For me the biggest benefit of default interface implementations is (binary) backwards compactability and interoperability with Java. And I think originally that was it's purpose.
When you use interfaces as contacts for extensions you would have to make a new interface to retain comparability with old implementers, and you cannot break all 2 week old extensions for some new features.
That's what I'm excited about.
I always thought as traits being a side-product. And as I read the discussions and the then-WIP spec of them it sounded like it being one
I have made an article myself in response to this article.
It outlines why I think that default interfaces are great. I actually agree about them not being useful as traits (because of confusion), but have a few other reasons that make me excited about them.
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