Re: Pre-PPC: class :abstract

That's only dangerous if the abstract implementer makes it so. In practice, 
default implementations with normal generic behavior would in some cases be 
reasonable, and in cases where they are not, the default implementation could 
throw a runtime exception, which would fail fast for users who update to the new 
abstract class and expect to use that unimplemented feature, while not affecting 
users who update to the new abstract class while keeping their usage unchanged. 
-- Darren Duncan

On 2025-12-20 7:08 a.m., Ovid wrote:
> On Sat, Dec 20, 2025 at 5:04 AM Darren Duncan 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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