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<body><div>On Tue, Dec 8, 2015, at 04:07 PM, Jordan Rose wrote:<br></div>
<blockquote type="cite"><div>Good idea, Kevin. Something like this seems reasonable (though I admit I just skimmed the message). In the mean time, maybe we can add a warning for implicit captures with a different strength?<br></div>
</blockquote><div> </div>
<div>A warning seems like a reasonable starting point. Maybe warn in 2.x, and then switch over to the different capture rules for 3.x? That way we wouldn't be changing the actual behavior of anyone's code until 3.x (though I'm willing to bet that nearly all affected cases are unintentional strong captures, but changing variable lifetimes can still have unexpected consequences).<br></div>
<div> </div>
<blockquote type="cite"><div>(I'd also be happy to treat "unowned" + "weak" as "error, be explicit" rather than "strong".)<br></div>
</blockquote><div> </div>
<div>Good idea.<br></div>
<div> </div>
<blockquote type="cite"><div>There is one case where an explicit capture behaves differently from an implicit one: capturing local variables (as opposed to local constants). There's an example of this in the Swift Programming Language book, in the reference section: "<a href="https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Expressions.html#//apple_ref/doc/uid/TP40014097-CH32-ID544">Capture Lists</a>".<br></div>
</blockquote><div> </div>
<div>Good point. I was thinking primarily about captures of self, but the rules would apply to other captures too.<br></div>
<div> </div>
<div>I'd actually like us to have some means by which to explicitly list a local variable in a capture list and still retain the capture-by-reference semantics. I'm just not sure what that would look like.<br></div>
<div> </div>
<div>That said, I'm not sure if this will be a problem for anyone. I'm trying to come up with examples of code that will need to add an explicit capture list to preserve current behavior, but that this capture list will break an implicit by-reference capture in a way that matters. And the best I can do is a contrived example that doesn't provide any explanation for why it wants to maintain current behavior. Specifically, the desired behavior here that breaks is having an outer closure capture the variable by-reference, and then the inner closure constructs the weak reference out of whatever value the variable has at the time the outer closure is invoked:<br></div>
<div> </div>
<div><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;">class Obj: CustomDebugStringConvertible {</span><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"><br></span></div>
<div><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"> var name: String</span><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"><br></span></div>
<div><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"> init(name: String) {</span><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"><br></span></div>
<div><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"> self.name = name</span><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"><br></span></div>
<div><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"> }</span><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"><br></span></div>
<div><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"> var debugDescription: String {</span><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"><br></span></div>
<div><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"> return "Obj(name: \(String(reflecting: name)))"</span><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"><br></span></div>
<div><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"> }</span><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"><br></span></div>
<div><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;">}</span><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"><br></span></div>
<div> </div>
<div><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;">var obj: Obj = Obj(name: "foo")</span><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"><br></span></div>
<div><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;">let f = {</span><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"><br></span></div>
<div><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"> let g = { [weak obj] in</span><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"><br></span></div>
<div><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"> print("g: \(obj)")</span><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"><br></span></div>
<div><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"> }</span><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"><br></span></div>
<div><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"> g()</span><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"><br></span></div>
<div><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;">}</span><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"><br></span></div>
<div><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;">obj = Obj(name: "bar")</span><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;"><br></span></div>
<div><span class="font" style="font-family: menlo, consolas, "courier new", monospace, sans-serif;">f()</span><br></div>
<div> </div>
<div>And even that can recover the existing behavior by just adding a `_ = obj` inside of the outer closure (as opposed to adding a capture list).<br></div>
