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

Exception objects

An exception is an object, like any other in Raku. When you write die 'something failed', Raku wraps your message in an exception object of the type X::AdHoc.

Inside a CATCH, you can inspect that object. Its type comes from .^name, and its text from .message:

{
    die 'something failed';

    CATCH {
        default {
            say .^name;   # X::AdHoc
            say .message; # something failed
        }
    }
}

Knowing the type lets you treat different errors differently. Raku’s built-in exceptions have specific types, whose names start with X::, and you can match on them with when, just as you matched values in a given/when block:

{
    die 'something failed';

    CATCH {
        when X::AdHoc {
            say 'an ad-hoc error: ' ~ .message;
        }
        default {
            say 'some other error';
        }
    }
}

This prints an ad-hoc error: something failed, because the exception matched the X::AdHoc type. Matching on the type is the basis of handling specific errors — the idea the section on custom exceptions builds upon.

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