Perl with Moo: How to call superclass's implementation of a method? - perl

Question

I have a class X with a subclass Y. X has a method calculate() that I'd like to override in Y with some additional behaviour, an if statement that, if it fails, call X.calculate(). In Python this would be accomplished with:
class X(object):
def calculate(self, my_arg):
return "Hello!"
class Y(X):
def calculate(self, my_arg):
if type(my_arg) is int and my_arg > 5:
return "Goodbye!"
return super(Y, self).calculate(my_arg)
How can I do this in Perl using the Moo module?

Answer 1

As the docs point out:
No support for super, override, inner, or augment - the author considers augment to be a bad idea, and override can be translated:
around foo => sub {
my ($orig, $self) = (shift, shift);
...
$self->$orig(#_);
...
};
(emphasis mine)
#!/usr/bin/env perl
use strict;
use warnings;
package X;
use Moo;
sub calculate {
return 'Hello!'
}
package Y;
use Moo;
extends 'X';
around calculate => sub {
my $orig = shift;
my $self = shift;
if ( $_[0] > 5 ) {
return $self->$orig(#_);
}
return 'Goodbye!';
};
package main;
my $y = Y->new;
print $y->calculate(3), "\n";
print $y->calculate(11), "\n";

Answer 2

This can be done in Perl via the SUPER:: pseudo-class, which is part of Perl's method resolution system. You just put it in front of the method-call. It does not work for class methods or function calls.
use strict;
use warnings;
use feature 'say';
package Foo;
use Moo;
sub frobnicate {
my $self = shift;
say "foo";
}
package Bar;
use Moo;
extends 'Foo';
sub frobnicate {
my $self = shift;
say "bar";
$self->SUPER::frobnicate;
}
package main;
Bar->new->frobnicate;
You can even use this to call each parent's method if you have multi-level inheritance.
package Grandparent;
sub foo { ... }
package Parent;
use parent 'Grandparent';
sub foo { $_[0]->SUPER::foo }
package Child;
use parent 'Parent';
sub foo { $_[0]->SUPER::foo }
This will subsequently call foo in Child, Parent and Grandparent.