operator for the job. See perlop.

Andrew.

AFAIK, the examples you gave still pose a problem. What if you were to assign the return value of open() to a variable:

$foo = open(FH, "filename.txt") || die $!;
# not the same as
$foo = open(FH, "filename.txt") or die $!;
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but they appear to be the same anyway ;) It's not the parentheses that's doing it, it's just the way it works. It's checking to see if the return of open() is true, and if not, tries to assign the return value of die to $foo, which never happens because it dies.

So it works the same either way, but it's actually doing two different things.

local $_ = "0A72656B636148206C72655020726568746F6E41207473754A";
while(s/..$//) { print chr(hex($&)) }
[download]