<div> </div>
<div>I'd be interested to know if anyone can come up with a practical example of code where this will be a problem.<br></div>
<div> </div>
<div>-Kevin Ballard</div>
<div> </div>
<blockquote type="cite"><div><blockquote type="cite"><div>On Dec 8, 2015, at 12:44, Kevin Ballard via swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>> wrote:<br></div>
<div> </div>
<div><div><div>In Swift code today, when using nested closures, if the inner closure weakly captures an object (e.g. `self`) that isn't otherwise captured by the outer closure, the outer closure implicitly strongly captures the object. This behavior is unlikely to be what the programmer intended, and results in unwanted object lifetime extension without making it obvious in the code.<br></div>
<div> </div>
<div>In practice, you'll find this happening in code that looks like<br></div>
<div> </div>
<div><span class="font" style="font-family:menlo, consolas, 'courier new', monospace, sans-serif">class SomeViewController: UIViewController {</span><br></div>
<div><span class="font" style="font-family:menlo, consolas, 'courier new', monospace, sans-serif"> // ...</span><br></div>
<div><span class="font" style="font-family:menlo, consolas, 'courier new', monospace, sans-serif"> func foo(url: NSURL) {</span><br></div>
<div><span class="font" style="font-family:menlo, consolas, 'courier new', monospace, sans-serif"> let task = NSURLSession.sharedSession().dataTaskWithURL(url) { data, response, error in</span><br></div>
<div><span class="font" style="font-family:menlo, consolas, 'courier new', monospace, sans-serif"> let result = // process data</span><br></div>
<div><span class="font" style="font-family:menlo, consolas, 'courier new', monospace, sans-serif"> dispatch_async(dispatch_get_main_queue()) { [weak self] in</span><br></div>
<div><span class="font" style="font-family:menlo, consolas, 'courier new', monospace, sans-serif"> self?.handleResult(result)</span><br></div>
<div><span class="font" style="font-family:menlo, consolas, 'courier new', monospace, sans-serif"> }</span><br></div>
<div><span class="font" style="font-family:menlo, consolas, 'courier new', monospace, sans-serif"> }</span><br></div>
<div><span class="font" style="font-family:menlo, consolas, 'courier new', monospace, sans-serif"> task.resume()</span><br></div>
<div><span class="font" style="font-family:menlo, consolas, 'courier new', monospace, sans-serif"> }</span><br></div>
<div><span class="font" style="font-family:menlo, consolas, 'courier new', monospace, sans-serif">}</span><br></div>
<div> </div>
<div>In here, at first glance it looks like the view controller is being weakly referenced. But in actuality, the view controller is being retained for the duration of the background processing, and is then only converted to a weak reference at the moment where it tries to hop back on to the main thread. So it's basically the worst of both worlds; the view controller lives far longer than intended, but it goes away right at the moment where it could be useful again. It's even worse if the programmer expected the view controller's deinit() to cancel the network task, as that can never happen. The fix for this code is to move the `[weak self]` up to the outer closure, but since the outer closure never actually touches self directly, it's not immediately obvious that this is required.<br></div>
<div> </div>
<div>My proposal is to change the rules so that whenever a closure captures an object only because a nested closure did so, then the outer closure should capture it using the same ownership semantics (this includes unowned(unsafe), unowned(safe), weak, and strong). If there are multiple nested closures that capture it, then we use the following rules:<br></div>
<div> </div>
<div>* If all nested captures use the same ownership, then the outer capture uses that ownership.<br></div>
<div>* If any nested capture is strong, the outer capture is strong.<br></div>
<div>* If at least one nested capture is weak, and at least one capture is unowned or unowned(unsafe), the outer capture is strong. This is because there's no (safe) way to convert from weak -> unowned, or from unowned -> weak, and we should not crash upon the creation of the nested closure, so the outer capture must be strong.<br></div>
<div>* If at least one nested capture is unowned(unsafe), and at least one nested capture is unowned(safe), then the outer capture is unowned(safe).<br></div>
<div> </div>
<div>This can be visualized with the following diagram, where the outer closures uses the right-most node that covers all the children:<br></div>
<div> </div>
<div><span class="font" style="font-family:menlo, consolas, 'courier new', monospace, sans-serif"> .--- weak <-------------------------------.</span><br></div>
<div><span class="font" style="font-family:menlo, consolas, 'courier new', monospace, sans-serif"> / \</span><br></div>
<div><span class="font" style="font-family:menlo, consolas, 'courier new', monospace, sans-serif">strong < + no capture</span><br></div>
<div><span class="font" style="font-family:menlo, consolas, 'courier new', monospace, sans-serif"> \ /</span><br></div>
<div><span class="font" style="font-family:menlo, consolas, 'courier new', monospace, sans-serif"> '--- unowned(safe) <--- unowned(unsafe) --'</span><br></div>
<div> </div>
<div>-Kevin Ballard<br></div>
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</blockquote><div> </div>
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