CODEWITHSUNDEEP
perlstringoopmethodssymbols
Ryan C. Thompson
Ryan C. Thompson

Reputation: 41990

In Perl, how can I call a method whose name I have in a string?

I'm trying to write some abstract code for searching through a list of similar objects for the first one whose attributes match specific values. In order to do this, I need to call a bunch of accessor methods and check all their values one by one. I'd like to use an abstraction like this:

sub verify_attribute {
    my ($object, $attribute_method, $wanted_value) = @_;
    if ( call_method($object, $attribute_method) ~~ $wanted_value ) {
        return 1;
    }
    else {
        return;
    }
}

Then I can loop through a hash whose keys are accessor method names and whose values are the values I'm looking for for those attributes. For example, if that hash is called %wanted, I might use code like this to find the object I want:

my $found_object;
FINDOBJ: foreach my $obj (@list_of_objects) {
    foreach my $accessor (keys %wanted) {
        next FINDOBJ unless verify_attribute($obj, $accessor, $wanted{$accessor});
    }
    # All attrs verified
    $found_object = $obj;
    last FINDOBJ;
}

Of course, the only problem is that call_method does not exsit. Or does it? How can I call a method if I have a string containing its name? Or is there a better solution to this whole problem?

Upvotes: 6

Views: 403

Answers (2)

xoid
xoid

Reputation: 1134

Functional way is cool, but for dummies like me eval rules:

test.pl

#!/usr/bin/perl -l
use F;
my $f = F->new();

my $fun = 'lol'; # method of F

eval '$f->'.$fun.'() '; # call method of F, which name is in $fun var

F.pm

package F;

sub new
{
    bless {};
}


sub lol
{
    print "LoL";
}
1;

[root@ALT-24 root]# perl test.pl

LoL

Upvotes: 0

tsee
tsee

Reputation: 5072

my $found_object;
FINDOBJ: foreach my $obj (@list_of_objects) {
  foreach my $accessor (keys %wanted) {
    next FINDOBJ unless $obj->$accessor() == $wanted{$accessor};
  }
  # All attrs verified
  $found_object = $obj;
  last;
}

Yes, you can call methods this way. No string (or any other) eval involved. Also, substitute == with eq or =~ depending on the type of the data...

Or, for some extra credits, do it the functional way: (all() should really be part of List::Util!)

use List::Util 'first';

sub all (&@) {
  my $code = shift;
  $code->($_) || return 0 for @_;
  return 1;
}

my $match = first {
                    my $obj = $_;
                    all { $obj->$_ == $attrs{$_} }
                      keys %wanted
                  } @list_of_objects;

Update: Admittedly, the first solution is the less obfuscated one, so it's preferable. But as somebody answering questions, you have add a little sugar to make it interesting for yourself, too! ;-)

Upvotes: 5

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