Generating similar functions in unrelated objects

[t'mo]

in reply to Re (tilly) 1: Why I hate File::Find and how I (hope I) fixed it
in thread Why I hate File::Find and how I (hope I) fixed it

Suppose you wanted to do something like this to a family of related objects. How would you assign the anonymous function to be a method of each class? (I can do that trick in JavaScript using ObjectName.prototpye.methodName, but how would I do that in Perl?)

For example, what I would like to do is:

...
sub ObjectToString {
    my $name = shift;
    return sub {
        my $s = $name . $self->getParametersAndValues();
        return $s;
    }
}
...
package Fu;
sub toString = ObjectToString("Fu");
...
package Bar;
sub toString = ObjectToString("Bar");
...
package Baz;
sub toString = ObjectToString("Baz");
...

Then I would use it like this:

my ($foo, $bar, $baz);

$foo = Fu->new();
# do stuff with foo...
print $foo->toString();

$bar = Bar->new();
# do stuff with bar...
print $bar->toString();
...

The obvious answer here is to use Inheritance™, but what if, for some reason, I don't want to use inheritance? Can I still accomplish what I want to?

[tilly]

You would want to do a typeglob assignment. If you assign an anonymous function to a typeglob, you define that function. Like this:

package Stringify;

sub objectToString {
  my $name = shift;
  return sub {
    $name . (shift)->getParametersAndValues();
  };
}

# Time passes
package Fu;
*toString = Stringify::objectToString("Fu");

But I should note that you can make this nicer with overload and an autogenerated method. For instance:

package Stringify;
use strict;
use Carp;

sub import {
  my $pkg = caller();
  my $code = qq(
    # A trick for custom error reporting - see perlsyn.
    \n# line 1 "'Autogenerated for $pkg by Stringify'"
    package $pkg;
    use overload '""' => sub {
      __PACKAGE__ . (shift)->getParametersAndValues();
    };
  );
  eval($code);
  if ($@) {
    confess("Cannot autogenerate stringification for package '$pkg': $
+@
      The code is:\n$code");
  }
}

1;

would allow you to simply:

package Fu;
use Stringify; # Use objects as strings, and they stringify

If you want you could create an autogenerated function called toString and pass a reference to that function to overload.

[t'mo]

You know what happened, don't you? Just as I was leaving the building, not more than five minutes after I asked the question, the answer came to me: symbol table. :-)

The import approach is interesting. It gives me more food for thought, especially in relation to this question I asked about writing your own import. Thanks.

[tilly]

Are custom imports a good idea?

Can I say it depends?

Most of what I was saying about AUTOLOAD at Re (tilly) 4: Perl and Objects, how do you resolve the two? applies to custom imports as well. A custom import is like a very good sword without a hilt. It can cut really well, but you need to be very cautious before using it.

My rule of thumb is this. If my custom import could be done with Exporter, I don't use it. If it makes for a significantly improved interface and works very differently, then I will consider it. But if I won't actually use it unless I am going to make it a meme in the body of code that I am working on. It doesn't matter how nice each customized rule is, a million and one customized high-level interfaces is unmaintainable.

Or, to put it a different way, It is OK to be clever, but only one person should be being clever at a time.