perlquestion
greengaroo
<p>Dear Monks,</p>
<p>I have a good question for you.</p>
<p>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.</p>
<p>Consider this module:</p>
<code>
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;
</code>
<p>Now this script:</p>
<code>
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( $hash{'test3'} );
print "\n";
</code>
<p>The result is:</p>
<code>
Real exists: 1
My exists: No!
Real exists:
My exists: No!
</code>
<p>Now, my real question is: How can you create a function that would act like the "exists" function? In other words, I need to build a function that is able to receive an expression and not a value (like "my_exists" actually does). According to perldoc, "exists" receives and "EXPR". How can I do that myself?</p>
<p>If I pass '$hash{"test1"}' as a string, then I could probably evaluate it right? Wrong! My function doesn't know what $hash is because its not declared in the Module!</p>
<p>I started to look at the prototypes for subroutines and I didn't find a solution. No luck with Google either.</p>
<p>I have a workaround. In short, I pass two arguments, first the HashRef: \%hash, then the string: '$hash{"test1"}'. It works but I would really like to know how to do it the "exists" way .</p>
<p>Thank you!</p>
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<i>There are no stupid questions, but there are a lot of inquisitive idiots.</i><br/>
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