What does this statement do: ();

Question

I've seen perl statements that look like this:

() unless $some_var;

What is that intended to achieve?

Answer 1

if executes the statement once if and only if the condition is true.

unless is the opposite, it executes the statement unless the condition is true (i.e., if the condition is false).

see for more detail: http://perldoc.perl.org/perlsyn.html

() is merely the empty list, so this code is providing a default value of the empty list for cases where $some_var is false (probably because $some_var is undefined).

Answer 2

It returns empty list if $some_var is false.

If I write @s = () unless $some_var;, I emptied @s if $some_var is false.

That is the only way this can be used I can think of.

Answer 3

It's probably used in implicit return, if $some_var evaluates to true, return it, otherwise return empty list.

Answer 4

On itself, this expression is a nop. But the value (empty list) can be automatically returned from a subroutine or do() block, if there's no explicit return statement and this value happens to be the last thing in the block.

Answer 5

There's a couple of things going on here, but most significant is the fact that this sort of line would usually appear as the last line of an executed block[1]. In that context, the value of the block is the value of the last executed statement[2]. Now a statement containing unless has the value of the left-hand side if the right-hand side is true and the value of the right-hand side otherwise, so this is a way of saying "the value of the block is $some_var if $some_var happens to be true, () otherwise".

() is merely the empty list, so this code is providing a default value of the empty list for cases where $some_var is false (probably because $some_var is undefined.)

It is more usual to achieve the same effect with or, as in the equivalent

$some_var or ();

Put together, we can see these cases:

use strict;
use warnings;

my @x = do { () unless 0 };
my @y = do { () unless 5 };

print "\@x: <@x>\n\@y: <@y>\n";

@x is the empty array because 0 is false, whereas @y is an array containing 5 because 5 is true.

Anywhere other than as the last statement of a block, () unless $some_var; is a no-op. It does suppress the once warning if $some_var is not otherwise mentioned, though.

[1]: That's a block evaluated by eval or do or supplied to (for example) map or grep, or a block called as a subroutine.

[2]: "The value of the last executed statement" is a weird phrase; mostly only expressions have values. But because Perl includes the concept that in the absence of an explicit return the value of a block is the value of the last statement, Perl needs this weird phrase.

Answer 6

Mystery solved:

GetOptions(
    "-interactive",
    "-request_file=s",
) or usage();

usage unless ($opt_request_file);
() unless ($opt_interactive);

It seems the '();' is misleading - it does nothing, and the purpose is just to mention the $opt_interactive variable once... as a way to suppress the perl warning 'Name "$var" used only once'.

Thanks to all who tried to shed some light!