<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div></div><div>I totally agree with Dave Sweeris. He has put the issue quite nicely.</div><div><br></div><div>Being able to add protocol conformance to a protocol would indeed solve this problem (having well designed protocols in the standard library would still be nice, though, so many thanks for the pointers, Dave A, that looks really interesting!).</div><div><br></div><div>-Thorsten</div><div><br>Am 20.04.2016 um 23:23 schrieb Dave via swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>>:<br><br></div><blockquote type="cite"><div><br class=""><blockquote type="cite" class="">On Apr 20, 2016, at 1:15 PM, Dave Abrahams via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""><br class=""><br class="">on Tue Apr 19 2016, Thorsten Seitz <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""><br class=""><blockquote type="cite" class="">I'd like to have something like Summable with 'add', 'adding' and 'zero' being a<br class="">separate protocol as well as somthing like Multiplicative with 'multiply',<br class="">'multiplied' and 'one' being a separate protocol, because these are universally<br class="">interesting for other cases, e.g. Summable would be useful for defining path<br class="">lengths in a graph library.<br class=""><br class="">Would you mind adding that to the proposal?<br class=""></blockquote><br class="">I suspect you may be headed into the realm of<br class="">protocols-as-bags-of-syntax.</blockquote><div class="">You say that like it’s a bad thing… Am I missing the point? (NOT a rhetorical question)</div><div class=""><br class=""></div><div class="">Speaking only for myself, I want stuff broken up into many simpler protocols (which `Arithmetic` and such conform to) because there are many algorithms which only require small parts of the larger protocol. What if I wanted to add a function that sums all the elements in a collection?</div><div class=""><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">extension</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">CollectionType</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">where</span><span style="font-variant-ligatures: no-common-ligatures" class=""> Generator.Element: Arithmetic {</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo; color: rgb(112, 61, 170);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">func</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> sum() -> </span><span style="font-variant-ligatures: no-common-ligatures" class="">Generator</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">.</span><span style="font-variant-ligatures: no-common-ligatures" class="">Element</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> {</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span><span style="font-variant-ligatures: no-common-ligatures" class=""> s = Generator.Element()</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">for</span><span style="font-variant-ligatures: no-common-ligatures" class=""> e </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">in</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">self</span><span style="font-variant-ligatures: no-common-ligatures" class=""> {</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> s.add(e)</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">return</span><span style="font-variant-ligatures: no-common-ligatures" class=""> s</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div></div><div class=""><br class=""></div><div class="">Yeah, it works, but if I want to get the sum of a collection of some custom type, I have to implement *all* of `Arithmetic`, as opposed to just the two parts of it (`init()` and `.add(:)`) that the algorithm actually uses. It’d be both simpler for the users and *more to the point of the function*, if it could be written like this:</div><div class=""><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: rgb(187, 44, 162);" class="">extension</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);" class="">CollectionType</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(187, 44, 162);" class="">where</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> Generator.Element: <span style="color: rgb(112, 61, 170);" class="">Addable</span> {</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="color: rgb(0, 132, 0); font-variant-ligatures: no-common-ligatures;" class="">// "Addable" instead of "Arithmetic"</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> ... </span><span style="font-variant-ligatures: no-common-ligatures" class="">// No change here</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo; color: rgb(112, 61, 170);" class="">}</div></div><div class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""><br class=""></span></div><div class="">Now, if Swift allowed you to add protocol conformance to protocols:</div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">protocol</span><span style="font-variant-ligatures: no-common-ligatures" class=""> Addable {</span></div><div style="margin: 0px; line-height: normal;" class=""><span style="font-family: Menlo; font-variant-ligatures: no-common-ligatures;" class=""> ... </span><font color="#008400" face="Menlo" class="">// Relevant subset of `Arithmetic`</font></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class="">}</div><div class=""><span style="font-family: Menlo; font-variant-ligatures: no-common-ligatures; color: rgb(187, 44, 162);" class="">#for</span><span style="font-family: Menlo; font-variant-ligatures: no-common-ligatures;" class=""> T </span><span style="font-family: Menlo; font-variant-ligatures: no-common-ligatures; color: rgb(187, 44, 162);" class="">in</span><span style="font-family: Menlo; font-variant-ligatures: no-common-ligatures;" class=""> <span style="color: rgb(112, 61, 170);" class="">Arithmetic</span> { </span><span style="color: rgb(0, 132, 0); font-family: Menlo;" class="">// "#for" because this is clearly macro-ish, and # seems to be what we've settled on for that</span></div><div class=""><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">extension</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class=""> : </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Addable</span><span style="font-variant-ligatures: no-common-ligatures" class=""> {} </span><span style="color: rgb(0, 132, 0);" class="">// Don't need anything here since `Arithmetic` already has everything in `Addable`</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div></div><div class="">… then this all becomes a somewhat moot point. If some generic function only needs a subset of a protocol’s functionality, said function’s author could do this “#for T …” song & dance, and their function would be defined with the minimum constraints.</div><div class=""><br class=""></div><br class=""><blockquote type="cite" class="">Look at pages 14 and 24 of<br class=""><<a href="http://www.cs.indiana.edu/pub/techreports/TR638.pdf" class="">http://www.cs.indiana.edu/pub/techreports/TR638.pdf</a>>—which does the job<br class="">right for C++—and you'll see why.</blockquote>COOL!!! Thanks for posting that, it looks like it’ll be a great read! :-D<div class=""><br class=""></div><div class="">- Dave Sweeris</div></div></blockquote><blockquote type="cite"><div><span>_______________________________________________</span><br><span>swift-evolution mailing list</span><br><span><a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a></span><br><span><a href="https://lists.swift.org/mailman/listinfo/swift-evolution">https://lists.swift.org/mailman/listinfo/swift-evolution</a></span><br></div></blockquote></body></html>