[OT] best way to determine existence of a function in a package

[Jonathan Rockway]

On Mon, 2007-11-19 at 11:16 +0000, Paul LeoNerd Evans wrote:
> You don't need a reference. I've always found that UNIVERSAL::can() works
> just as well:
> 
>   if( my $subref = UNIVERSAL::can( "Package", "function" ) ) {
>      $subref->( args, here );
>   }
> 

Everyone was quick to jump down your throat on this one, so I thought
I'd explain why.  Consider this case:

   package My::Module;
   our $AUTOLOAD;
   sub AUTOLOAD { return 42 if $AUTOLOAD =~ /forty_?two/ }

Now you have some calling code that wants to see if My::Module can
"fortytwo", it will say something like "My::Module->can('fortytwo')" and
get undef back, since there is no fortytwo method in your module.  You
can rectify the situation by defining can:

  sub My::Module::can {
     return sub { $_[0]->forty_two } if $_[1] =~ /forty_?two/;
     return;
  }

Now My::Module->can('fortytwo') will return true, and all is well!
However, if you called UNIVERSAL::can('My::Module', 'fortytwo'), your
My::Module::can would never get called, and hence the can call would
return inaccurate results.

Anyway, please read man UNIVERSAL and UNIVERSAL::isa (the CPAN version)
for more information.

Regards,
Jonathan Rockway