To circle back on the structural nature of observable types - there are definitely use cases for things to be able to be Observable and interplay with observation that are structures; however as folks have raised there are some sharp edges. Namely of which - what happens when you copy a structure that is marked with @Observable. To go into this ...

Making the observation tracking data copy-on-write for value types seems better than making it truly shared, but it still doesn't seem right. I don't think I can explain what I see as the right thing here without a little bit of theory. I think folks might be confusing values and locations. These are both basic formal concepts of languages, t...