Course of Raku / Objects, I/O, and exceptions / Exceptions

Custom exceptions

Besides the built-in X:: exceptions, you can define your own. A custom exception is a class that inherits from the built-in Exception class and provides a message method:

class TooBig is Exception {
    has $.value;

    method message {
        "Value $.value is too big";
    }
}

TooBig is a normal class — it can have attributes such as value — but because it is Exception, it can be thrown. You throw it by creating an object and calling .throw on it:

my $value = 20 * 30;

TooBig.new(value => $value).throw if $value > 50;

CATCH {
    when TooBig {
        say .message;
    }
}

The program prints:

Value 600 is too big

Inside the CATCH, when TooBig matches because the thrown object is of that type. A custom exception carries whatever data you give it (here, the value), so the handler can build a precise message or react in a specific way — far more useful than a plain string passed to die.

This brings together everything in this part: a custom exception is a class (with attributes and a method) that takes part in the exception machinery, ready to signal exactly what went wrong.

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