Course of Raku / Objects, I/O, and exceptions / Classes and objects / Roles / Exercises / A sized box

Solution: A sized box

Here is a possible solution to the task.

Code

role Sized {
    method describe {
        'size is ' ~ self.size;
    }
}

class Box does Sized {
    has $.size;
}

my $b = Box.new(size => 10);
say $b.describe;
say $b ~~ Sized;

🦋 You can find the source code in the file greetable.raku.

Output

size is 10
True

Comments

1. The role provides the describe method, and the class composes it with does Sized. The method relies on self.size, which is supplied by the Box class — the role and the class fit together to form the complete object.

2. Because Box does the role, the smartmatch $b ~~ Sized is True: an object is recognised as having every role its class composes, which is useful for checking what an object can do before calling a role’s method.

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