Detect how a subroutine is called in Perl

Question

I would like to detect how a subroutine is called so I can make it behave differently depending on each case:

# If it is equaled to a variable, do something:
$var = my_subroutine();

# But if it's not, do something else:
my_subroutine();

Is that possible?

Declarer

Answer 1

Use wantarray

if(not defined wantarray) {
    # void context: foo()
}
elsif(not wantarray) {
    # scalar context: $x = foo()
}
else {
    # list context: @x = foo()
}

Maxinemaxiskirt

There is also p3rl.org/Want module, but for this case it might be overkill. – Piapiacenza

Answer 2

Yes, what you're looking for is wantarray:

use strict;
use warnings;

sub foo{
  if(not defined wantarray){
    print "Called in void context!\n";
  }
  elsif(wantarray){
    print "Called and assigned to an array!\n";
  }
  else{
    print "Called and assigned to a scalar!\n";
  }
}

my @a = foo();
my $b = foo();
foo();

This code produces the following output:

Called and assigned to an array!
Called and assigned to a scalar!
Called in void context!

Epicycloid

Answer 3

There is a powerful and very useful XS module called Want developed by Robin Houston. It extends what the core perl function wantarray() offers and enables you to find out if your method for example was called in an object context.

$object->do_something->some_more;

Here in do_something you could write before returning:

if( want('OBJECT') )
{
    return( $object );
}
else
{
    return; # return undef();
}

Parochialism