CODEWITHSUNDEEP
perl
new_perl
new_perl

Reputation: 7735

Why I can't omit the () here?

SaveImages @img_sources;

The above will report:

Array found where operator expected

Why can't I omit the () here?

Upvotes: 6

Views: 332

Answers (5)

Francisco R
Francisco R

Reputation: 4048

You can omit () with built-in functions (see perlfunc) because built-in functions are keywords of the language and don't need parentheses to be recognized as functions.

Some imported functions (as max from List::Util), usually from core modules, may be called without parenthesis also.

If a subroutine is declared before being called, parentheses can be omitted too, although Perl Best Practices (chapter 2, section 4) recommends to avoid it in order to distinguish between calls to subroutines and built-ins.

Upvotes: -2

Joel Berger
Joel Berger

Reputation: 20280

Many good points here, just one more: see also the subs pragma. Used like use subs qw/SaveImage/; before your function call (probably near the top with the other use calls) it should nicely predeclare your sub in a less obtrusive way.

Upvotes: 4

salva
salva

Reputation: 10234

Perl can parse calls to subroutines without parens when those have been previously declared (or defined). For instance:

sub SaveImages;

SaveImages @img_sources;

Upvotes: 6

DavidO
DavidO

Reputation: 13942

From perlsub:

To call subroutines:

NAME(LIST); # & is optional with parentheses.
NAME LIST; # Parentheses optional if predeclared/imported.
&NAME(LIST); # Circumvent prototypes.
&NAME; # Makes current @_ visible to called subroutine.

Usually subs are not pre-declared in practice. That's not usually a problem, as people are generally accustomed to using parens with programmer-created subs.

Perl::Critic (A module that supports Damien Conway's model of Perltopia as set forth in Perl Best Practices) suggests the following treatments for subs:

  • Prohibit ampersand sigils.
  • Prohibit subroutine prototypes.
  • Prohibit parens with built-ins.

One of the reasons for not using parens with built-ins is to make them visually distinct from program-defined functions, which traditionally DO use parens. Since it's unusual to predeclare subs, and it's discouraged to use ampersand (because it alters how @_ may be treated), or prototypes (because, well, it's a long story), that leaves a very strong background for using parens with script-defined subs.

Upvotes: 6

Nylon Smile
Nylon Smile

Reputation: 9436

because your SaveImages subroutine is declared after the call. Parentheses are not necessary if a subroutine is declared before the call.

example: 

use strict;
use warnings;
use Data::Dumper;
my @ar = (1, 2);
fn @ar;
sub fn
{
    print Dumper \@_;
}

does not work, while

use strict;
use warnings;
use Data::Dumper;
my @ar = (1, 2);
sub fn
{
    print Dumper \@_;
}
fn @ar;

works.

This is an expected behavior and is pointed out in the camel book.

Upvotes: 13

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