What is the difference between Perl's ( or, and ) and ( ||, && ) short-circuit operators?

Question

Which of these subroutines is not like the other?

sub or1 {
    my ($a,$b) = @_;
    return $a || $b;
}

sub or2 {
    my ($a,$b) = @_;
    $a || $b;
}

sub or3 {
    my ($a,$b) = @_;
    return $a or $b;
}

sub or4 {
    my ($a,$b) = @_;
    $a or $b;
}

I came to Perl 5 from C and Perl 4 and always used || until I saw more scripts using or and I liked the way it looked. But as the above quiz shows, it's not without its pitfalls for the unwary. For people who use both constructs or who use a lot of or, what rules of thumb do you use to decide which construct to use and make sure the code is doing what you think it is doing?

Answer 1

My guess is that or3 is different.

I'm not really a Perl guy, but it looks like 1, 2, and 4 all explicitly return booleans. I'm guessing 3 has side effects, such as returning $a or something crazy like that.

looks down

Hey, I was right.

Answer 2

Both versions are short-circuiting in Perl, but the 'textual' forms ('and' and 'or') have a lower precedence than their C-style equivalents.

http://www.sdsc.edu/~moreland/courses/IntroPerl/docs/manual/pod/perlop.html#Logical%5FAnd

Answer 3

In Perl 5, "or" and "and" have lower precedence than "||" and "&&". Check out this PerlMonks thread for more info:

http://www.perlmonks.org/?node_id=155804

Answer 4

Due to the low precedence of the 'or' operator, or3 parses as follows:

sub or3 {
    my ($a,$b) = @_;
    (return $a) or $b;
}

The usual advice is to only use the 'or' operator for control flow:

@info = stat($file) or die;

For more discussion, see the perl manual: http://www.perl.com/doc/manual/html/pod/perlop.html#Logical%5For%5Fand%5FExclusive%5FOr

Answer 5

What rules of thumb do you use to decide which construct to use and make sure the code is doing what you think it is doing

The operator precedence rules.

|| binds tightly, or binds weakly. There is no "rule of thumb".

If you must have a rule of thumb, how about "only use or when there is no lvalue":

or:

open my $fh, '>', 'file' or die "Failed to open file: $!"

||:

my $greeting = greet() || $fallback || 'OH HAI';

I agree with MJD about avoiding parens; if you don't know the rules, look them up... but don't write (open(my $fh, '>', 'file')) or (die("Failed to open file: $!")) "just to be sure", please.