As other people have pointed out, checking that you are a specific type is just not-Perl. In fact, its not OO either, because it disables inheritance.

C++/Java approach would be to define base classes (as interfaces) and inherit from them. The you can say 

sub foo
{
  my ($self) = @_;
  croak "..." unless ref($self) && $self->isa("AnInterface")
}

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But this would have problems: First, you may need to check multiple interfaces; second you have to include them in your ISA list; third, its all getting very verbose, and not-Perl.

So lets turn it around a bit: 

sub foo # requires interfaces InterfaceA, InterfaceB
{
  my ($self) = @_;
  InterfaceA::validate($self);
  InterfaceB::validate($self);
}

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This client code is much simpler: you are passing the implementation of the check down to a module that knows what it means to be that module. An you no longer require explicit inheritance: 
sub InterfaceA::validate
{
  my ($candidate) = @_;
  error unless ref($candidate);
  error unless $candidate->isa("InterfaceA"); # explicit ISA
}

sub InterfaceB::validate
{
  my ($candidate) = @_;
  error unless ref($candidate);
  error unless $candidate->can("fn1"); # implicit ISA
  error unless $candidate->can("fn2");
}

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Ignoring the issue of naming, it is clear that having an interface know what methods are needed is more powerful than requiring explicit inheritance. If an object supports all the methods in an interface, then it can be said to implement that interface.

One last thing: the names I've chosen are non-optimal, but if validation is common then you'd think that you want concise names. But you can work around this. We might name our validation methods "is_implemnted_by", and create a wrapper function (in a shared utilities module): 

sub validate_object
{
  my ($self, @interfaces) = @_;
  error unless ref($self);
  foreach my $interface (@interfaces)
  {
    my $fn = $interface . "::is_implemnted_by";
    error unless $fn->($self);
  }
}

#...

sub foo
{
  my ($self) = @_;
  validate_object($self, qw/InterfaceA InterfaceB InterfaceC/);
}

sub bar
{
  my ($self) = @_;
  validate_object($self, map {"Interface$_"} qw/A B D/);
}

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--Dave

In reply to Re: OO-style modifiers for 'sub' ? by dpuu
in thread OO-style modifiers for 'sub' ? by Gilimanjaro

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