The following little idea seems so simple that I'm surprised that I haven't seen this before.

The following snippet may not work as you expect:

  use Test::More tests => 3;
                                                                      
+                                                                     
+                 
  use_ok('CGI') or die;
  can_ok('CGI', 'start_html');
  ok(CGI->start_html, '... and calling it should succeed');
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Running that produces the following output:

  1..3
  ok 1 - use CGI;
  not ok 2 - CGI->can('start_html')
  #     Failed test (test.pl at line 4)
  #     CGI->can('start_html') failed
  ok 3 - ... and calling it should succeed
  # Looks like you failed 1 tests of 3.

What gives? The test says that CGI.pm does not have a start_html() method, but calling that method succeeds. AUTOLOAD() can cause strange behaviors.

Or what about trying to validate the interface of delegated object methods? Just because I can stick any 'ol object in a slot doesn't mean that that object has the methods that I want, but if methods are AUTOLOADed, I can't easily validate them (I was also thinking about this in reference to traits validating required methods, but I don't know how best to implement that yet).

The solution that occurred to me was to provide stub methods:

  package Foo;
  use stub qw'this that';

  sub AUTOLOAD {...} # required
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Then, when another object needs to validate the interface:

  sub _delegate {
      # this is not a complete method.  It's just here for illustratio
+n
      my ($self, $slot, $object) {
      $self->{$slot}{object} = $object;
      my @missing;
      foreach my $method (@{$self->{$slot}{methods}}) {
          push @missing => $method unless UNIVERSAL::can($object, $met
+hod);
      }
      die "Interface required @missing" if @missing;
      return $self;
  }
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Now we can validate the interface even if the methods don't exist.

The actual stub package:

  package stub;
                                                                      
+                                                                     
+                 
  sub import {
      my @stub_methods = @_;
      my ($package)    = caller;
      foreach my $stub (@stub_methods) {
          *{"$package::$stub"} = sub { goto &AUTOLOAD };
      }
  }
                                                                      
+                                                                     
+                 
  1;
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The package using stubs would be expected to have an AUTOLOAD method capable of handling all of the stub methods.

Did I miss anything? Is this even worth the trouble?