Immutable Message Passing

[Graham St Jack]

On 05/12/11 17:51, Jacob Carlborg 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) ?
>
I always use arrays or structs. Until the tail-const thing (or something 
like it) happens, classes don't seem to be viable in messages between 
threads.

-- 
Graham St Jack