<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Sep 19, 2017, at 9:48 AM, David Zarzycki <<a href="mailto:dave@znu.io" class="">dave@znu.io</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div style="font-family: Helvetica; font-size: 12px; 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=""><br class="Apple-interchange-newline"><br class=""><blockquote type="cite" class=""><div class="">On Sep 19, 2017, at 12:31, Joe Groff via swift-dev <<a href="mailto:swift-dev@swift.org" class="">swift-dev@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><br class=""><div class=""><br class=""><blockquote type="cite" class=""><div class="">On Sep 19, 2017, at 5:19 AM, David Zarzycki via swift-dev <<a href="mailto:swift-dev@swift.org" class="">swift-dev@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="" style="font-family: Helvetica; font-size: 12px; 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;"><br class="Apple-interchange-newline"><br class=""><blockquote type="cite" class=""><div class="">On Sep 18, 2017, at 17:54, Ben Cohen via swift-dev <<a href="mailto:swift-dev@swift.org" class="">swift-dev@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><br class=""><div class=""><br class=""><blockquote type="cite" class=""><div class="">On Sep 13, 2017, at 1:06 PM, David Zarzycki via swift-dev <<a href="mailto:swift-dev@swift.org" class="">swift-dev@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><br class="" style="font-family: Helvetica; font-size: 12px; 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;"><br class="" style="font-family: Helvetica; font-size: 12px; 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;"><blockquote type="cite" class="" style="font-family: Helvetica; font-size: 12px; 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;">On Sep 13, 2017, at 15:23, Matthew Johnson via swift-dev <<a href="mailto:swift-dev@swift.org" class="">swift-dev@swift.org</a>> wrote:<br class=""><br class=""><br class=""><br class="">Sent from my iPhone<br class=""><br class=""><blockquote type="cite" class="">On Sep 13, 2017, at 11:56 AM, David Zarzycki via swift-dev <<a href="mailto:swift-dev@swift.org" class="">swift-dev@swift.org</a>> wrote:<br class=""><br class=""><br class=""><br class=""><blockquote type="cite" class="">On Sep 13, 2017, at 13:53, David Sweeris <<a href="mailto:davesweeris@mac.com" class="">davesweeris@mac.com</a>> wrote:<br class=""><br class=""><br class=""><blockquote type="cite" class="">On Sep 13, 2017, at 09:54, David Zarzycki via swift-dev <<a href="mailto:swift-dev@swift.org" class="">swift-dev@swift.org</a>> wrote:<br class=""><br class="">Hello,<br class=""><br class="">As a part of a research project that I’m working on, I’ve started bumping into the need for value-type bound protocols (as opposed to the existing class bound protocols). Is this something that would be worth proposing formally? Or should I just keep the patch I have on my research branch?<br class=""></blockquote><br class="">I think it'd be worth a proposal, especially if can talk about why you needed it.<br class=""></blockquote><br class="">While I look forward to talking about my research, I’m not ready to do in the near future.<br class=""><br class="">That being said, value-type bound protocols seem independently useful and that is why I emailed the list.<br class=""><br class="">I think the use case for this is generic algorithms. Why? Because it can be hard to impossible to write *robust* generic code when you don’t know whether an abstract type copies by value or by reference during assignment/initialization. With class-bound protocols, you can guarantee reference semantics, but there is no analogous feature for ensuring value semantics. I have a small (~150 line) patch that fixes this.<br class=""></blockquote><br class="">Value types and value semantics are not the same. Most people who have asked for this capability actually want a constraint for value semantics, not value types. Is that what you're asking for as well?<br class=""></blockquote><br class="" style="font-family: Helvetica; font-size: 12px; 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;"><span class="" style="font-family: Helvetica; font-size: 12px; 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; float: none; display: inline !important;">The patch that I’m ready to put forth is only a value-type bound. In other words only structs and enums would be able to conform to a value-type bound protocol. Enforcing value semantics is arguably a separable language goal.</span><br class="" style="font-family: Helvetica; font-size: 12px; 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;"><br class="" style="font-family: Helvetica; font-size: 12px; 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;"></div></blockquote><div class=""><br class=""></div><div class="">But knowing something is a value type isn’t particularly useful, given it doesn’t guarantee value semantics. It could even do more harm than good, by being confusable with enforcing value semantics. </div><div class=""><br class=""></div><div class="">Can you go into the use cases you have where you would use the knowledge that a type is a value type?</div></div></div></div></blockquote></div><div class="" style="font-family: Helvetica; font-size: 12px; 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;"><br class=""></div><div class="" style="font-family: Helvetica; font-size: 12px; 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;">Hi Ben,</div><div class="" style="font-family: Helvetica; font-size: 12px; 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;"><br class=""></div><div class="" style="font-family: Helvetica; font-size: 12px; 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;">As a part of a much larger goal, I’m experimenting with enforced value *semantics* and I found that value-type bound protocols are a wholly separable and independently useful prerequisite. Here is a contrived but representative example:</div><div class="" style="font-family: Helvetica; font-size: 12px; 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;"><br class=""></div><div class="" style="font-family: Helvetica; font-size: 12px; 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;"><font face="Courier" class="">protocol ValueThingy : !class { // From the patch sent to the list</font></div><div class="" style="font-family: Helvetica; font-size: 12px; 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;"><font face="Courier" class=""> mutating func increment()</font></div><div class="" style="font-family: Helvetica; font-size: 12px; 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;"><font face="Courier" class="">}</font></div><div class="" style="font-family: Helvetica; font-size: 12px; 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;"><font face="Courier" class=""><br class=""></font></div><div class="" style="font-family: Helvetica; font-size: 12px; 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;"><div class=""><font face="Courier" class="">func incrementByCopy<T : </font><span class="" style="font-family: Courier;">ValueThingy</span><font face="Courier" class="">>(_ arg : T) -> T {</font></div><div class=""><font face="Courier" class=""> var copy = arg</font></div><div class=""><font face="Courier" class=""> copy.increment()</font></div><div class=""><font face="Courier" class=""> return copy</font></div><div class=""><font face="Courier" class="">}</font></div><div class=""><br class=""></div><div class="">Without value-type bound protocols, generic code cannot ensure that required copies are actually happening. This is independently useful and good.</div></div></div></blockquote><br class=""></div><div class="">As others have noted, this doesn't by itself guarantee anything.</div></div></div></blockquote><div class=""><br class=""></div>…</div><div style="font-family: Helvetica; font-size: 12px; 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=""><br class=""><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><blockquote class="" style="margin: 0px 0px 0px 40px; border: none; padding: 0px;"><div class=""><br class=""></div></blockquote>The more fundamental thing I think we're looking for in this space is a "pure" restriction for functions and methods, meaning they only access non-shared-mutable data. Any annotation at the type level is not going to give strong enough guarantees to build sound abstractions on top of.</div></div></blockquote><br class=""></div><div style="font-family: Helvetica; font-size: 12px; 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="">Hi Joe,</div><div style="font-family: Helvetica; font-size: 12px; 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=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px; 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="">I know that this doesn’t enforce value semantics. I thought I was being fairly clear about that. As I wrote, I see this change as a separable prerequisite to enforced value semantics. Are you suggesting that the core team views enforced value semantics as “all or nothing”? I.e. no incremental enforcement?</div></div></blockquote></div><div><br class=""></div><div>My concern is that "not class" by itself doesn't really enforce anything interesting over no constraint at all. As Ben noted, it may also be harmfully misleading to people who think it means more than it does.</div><div><br class=""></div><div>-Joe</div><br class=""></body></html>