On Nov 4, 2021, at 3:25 PM, Remi Forax <fo...@univ-mlv.fr<mailto:fo...@univ-mlv.fr>> wrote:
I don't think a second bifurcation is needed. At runtime bucket 2 and bucket 3 behave the same apart from null. Given that IdentitylessObject (or whatever the name we choose) is an interface, it always accept null, so if they are typed as that interface, B2 and B3 behave exactly the same. Piling on: The marker interfaces are useful for testing and bounding *reference types*. But a primitive type is not a reference type, so it cannot be (directly) tested or bounded as a reference. There *is* a difference between a reference of the form B3.ref (B3.box, B3? whatever) and B2. But it’s not an interesting difference, because when you box a B3 primitive you get something which has (as Brian says) all the affordances of reference, but without object identity. That’s exactly what a B2 type is. The only difference between a reference to a B3 type and a B2 type is the syntax by which they were declared and derived. This looked pretty clear to me when I did my diagram, where B3 types have ref projections that bubble into the B2 swath of types. Once there, they behave exactly like native B2 types. The diagram has three swathes for concrete types (PRIM, NOID, IDOSAUR), plus a separate upper quadrant for non-concrete reference types. The PRIM swath has a little excrescence into the NOID swath where the P.ref types pop out. http://cr.openjdk.java.net/~jrose/values/type-kinds-venn.pdf All that suggests to me that we won’t want a marker interface to specially distinguish the B3 excrescences. It does also suggest that we are not done bike-shedding terms: What’s the collective term for “B2 refs + B3 boxes”? (I used NOID.) Or, is a B3 box a “pure object” like any B2 pure object, whose class happens to be a primitive class? I dunno. It remains true (and I hope will continue to be true) that a B3 class defines two types, one reference and one non-reference, while a B2 class defines one reference type. But maybe those two reference types are both to “pure objects”? I’ll bet Dan has a take on this.
