I've never really done any operator overloading in perl, so I was very interesting when I saw this article on perl.com that uses Number::Fraction as an example. But as interesting as the info on overloading was, I couldn't help but get hung up on a particluar line of (non operator related) code ... 

sub add {
  my ($l, $r, $rev) = @_;

  if (ref $r) {
    if (UNIVERSAL::isa($r, ref $l)) {
      return (ref $l)->new($l->{num} * $r->{den} + $r->{num} * $l->{de
+n},
               $r->{den} * $l->{den});
    } else {
...

[download]

The key thing that jumps out at me here (unless I'm really missing something about how UNIVERSAL::isa works) is that even though this code will allow the 'add' method to keep working if $l is a Number::Fraction, and $r is a subclass of N::F, or if $l is a subclass of N::F, and $r is a subclass of the subclass, it should still work -- but what if $r and $l are instances of seperate subclasses of N::F ?

Okay, so maybe I'm babling. Consider this (contrived) hierarcy... 

                 Number::Fraction
                  /           \
 Number::MixedFraction       Number::Fraction::Tk

Assuming Number::MixedFraction, and Number::Fraction::Tk are both happy to inherit the 'add' method from Number::Fraction (and get a Number::Fraction as the result) then wouldn't it make sense for Number::Fraction::add to be general enough to work in that scenerio?

Isn't this a case where hardcoding 'Number::Fraction' in the class makes sense? ... 

sub add {
  my ($l, $r, $rev) = @_;

  if (ref $r) {
    if (UNIVERSAL::isa($r, 'Number::Fraction')) {
      return (ref $l)->new($l->{num} * $r->{den} + $r->{num} * $l->{de
+n},
               $r->{den} * $l->{den});
    } else {
...

[download]

?

In reply to if (UNIVERSAL::isa($r, ref $l)) by hossman

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