The threading story here is what grabbed my attention. Pass-by-value with copy-on-write means you get data-race immunity without any locks or channels. You just pass data to a thread and mutations stay local. That's a genuinely useful property. I've worked on systems where we spent more time reasoning about shared state than writing actual logic. The typical answer is "just make everything immutable" but then you los…
Swift actually does this copy-on-mutation strategy as well for its standard library’s container types like Array and Dictionary, and it does indeed make multithreaded programming much easier without requiring full copies. The downside is that you pay reference-counting overhead.