Re: Pre-PPC: class :abstract

On Sat, Dec 20, 2025 at 5:04 AM Darren Duncan <[email protected]>
wrote:

> On 2025-12-19 2:59 a.m., Ovid wrote:
> > For abstract classes, they have uses, too, such as being the base class
> for a
> > factory and ensuring that every factory class has the required
> interface. They
> > do have one notable fragility, though. If you add a new abstract method,
> all
> > subclasses are immediately broken until they can be updated. Maybe not
> so bad
> > for closed source work, but bad for open source.
>
> That problem is avoided easily enough by the abstract class having a
> default
> implementation, which in likely most cases would be reasonable. -- Darren
> Duncan
>

That is a very dangerous assumption. The following is a trivial example:

use v5.40.0;

class StorageHandler {
    method connect() {
        say "Connecting to generic backend...";
    }

    # If a subclass forgets to override this, the app thinks data
    # is being deleted when it isn't. This violates the "fail-fast"
principle.
    method delete_all_logs($user_id) {
        # Doing nothing here is dangerous.
        return 1;
    }
}

class CloudStorage :isa(StorageHandler) {
    method connect() {
        say "Connecting to S3...";
    }
    # Developer forgot to implement delete_all_logs!
}

my $storage = CloudStorage->new;
$storage->connect;

# The dev thinks they are clearing sensitive data:
$storage->delete_all_logs($user_id);
say "Cleanup complete!"; # Oops

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