On Wednesday, 17 February 2016 at 02:20:15 UTC, Dicebot wrote:
One example of a existing guarantee you won't be able to keep,
for example, is that any immutable allocation can be completely
put into separate read-only memory.
Yes, and it would be rejected statically (rule 2). I therefor
don't consider this a problem.
A very important property for building optimized allocators if
you keep in mind sharing goals of immutability.
This is considered too (rule 3). An object can only be immutable
if all its embedded @mutable members are marked as shared.
For example, it's always possible to use a global mutable
associative array to store additional data connected with an
immutable or const object (ignoring purity issues for the
moment). That's safe because from the outside, there's no
observable change to the state of the object itself, and the
global AA's type (shared/thread-local) prevents race
conditions.
Yes and this is how I believe it must be done.
There's no reason why we can't have the same guarantees for
embedded members, without resorting to an external AA. Have a
look at my DIP [1] for lazy initialization of const members,
which I now realize can be adapted to this use case.
Basically, just replace `lazy` by `@mutable`, and most of the
DIP is still valid. I'll try updating it when I find the time.
[1] http://wiki.dlang.org/DIP85
The problem with this DIP is that it speaks about type system
semantics and what matters first is memory model (which is
currently very under-defined as soon as you step from a "the
GC" world).
I'd say most of the memory model issues are the same as those
with shared, which is just as undefined currently. That leaves
possible reordering of accesses to immutable data. I don't have a
complete answer to that at the moment, but most of it is likely
also covered by the fact that the @mutable members must be shared
in this case.
Physical immutability is demanding but it also has great value
in its simplicity and being hard to fool. Any language change
that is going to reject this notion has to be really strongly
justified in terms of what is gained and what is lost and not
come simply because expressing such semantics is possible.
