CONTROL or once messes with last?

Question

This loop never stops:

class CX::Vaya does X::Control {
    has $.message
}

loop {
    once {
        CX::Vaya.new( message => "I messed up!" ).throw;
    }
    last;
    CONTROL {
        default {
            say "Controlled { .^name }: { .message }"
        }
    }
}

It keeps writing

Controlled CX::Last: <last control exception>
Controlled CX::Last: <last control exception>
Controlled CX::Last: <last control exception>
Controlled CX::Last: <last control exception>
Controlled CX::Last: <last control exception>
Controlled CX::Last: <last control exception>
Controlled CX::Last: <last control exception>
Controlled CX::Last: <last control exception>
...

It could be the once bit, because CONTROL phaser with last does finish:

loop { say "Hey"; last; CONTROL { default: .message.say } }
# OUTPUT: «Hey␤<last control exception>␤»

But I'm not really sure.

Answer 1

Loop flow control in Perl 6 is implemented using control exceptions. Thus, last is actually throwing a CX::Last control exception. Since the CONTROL block uses default, it is therefore catching the CX::Last thrown by last, meaning that control is never transferred out of the loop.

The fix is to instead specify which control exception to catch, using when:

loop {
    once {
        CX::Vaya.new( message => "I messed up!" ).throw;
    }
    last;
    CONTROL {
        when CX::Vaya {
            say "Controlled { .^name }: { .message }"
        }
    }
}