Perl 6 Deep Dive

Using child class instances as objects of a base class

An object of the ModernHouse type is also a House. Consider a function f that takes the argument and thinks that it is an object of the House type. The signature of the function applies a restriction to the argument:

sub f(House $h) {    say "There are {$h.rooms} rooms in this house.";}

Now, let's create two different houses and call the function with them:

my $house = House.new(rooms => 2);my $modern_house = ModernHouse.new(rooms => 3, power => 100);f($house);f($modern_house);

This code works perfectly  and prints the expected strings:

There are 2 rooms in this house.There are 3 rooms in this house.

The function is only using the attribute, which is defined in the parent class, so it ...