typed objects and value types

Dmitry Lomov dslomov at chromium.org
Wed Apr 2 12:11:50 PDT 2014

On Wed, Apr 2, 2014 at 8:26 PM, Mark S. Miller <erights at google.com> wrote:

> We could specify that WeakMaps can use typed objects as keys. The current
> discussion in your article confuses semantics with implementation when it
> speaks of typed objects comparing structurally. Typed objects have
> identity, characterized by the four-tuple you explain. Just as all the bits
> of a thin pointer are significant when comparing two thin pointers to see
> if they point at the same semantic object identity, so are all the bits in
> your fat pointer significant. Don't get confused by the bits in the pointer
> to the fat pointer, when the fat pointer happens to be boxed.

It is unclear to me how WeakMaps can ever use typed objects as keys, since
typed objects do not have their unique identity. Or rather, the only
possible semantics I see is "hold to the value forever". So for example:

   w = new WeakMap();
   w[Point(buffer, 0)] = "abc"; // 1

   w[Point(buffer,0)] = ? // 2

I believe that in line 2, result should always be "abc" (since
Point(buffer, 0) === Point(buffer, 0)).
This is something we can spec, but it is probably not behavior we want.

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