Immutable Message Passing

[Andrew Wiley]

On Sun, Dec 4, 2011 at 6:19 PM, Timon Gehr <timon.gehr at gmx.ch> wrote:
> On 12/04/2011 11:32 PM, Andrew Wiley wrote:
>>
>> On Sun, Dec 4, 2011 at 4:23 PM, Andrei Alexandrescu
>> <SeeWebsiteForEmail at erdani.org>  wrote:
>>>
>>> On 12/4/11 4:16 PM, Andrew Wiley wrote:
>>>
>>>>
>>>> So it looks like right now, message passing is copying objects, which
>>>> seems very bad. Check this out:
>>>> --------
>>>> import std.stdio;
>>>> import std.concurrency;
>>>>
>>>> class Bob {
>>>> }
>>>>
>>>> void main() {
>>>>
>>>>        auto tid = spawn(&func);
>>>>        auto bob = new shared(Bob)();
>>>>        writeln("bob is currently at ", cast(void*)(&bob));
>>>
>>>
>>>
>>> This is the address of the reference, not that of the object. Use
>>> cast(void*)(bob).
>>>
>>> Andrei
>>
>>
>> Ah, I am covered with shame.
>>
>> In that case, no object copying is occurring, and I have message
>> passing for immutable objects working, although my current solution is
>> basically to check whether the object is immutable, and if so, memcpy
>> the reference. That breaks immutability, but only for the reference,
>> and I don't think there's any alternative unless we get tail
>> const/immutable into the language.
>>
>> I'm guessing this is too hackish to get merged into std.variant?
>
>
> You might want to have a look at std.typecons.Rebindable.

The problem with Rebindable is that internally, it looks like this:
union {
T value;
immutable(T) visibleValue;
}

This fails the check in std.concurrency for aliases to thread-local
mutable data. I *could* cast to and from shared to avoid this, but
then I've just exchanged a local hack in variant for one that crosses
thread boundaries.