Something needs to happen with shared, and soon.

[Michel Fortin]

On 2012-11-14 10:30:46 +0000, Timon Gehr <timon.gehr at gmx.ch> said:

> On 11/14/2012 04:12 AM, Michel Fortin wrote:
>> On 2012-11-13 19:54:32 +0000, Timon Gehr <timon.gehr at gmx.ch> said:
>> 
>>> On 11/12/2012 02:48 AM, Michel Fortin wrote:
>>>> I feel like the concurrency aspect of D2 was rushed in the haste of
>>>> having it ready for TDPL. Shared, deadlock-prone synchronized classes[1]
>>>> as well as destructors running in any thread (thanks GC!) plus a couple
>>>> of other irritants makes the whole concurrency scheme completely flawed
>>>> if you ask me. D2 needs a near complete overhaul on the concurrency
>>>> front.
>>>> 
>>>> I'm currently working on a big code base in C++. While I do miss D when
>>>> it comes to working with templates as well as for its compilation speed
>>>> and a few other things, I can't say I miss D much when it comes to
>>>> anything touching concurrency.
>>>> 
>>>> [1]: http://michelf.ca/blog/2012/mutex-synchonization-in-d/
>>> 
>>> I am always irritated by shared-by-default static variables.
>> 
>> I tend to have very little global state in my code,
> 
> So do I. A thread-local static variable does not imply global state. 
> (The execution stack is static.) Eg. in a few cases it is sensible to 
> use static variables as implicit arguments to avoid having to pass them 
> around by copying them all over the execution stack.
> 
> private int x = 0;
> 
> int foo(){
>      int xold = x;
>      scope(exit) x = xold;
>      x = new_value;
>      bar(); // reads x
>      return baz(); // reads x
> }

I'd consider that poor style. Use a struct to encapsulate the state, 
then make bar, and baz member functions of that struct. The resulting 
code is cleaner and easier to read:

pure int foo() {
	auto state = State(new_value);
	state.bar();
	return state.baz();
}

You could achieve something similar with nested functions too.


> Unfortunately, this destroys 'pure' even though it actually does not.

Using a local-scoped struct would work with pure, be more efficient 
(accessing thread-local variables takes more cycles), and be less 
error-prone while refactoring.

-- 
Michel Fortin
michel.fortin at michelf.ca
http://michelf.ca/