What To Do About Shared?

[Don]

dsimcha wrote:
> == Quote from dsimcha (dsimcha at yahoo.com)'s article
>> On 3/23/2011 9:09 AM, Jason House wrote:
>>> dsimcha Wrote:
>>>
>>>> Some discussions about std.parallelism have prompted an examination of
>>>> how far D's guarantees against low level data races should extend and
>>>> how safety and practicality should be balanced.
>>> I didn't follow the review of std.parallelism, can you give some specific
> examples?
>>> Users of languages look to standard libraries as a model for how to write
> their own apps. I don't like your proposal and think std.parallelism should use
> shared properly. I'd like to understand better what your issues with shared were.
> I've done a descent amount of shared-correct code, so I'm pretty sure it's usable.
> In fact, the only really nasty bug I had could have been caught if std.thread had
> been shared-correct...
>> I have already decided that, unless shared is drastically improved in
>> ways I don't foresee (I'm not even sure exactly how, this would need to
>> be discussed), I will not be making std.parallelism shared correct.
> 
> One small clarification/relaxation of this position:  I will seriously consider
> making individual artifacts of std.parallelism shared correct if this can
> demonstrably be done without affecting efficiency, flexibility or ease of use.
> For example, certain uses of future/promise parallelism via task() are
> shared-correct, marked as @safe/@trusted and documented as such.
> 
> However, I will not make shared correctness a higher priority than efficiency,
> ease of use or flexibility and I will not cripple or remove artifacts that cannot
> reasonably be made shared correct.

Seems to me, that you're making use of some primitive that I'll call a 
'DivisableArray' -- an array that can be sliced up (into other 
DivisibleArrays), and different DivisableArrays can be sent to different 
threads. You can extract a normal array slice from a DivisibleArray, but 
cannot send that slice to other threads: only DivisibleArrays can do that.
In debug mode, a DivisibleArray could keep track of how it has been 
sliced, and disallow overlapping slices. The DivisibleArray could even 
ensure that all slices lie on word/paragraph boundaries, thus dealing 
with word tearing.