Dropping the ampersand

[throop]

Brethern

Until recently, I've written calls to my subroutines with an ampersand, e.g.

@livesys = sort &Con::liveones;

But Perl Best Practices (p 175) sez Call subroutines with parentheses but without a leading &

I'm running a decrepit Perl, for reasons beyond my control - (5.0 patchlevel 4 subversion 4) and this may be part of my problem, but when I write

@livesys = sort Con::liveones();

I get an empty array. I can work around this (starting with 'If its not broke dont fix it' - leaving the code alone. Or putting the result in a temporary variable, etc.) But what's the Best Practice way?

throop

[ikegami]

sort treats the first parameter specially if it looks like a function name. It uses the function by the specified name as the compare function during sorting. You can use a unary-+ to disambiguate.

@livesys = sort +Con::liveones;
is the same
@livesys = sort { $a cmp $b } Con::liveones;
with no lost efficiency.

It's no better and no worse than
@livesys = sort &Con::liveones;

Both are better than
@livesys = sort @{[Con::liveones]};

[diotalevi]

You don't lose any efficiency with sort { $a cmp $b }. It's special cased along with several other similar kinds of sorts.

[kyle]

I can't tell whether this affects you without seeing the rest of your code, but there's a small difference between &foo and foo(). Specifically, &foo will get the current contents of @_ while foo() will not. If Con::liveones is expecting that, then that may be your problem.

bar( qw( testing is fun ) );

sub bar {
    &foo;
    foo();
    &foo();
    foo( 'woohoo' );
}
sub foo {
    print 'foo: ';
    print join q{,}, map { "[$_]" } @_;
    print "\n";
}

Outputs:

foo: [testing],[is],[fun]
foo:
foo:
foo: [woohoo]

[imp]

Perl is very particular about the arguments to sort. In this case perl is interpreting your code to mean that it should use Con::liveones to sort the empty list ().

You should just store the list in a temporary array, otherwise you'll have to do something hideous like this:

my @livesys = sort @{[Con::liveones()]};

[kwaping]

To illustrate and reinforce imp and ikegami's posts:

% perl -MO=Deparse -le '@livesys = sort &Con::liveones;'
BEGIN { $/ = "\n"; $\ = "\n"; }
@livesys = sort(&Con::liveones);
-e syntax OK

% perl -MO=Deparse -le '@livesys = sort Con::liveones();'
BEGIN { $/ = "\n"; $\ = "\n"; }
@livesys = (sort Con::liveones ());
-e syntax OK