<div><br><div class="gmail_quote"><div>On Fri, Jun 9, 2017 at 06:56 Gor Gyolchanyan <<a href="mailto:gor@gyolchanyan.com">gor@gyolchanyan.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word;line-break:after-white-space"><div>The type of `self` could remain `inout Self` inside the failable initializer. The ability to assign nil would be a compiler magic (much like `return nil` is compiler magic) that is meant to introduce uniformity to the initialization logic.</div><br><div>The idea is to define all different ways initialization can take place and expand them to be used uniformly on both `self` and all its members, as well as remove the ways that do not make sense for their purpose.</div><div><br></div><div>Currently, there are 3 ways of initializing self as a whole:</div><div><span class="m_-8492261585337030922Apple-tab-span" style="white-space:pre-wrap"> </span>1. delegating initializer</div><div><span class="m_-8492261585337030922Apple-tab-span" style="white-space:pre-wrap"> </span>2. assigning to self</div><div><span class="m_-8492261585337030922Apple-tab-span" style="white-space:pre-wrap"> </span>3. returning nil</div><div><div><br></div><div>#1: The delegating initializer is pretty much perfect at this point, in my opinion, so no changes there.</div><div><br></div><div>#2: The only exception in assigning to self is the `nil` inside failable initializers.</div><div><br></div><div>#3: The only thing that can be returned from an initializer is `nil`, which is compiler magic, so we can thing of it as a misnomer (because we aren't really **returning** anything).</div><div><br></div><div>If, for a second, we forget about potential factory initializers, returning anything from an initializer doesn't make much sense, because an initializer is conceptually meant to bring an existing object in memory to a type-specific valid state. This semantic was very explicitly in Objective-C with `[[MyType alloc] init]`. Especially since even syntactically, the initializer does not specify any return type, the idea of returning from an initializer is counter-intuitive both syntactically and semantically.</div><div><br></div><div>The actual *behavior* of `return nil` is very sensible, so the behavior, I imagine `self = nil`, would largely mean the same (except not needed to return immediately and allowing non-self-accessing code to be executed before return). Being able to assign `nil` to a non-optional (ExpressibleByNilLiteral doesn't count) may feel a bit wonky,</div></div></div></blockquote><div><br></div><div>What happens when Self is ExpressibleByNilLiteral and you want to initialize self to nil? That is what `self = nil` means if `self` is of type `inout Self`. If `self` is of type `inout Self` and Self is not ExpressibleByNilLiteral, then it must be an error to assign nil to self. Anything else does not make sense, unless `self` is of type `inout Self?`.</div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word;line-break:after-white-space"><div><div>but not as wonky as returning nil from something that is meant to initialize an object in-place and doesn't look like it should return anything.</div><div><br></div><div># Factory Initializers</div><div><br></div><div>In case of factory initializers, the much discussed `factory init` syntax could completely flip this logic, but making the initializer essentially a static function that returns an object. In this case the initializer could be made to specify the return type (that is the supertype of all possible factory-created objects) and assigning to self would be forbidden because there is not self yet:</div><div><br></div><div>extension MyProtocol {</div><div><br></div><div><span class="m_-8492261585337030922Apple-tab-span" style="white-space:pre-wrap"> </span>public factory init(weCool: Bool) -> MyProtocol {</div><div><span class="m_-8492261585337030922Apple-tab-span" style="white-space:pre-wrap"> </span>self = MyImpl() // error: cannot assign to `self` in a factory initializer</div><div><span class="m_-8492261585337030922Apple-tab-span" style="white-space:pre-wrap"> </span>self.init(...) // error: cannot make a delegating initializer call in a factory initializer</div><div><span class="m_-8492261585337030922Apple-tab-span" style="white-space:pre-wrap"> </span>if weCool {</div><div><span class="m_-8492261585337030922Apple-tab-span" style="white-space:pre-wrap"> </span>return MyCoolImpl()</div><div><span class="m_-8492261585337030922Apple-tab-span" style="white-space:pre-wrap"> </span>} else {</div><div><span class="m_-8492261585337030922Apple-tab-span" style="white-space:pre-wrap"> </span>return MyUncoolImpl()</div><div><span class="m_-8492261585337030922Apple-tab-span" style="white-space:pre-wrap"> </span>}</div><div><span class="m_-8492261585337030922Apple-tab-span" style="white-space:pre-wrap"> </span>}</div><div><br></div><div>}</div><div><br></div><div># In-place Member Initializers</div><div><br></div><div>In addition, member initialization currently is only possible with #2 (as in `self.member = value`), which could be extended in a non-factory initializer to be initializable in-place like this:</div><div><br></div><div>self.member.init(...)