Arr-D observable in any way?

[olydis]

From the handful of occurrences of "Arr-D" in the spec, I don't see any way "Arr-D" should be distinguishable from "Arr" (IIRC the wording rather suggests it shouldn't be). In that case, it would be a mere implementation detail, so why mention it at all? One could even discuss whether the entire copy-on-write has a purpose when describing the semantics of Hack. I was surprised since at the same time the spec seems fairly liberal about related concepts like memory management or object lifetime; reads like some bits of a runtime spec/doc have found their way into a language spec. 🙂