[MacRuby-devel] locals & dispatch: is using a pointer the best practice?

[Michael Johnston]

When I need to get a queue-protected result into a local in code that is concurrent (so I can't use an ivar) is a pointer the best (only) way to get the result of a sync block? Assuming I don't want to factor out  a method object.


ex:

result_p = Pointer.new(:id)
some_queue.sync do
	result_p.assign(value_protected_by_queue)
end
result = result_p[0]

it's not very ruby-ish...

Cheerio,

Michael Johnston
lastobelus at mac.com




On 2011-01-26, at 2:50 PM, Laurent Sansonetti wrote:

> On Jan 26, 2011, at 12:49 AM, Charles Oliver Nutter wrote:
>> On Mon, Jan 24, 2011 at 11:13 AM, Joshua Ballanco <jballanc at gmail.com> wrote:
>>> Regarding the potential bug...well...where to begin? So, yes, MacRuby blocks
>>> have the same semantics as regular Ruby blocks, *except* when they are
>>> dispatched through GCD. In that case, ivars are shared, but local variable
>>> get copied. I'm sure Laurent can explain why better than I, but it has to do
>>> with the semantics of libdispatch and the uncertainty inherent in when a
>>> dispatched block or function will execute (i.e. if local variables were not
>>> copied during dispatch, they might go out of scope and be collected before
>>> GCD ever gets around to running the code).
>> 
>> Wow, that's very surprising. I'm not sure I agree with bending Ruby
>> semantics so drastically, even to help concurrency. Or at least, I'd
>> expect other threaded scenarios to be consistent:
>> 
>> ~ ➔ ruby -ve "a = 0; Thread.new { a += 1 }.join; p a"
>> MacRuby 0.8 (ruby 1.9.2) [universal-darwin10.0, x86_64]
>> 1
>> 
>> ~ ➔ ruby -ve "a = 0; q = Dispatch::Queue.concurrent; q.sync {a += 1}; p a"
>> MacRuby 0.8 (ruby 1.9.2) [universal-darwin10.0, x86_64]
>> 0
>> 
>> The implicitness in being able to mutate the surrounding scope is
>> certainly problematic. This is one reason Java's anonymous inner
>> classes require that referenced variables be declared final...to
>> indicate they can't be mutated by the body of the anonymous inner
>> class's methods.
>> 
>> The result is that people end up using one-element arrays and the
>> like, but people find ways around anything.
>> 
>> So I suppose this applies to anything in the surrounding scope,
>> including visibility, $~, and so on?
> 
> No, only locals and dynamic (block) variables.
> 
> To be honest I always disliked this semantic change too. I think it was a mistake to add it. It will probably be reverted for 1.0.
> 
> Laurent
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