</div><div><br></div><div>This would compliment the delegating initialization syntax, while giving a more reliable performance guarantee that this member will not be copy-initialized.</div></div></div><div style="word-wrap:break-word;line-break:after-white-space"><div><div><br><blockquote type="cite"><div>On Jun 9, 2017, at 1:32 PM, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" target="_blank">xiaodi.wu@gmail.com</a>> wrote:</div><br class="m_-8492261585337030922Apple-interchange-newline"><div>If `self` is not of type `inout Self?`, then what is the type of `self` such that you may assign it a value of `nil`?<br><br>It certainly cannot be of type `inout Self`, unless `Self` conforms to `ExpressibleByNilLiteral`, in which case you are able to assign `self = nil` an unlimited number of times–but that has a totally different meaning.<br><br>Could `self` be of type `inout Self!`? Now that implicitly unwrapped optionals are no longer their own type, I’m not sure that’s possible. But even if it were, that seems unintuitive and potentially error-prone.<br><br>So I think Greg is quite right that, to enable this feature, `self` would have to be of type `inout Self?`–which is intriguing but potentially more boilerplatey than the status quo.<br><div class="gmail_quote"><div>On Fri, Jun 9, 2017 at 05:24 Gor Gyolchanyan via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word;line-break:after-white-space">Good point, but not necessarily.<br><div>Since you cannot access `self` before it being fully initialized and since `self` can only be initialized once, this would mean that after `self = nil`, you won't be allowed to access `self` in your initializer at all.You'll be able to do any potential, cleanup though.</div><div>Also, since there can be only one `self = nil`, there's no reason to treat `self` as `inout Self?`, because the only place it can be `nil` is the place it cannot be accessed any more.</div></div><div style="word-wrap:break-word;line-break:after-white-space"><div><br><div><br><blockquote type="cite"><div>On Jun 9, 2017, at 7:45 AM, Greg Parker <<a href="mailto:gparker@apple.com" target="_blank">gparker@apple.com</a>> wrote:</div><br class="m_-8492261585337030922m_1716065582357142928Apple-interchange-newline"><div><div style="word-wrap:break-word;line-break:after-white-space"><br><div><blockquote type="cite"><div>On Jun 8, 2017, at 5:09 AM, Gor Gyolchanyan via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>> wrote:</div><div><div><br>1. Arbitrary `self` Assignments In Intializers<br><br>The first ideas is to allow `self = nil` inside failable initializers (essentially making `self` look like `inout Self?` instead of `inout Self` with magical `return nil`), so that all initializers uniformly can be written in `self = ...` form for clarity and convenience purposes. This should, theoretically, be nothing but a `defer { return nil }` type of rewrite, so I don't see any major difficulties implementing this. This is especially useful for failable-initializing enums where the main switch simply assigns to self in all cases and the rest of the initializer does some post-processing.<br></div></div></blockquote><div><br></div><div>I don't see how to avoid source incompatibility and uglification of failable initializer implementations here. Allowing `self = nil` inside a failable initializer would require `self` to be an optional. That in turn would require every use of `self` in the initializer to be nil-checked or forced. I don't think that loss everywhere outweighs the gain of `self = nil` in some places.</div></div><br><div><br></div><div>-- </div><div>Greg Parker <a href="mailto:gparker@apple.com" target="_blank">gparker@apple.com</a> Runtime Wrangler</div><div><br></div><div><br></div></div></div></blockquote></div><br></div></div>_______________________________________________<br>
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