<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">Le 19 août 2017 à 11:38, Matthew Johnson <<a href="mailto:matthew@anandabits.com" class="">matthew@anandabits.com</a>> a écrit :</div><br class="Apple-interchange-newline"><div class=""><div style="font-family: SourceSansPro-Regular; font-size: 15px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span class=""></span></div><div style="font-family: SourceSansPro-Regular; font-size: 15px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><div class=""><br class=""><br class="">Sent from my iPad</div><div class=""><br class="">On Aug 19, 2017, at 8:16 AM, Michel Fortin via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""><br class=""></div><blockquote type="cite" class=""><div class=""><div class=""><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class=""><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="">For instance, has Array<UIView> value semantics?<span class="Apple-converted-space"> </span></div></div></div></blockquote><div class=""><br class=""></div><div class="">By the commonly accepted definition, Array<UIView> does not provide value semantics.</div></div></div></div></blockquote><div class=""><blockquote type="cite" class=""><br class=""></blockquote></div><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class=""><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="">You might be tempted to say that it does not because it contains class references, but in reality that depends on what you do with those UIViews.</div></div></div></blockquote><div class=""><br class=""></div><div class="">An aspect of the type (“does it have value semantics or not”) should not depend on the clients. By your definition, every type has value semantics if none of the mutating operations are called :-)</div></div></div></div></blockquote><br class=""><div class="">No, not mutating operations. Access to mutable memory shared by multiple "values" is what breaks value semantics. You can get into this situation using pointers, object references, or global variables. It's all the same thing in the end: shared memory that can mutate.</div><div class=""><br class=""></div><div class="">For demonstration's sake, here's a silly example of how you can give Array<Int> literally the same semantics as Array<UIView>:</div><div class=""><br class=""></div><div class=""><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>// shared UIView instances in global memory</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>var instances: [UIView] = []</div><div class=""><br class=""></div></div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>extension Array where Element == Int {</div><div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>// append a new integer to the array pointing to our UIView instance</div><div class=""><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>func append(view: UIView) {</div><span class="Apple-tab-span" style="white-space: pre;"> </span>self.append(instances.count)</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>instances.append(newValue)<div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>}</div><div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>// access views pointed to by the integers in the array</div></div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>subscript(viewAt index: Int) -> UIView {</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>get {</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>return instances[self[index]]</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>}</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>set {</div><span class="Apple-tab-span" style="white-space: pre;"> </span>self[index] = instances.count<div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>instances.append(newValue)</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>}</div><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>}</div></div><div class=""><div class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>}</div><div class=""><br class=""></div><div class="">And now you need to worry about passing Array<Int> to other thread. ;-)</div></div></div></blockquote><blockquote type="cite" class=""><div class=""><div class=""><div class=""><br class=""></div><div class="">It does not really matter whether the array contains pointers or wether it contains indices into a global table: in both cases access to the same mutable memory is accessible through multiple copies of an array, and this is what breaks value semantics.</div><div class=""><br class=""></div><div class="">Types cannot enforce value semantics. Its the functions you choose to call that matters. This is especially important to realize in a language with extensions where you can't restrict what functions gets attached to a type.</div></div></div></blockquote><div class=""><br class=""></div><div class="">This gets deeper into the territory of the conversation Dave A and I had a while ago. I think this conflates value semantics with pure functions, which I think is a mistake. </div><div class=""><br class=""></div><div class="">I agree that if you assume away reference counting a function that takes Array<UIView> but never dereferences the pointers can still be a pure function. However, I disagree that Array<UIView> has value semantics.</div></div></div></blockquote><div><blockquote type="cite" class=""><br class=""></blockquote></div></div><div><blockquote type="cite" class=""><div class=""><div style="font-family: SourceSansPro-Regular; font-size: 15px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><div class="">The relationship of value semantics to purity is that value semantics can be defined in terms of the purity of the "salient operations" of the type - those which represent the meaning of the value represented by the type. The purity of these operations is what gives the value independence from copies in terms of its meaning. If somebody chooses to add a new impure operation in an extension of a type with value semantics it does not mean that the type itself no longer has value semantics. The operation in the extension is not "salient".</div><div class=""><br class=""></div><div class="">This still begs the question: what operations are "salient"? I think everyone can agree that those used in the definition of equality absolutely must be included. If two values don't compare equal they clearly do not have the same meaning. <span style="background-color: rgba(255, 255, 255, 0);" class="">Thread safety is also usually implied for practical reasons as is the case in Chris's manifesto. These properties are generally considered necessary for value semantics.</span></div></div></div></blockquote><div><blockquote type="cite" class=""><br class=""></blockquote></div><blockquote type="cite" class=""><div class=""><div style="font-family: SourceSansPro-Regular; font-size: 15px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><div class="">While these conditions are *necessary* for value semantics I do not believe they are *sufficient* for value semantics. Independence of the value is also required. When a reference type defines equality in terms of object identity copies of the reference are not truly independent. </div><div class=""><br class=""></div><div class="">This is especially true in a language like Swift where dereference is implicit. I argue that when equality is defined in terms of object identity copies of the reference are *not* independent. The meaning of the reference is inherently tied up with the resource it references. The resource has to be considered "salient" for the independence to be a useful property. On the other hand, if all you really care about is the identity and not the resource, ObjectIdentifier is available and does have value semantics. There is a very good reason this type exists.</div></div></div></blockquote><div><br class=""></div><div><div>The reason we're discussing value semantics here is because they are useful making concurrency safer. If we define the meaning of value semantics as "a type where a subset of the member functions (the important ones) can be used with concurrency" then that definition of value semantics lose quite a bit of its value for solving the problem at hand. It's too vague.</div><div><br class=""></div><div>I'm not actually that interested in the meaning of value semantics here. I'm debating the appropriateness of determining whether something can be done in another thread based on the type a function is attached to. Because that's what the ValueSemantical protocol wants to do. ValueSemantical, as a protocol, is whitelisting the whole type while in reality it should only vouch for a specific set of safe functions on that type.</div><div><br class=""></div></div><blockquote type="cite" class=""><div class=""><div style="font-family: SourceSansPro-Regular; font-size: 15px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><blockquote type="cite" class=""><div class=""><span class=""></span></div></blockquote></div></div></blockquote></div><br class=""><div class="">
<div style="color: rgb(0, 0, 0); letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div style="color: rgb(0, 0, 0); letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div style="color: rgb(0, 0, 0); letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div style="color: rgb(0, 0, 0); font-family: Helvetica; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><span class="Apple-style-span" style="border-collapse: separate; font-variant-ligatures: normal; font-variant-east-asian: normal; font-variant-position: normal; line-height: normal; border-spacing: 0px;"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; border-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-stroke-width: 0px;"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; border-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-stroke-width: 0px;"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">-- <br class="">Michel Fortin</div><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><span style="text-align: -webkit-auto;" class=""><a href="https://michelf.ca" class="">https://michelf.ca</a></span></div></span></div></span></div></span></div></div></div></div>
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