[dmd-concurrency] Defining shared delegates

[Robert Jacques]

On Tue, 19 Jan 2010 19:31:04 -0500, Andrei Alexandrescu  
<andrei at erdani.com> wrote:
> Robert Jacques wrote:
>> On Tue, 19 Jan 2010 16:20:43 -0500, Andrei Alexandrescu  
>> <andrei at erdani.com> wrote:
>>> Michel Fortin wrote:
>>>> Le 2010-01-19 à 15:20, Andrei Alexandrescu a écrit :
>>>>
>>>>> One suggestion about shared delegates: should we take the executive  
>>>>> decision to not define them at all? There's nothing you can do with  
>>>>> delegates that you can't do with classes. Although generally  
>>>>> delegates do justify their existence because they're more comfy than  
>>>>> e.g. local classes, it doesn't strike me as obvious we'll need  
>>>>> comparable convenience when dealing with shared classes.
>>>>  The reason I wrote all this is because I think delegates can be much  
>>>> easier to work with than classes. That's what I tried to demonstrate.
>>>>  That's because delegates completely encapsulate the "sharedness" of  
>>>> what they touch, which means that contrary to other types you can  
>>>> implicitly convert a shared delegate to a thread-local one. This  
>>>> should make them much easier to share than most other types,  
>>>> including classes.
>>>
>>> I may be missing something, but passing a delegate object across  
>>> threads begets arbitrary sharing of that delegate's state across  
>>> threads - because the delegate can do anything to its data. In fact,  
>>> many functional languages define poor man's classes as delegates that  
>>> dispatch the action depending on arguments. There are examples in e.g.  
>>> SICP (http://mitpress.mit.edu/sicp/).
>>>
>>> Andrei
>>  Yes, I believe you are. You described a regular delegate that is  
>> shared between threads. What Michel and I have been talking about is a  
>> shared delegate, which can be safely passed between thread. A good  
>> example of what a shared delegate would be is the member function of a  
>> shared class, while a regular delegate is the member function of a  
>> normal class. i.e. the delegate's state would be protected in the same  
>> way any other shared object would be.
>
> I understand now. So is it correct to assess that if we get the rules  
> for classes straight we also have delegates covered?
>
> Andrei

Yes. The only extra things would be to A) implement them in the type  
system and B) allow automatic conversion to less restrictive delegate  
types.