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Date: Thu, 15 Jun 2023 23:36:32 +0200
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Subject: Re: [PHP-DEV] [RFC] Interface Default Methods
From: tovilo.ilija@gmail.com (Ilija Tovilo)

Hi Levi

> I am moving my RFC for interface default methods to discussion:
> https://wiki.php.net/rfc/interface-default-methods.

This or a similar concept makes sense to me. The proposal seems
similar to Swift protocol extensions, or Rust traits, with the
exception that function default implementations may only be defined in
the interface itself.

Note that there's a large overlap between this proposal and extending
traits to allow implementing interfaces.
https://wiki.php.net/rfc/traits-with-interfaces The main difference is
how you would use the feature from a given class, i.e. using an
interface implementation or a trait usage. Implementing interfaces
from traits would require declaring both a trait and an interface. I
do think your proposal is the more natural approach.

The redundancy of interfaces and traits after this RFC are also
somewhat unfortunate. Both interfaces and traits could inject default
behavior into classes. Both could enforce implementation of methods in
classes (traits through abstract methods). My intuition is that
interface default implementations should be used for public APIs
(because this provides an abstracted interface), while traits should
be used for protected/private ones (because non-public methods can't
be added to interfaces). The other obvious difference is that
interfaces don't allow manual conflict resolution while traits do.

The RFC doesn't mention default implementations for static methods.
I'm not sure there's a use case but it might make sense to explicitly
mention whether they are supported.

Ilija