Dear Monks,

I have a good question for you.

Lets say I want to re-program the "exists" function. Of course, that is not my end goal, but I will use this as an example because it will be easier to explain.

Consider this module:

package MyExists;

use Exporter qw( import );

@EXPORT = qw(my_exists);


sub my_exists {
  
  my ($string) = @_;
  
  if ( eval("exists($string)") ) {
   
    return "Yes!";
  }
  else {
    
    return "No!";
  }
  
}

1;
[download]

Now this script:

use MyExists qw(my_exists);

my %hash = ( 'test1' => 'value1', 'test2' => 'value2' );

print "Real exists: ";
print exists( $hash{'test1'} );
print "\n";

print "My exists: ";
print my_exists( $hash{'test1'} );
print "\n";

print "Real exists: ";
print exists( $hash{'test3'} );
print "\n";

print "My exists: ";
print my_exists( $hash{'test3'} );
print "\n";
[download]

The result is:

Real exists: 1
My exists: No!
Real exists:
My exists: No!
[download]

Now, my real question is: How can you create a function that would act like the "exists" function? In other words, I need to build a function that is able to receive an expression and not a value (like "my_exists" actually does). According to perldoc, "exists" receives and "EXPR". How can I do that myself?

If I pass '$hash{"test1"}' as a string, then I could probably evaluate it right? Wrong! My function doesn't know what $hash is because its not declared in the Module!

I started to look at the prototypes for subroutines and I didn't find a solution. No luck with Google either.

I have a workaround. In short, I pass two arguments, first the HashRef: \%hash, then the string: '$hash{"test1"}'. It works but I would really like to know how to do it the "exists" way .

Thank you!

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