Immutable Message Passing

[Graham St Jack]

On 11/12/11 12:59, Andrew Gough wrote:
> On Mon, 05 Dec 2011 08:21:45 +0100
> Jacob Carlborg<doob at me.com>  wrote:
>
>> On 2011-12-04 21:46, Andrew Wiley wrote:
>>> On Sun, Dec 4, 2011 at 1:42 AM, Jonathan M
>>> Davis<jmdavisProg at gmx.com>   wrote:
>>>> On Sunday, December 04, 2011 01:24:02 Andrew Wiley wrote:
>>>>> This should work, right? I'm not just going crazy or something?
>>>>>
>>>>> import std.concurrency;
>>>>> import std.stdio;
>>>>>
>>>>> class Bob {
>>>>> }
>>>>>
>>>>> void main() {
>>>>>       auto tid = spawn(&b);
>>>>>       tid.send(new immutable(Bob)());
>>>>> }
>>>>>
>>>>> void b() {
>>>>>       receive(
>>>>>           (immutable(Bob) b) {
>>>>>               writeln("got it");
>>>>>           }
>>>>>       );
>>>>> }
>>>>>
>>>>>
>>>>> I'm getting this from both GDC (trunk) and DMD (2.056 and trunk -
>>>>> I can't seem to get 2.055 to run):
>>>>> core.exception.AssertError@/usr/include/d/std/variant.d(286):
>>>>> immutable(Bob)
>>>>>
>>>>> Seems like some type conversion isn't working properly and
>>>>> variant is flagging immutable types as not assignable?
>>>> Well, that particular assertion seems to be because of the
>>>> immutability. It seems that Variant can't handle it - the comment
>>>> with it being
>>>>
>>>> // type is not assignable
>>>>
>>>> But I'd have to go digging to see why on earth this is failing
>>>> (and that's assuming that it's not a compiler bug, since then it
>>>> would be completely out of my league). It _looks_ like it should
>>>> work.
>>>>
>>>> - Jonathan M Davis
>>> Actually, it looks like I am going crazy. I went back to dmd 2.052
>>> (and every version in between), and this doesn't seem to have ever
>>> worked.
>>> However, not allowing message passing with immutable messages
>>> eliminates one of the main use cases for immutable. I'll see if I
>>> can find a workaround.
>>>
>>> The actual issue is that once Variant finds a matching type, it
>>> tries to assign `immutable(Bob) = immutable(Bob)` , which is
>>> illegal. However, all it needs to do is assign a *reference* to an
>>> immutable object. Unfortunately, we don't support that.
>>> Incidentally, this makes a compelling argument for something like
>>> immutable(Object) ref.
>> Have message passing ever worked for other than primitive types
>> (including strings) ?
>>
> It works for shared classes
>
Yes, but using shared classes brings its own problems that aren't easy 
to solve without something like tail shared/const/immutable.

-- 
Graham St Jack