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

22 Oct 2011

      Very strange, your example works for me in irb too. Irb must do something odd with locals. Have you tried this outside of irb, though? If you wrap with a method def'n in irb, it behaves differently, and the fact that locals are copied has been discussed lots on the list. But Laurent (I think) said this will likely change, has it already? I'm on 0.10

$ macirb --noprompt
def check
    print "can I change a local scalar? "
    maybe = "nope"
    $q.sync do
        maybe  = "yep"
  end
    puts maybe
end
=> nil
def check_p
    print "can I assign to a pointer? "
    maybe_p = Pointer.new(:id)
    maybe_p.assign "nope"
    $q.sync do
        maybe_p.assign "yep"
  end
    puts maybe_p[0]
end
=> nil
def check_w
    print "can I assign to a wrapper attr? "
    maybe = ResultWrapper.new("nope")
    $q.sync do
        maybe.value = "yep"
  end
    puts maybe.value  
end
=> nil
ResultWrapper = Struct.new(:value)
=> ResultWrapper
$q= Dispatch::Queue.new('q')
=> q
check
can I change a local scalar? nope
=> nil
# but using a pointer works:
=> nil
check_p
can I assign to a pointer? yep
=> nil
# or a wrapper (but more expensive for tight loops):
=> nil
check_w
can I assign to a wrapper attr? yep
=> nil
$q= Dispatch::Queue.concurrent
=> com.apple.root.default-priority
# double-checking it isn't different for the parallel queues:
=> nil
check
can I change a local scalar? nope
=> nil
check_p
can I assign to a pointer? yep
=> nil
check_w
can I assign to a wrapper attr? yep
=> nil

Cheerio,

Michael Johnston
lastobelus@mac.com

On 2011-10-22, at 12:32 AM, Joshua Ballanco wrote:
...
On Saturday, October 22, 2011 at 1:35 AM, Michael Johnston wrote:
...
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...
There's no restriction on not using ivars in block. Indeed, even locals are in scope in a block (and with a #sync dispatched block such as the one you provided, there aren't even any threading issues):
result = nil
Dispatch::Queue.concurrent.sync do
  result = true
end
p result #=> true 
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