From: Jeff 'japhy' Pinyan <jeffp@...>
Date: 2002-02-14T01:14:36+09:00
Subject: Re: qualms about respond_to? idiom

On Feb 13, Dave Thomas said:

>David Alan Black <dblack@candle.superlink.net> writes:
>
>> I'm cool with Ruby's emphasis on object interface rather than type.
>> The only thing is...  I just can't get myself to be happy about this
>> idiom:
>> 
>>    if obj.respond_to? "method" then obj.method end
>
>This is an interesting topic. Just to make sure we're talking about
>the same thing, can I start with a question.
>
>When do you need to check the protocol of an object like this?

I believe the functionality was borrowed from Perl's universal object
method, "can".  Perl code:

  if ($object->can($method_name)) {
    $object->$method_name(@args);
  }

  # you can also store the return value of can() into a scalar
  # which holds the function reference and call it instead of
  # doing a dynamic method call:
  #   $object->$code_ref(@args)

which is like the Ruby code:

  if object.responds_to?("foo") then object.foo(...) end

As to *WHY* it's needed, it's there primarily because in Perl, classes can
inherit from multiple other classes, and dynamically, too.  And if you
have to choose one of a set of classes, some of which are more capable
than others (this might be reaching...) it's nice to be able to say:

  my $object = AnyMailer->new();

  if ($code = $object->can("PGP")) {
    $object->$code($my_pgp_stuff);
  }

  # etc.

It's also good for dispatch tables.

-- 
Jeff "japhy" Pinyan      japhy@pobox.com      http://www.pobox.com/~japhy/
RPI Acacia brother #734   http://www.perlmonks.org/   http://www.cpan.org/
** Look for "Regular Expressions in Perl" published by Manning, in 2002 **
<stu> what does y/// stand for?  <tenderpuss> why, yansliterate of course.
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