<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="">On Feb 21, 2017, at 8:57 PM, Michel Fortin via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="">(this was accidentally sent off-list, reposting here a day later)<br class=""></div></div></blockquote><div><br class=""></div>And this was my reply.</div><div><br class=""><blockquote type="cite" class=""><div class=""><div class=""><br class=""><blockquote type="cite" class="">Le 20 févr. 2017 à 12:17, Matthew Johnson <<a href="mailto:matthew@anandabits.com" class="">matthew@anandabits.com</a>> a écrit :<br class=""><br class=""><blockquote type="cite" class="">e) Generic Containers:<br class="">Generic containers that impose requirements on their elements will pose some additional problems, for instance: `Set.insert` needs to call `hashValue` and `==` on its elements, making the purity of `Set.insert` constrained by the purity of those functions. Without a way to express this kind of conditional purity, `Set` and `Dictionary` cannot be pure.<br class=""></blockquote><br class="">Could we use a mechanism similar to `rethrows` to address this kind of transitive purity? We have already discussed something along those lines for handling functions passed as arguments. Maybe that mechanism could be designed to handle this use case as well.<br class=""></blockquote><br class="">Similar, yes. But more complicated too. In pseudo code, this is what you'd have to express for `Set.insert`:<br class=""><br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>pure(where: Element.hashValue is pure, Element.== is pure)<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>func insert(_ element: Element) { ... }<br class=""><br class="">Then the compiler can enforce that `insert` only does things that are allowed in a pure function, but can ignore the purity of Element.hashValue and Element.== because the caller will check for that that.<br class=""></div></div></blockquote><div><br class=""></div><div><blockquote type="cite" class=""><blockquote type="cite" class=""></blockquote></blockquote>Yep, that’s the kind of thing I had in mind.<br class=""><blockquote type="cite" class=""><blockquote type="cite" class=""></blockquote></blockquote><font color="#00afcd" class=""><br class=""></font><blockquote type="cite" class=""><blockquote type="cite" class=""></blockquote></blockquote>I can imagine being this explicit could get pretty cumbersome though. I wonder if there is a way to streamline this. For example, in this case if we could just talk about `Element`’s conformance to `Hashable` and we could talk about a `pure` conformance (all requirements are met with pure implementations) then we could just say something like this:<br class=""><blockquote type="cite" class=""><blockquote type="cite" class=""></blockquote></blockquote><font color="#00afcd" class=""><br class=""></font><blockquote type="cite" class=""><blockquote type="cite" class=""></blockquote></blockquote>pure(Element.Hashable)<br class=""><blockquote type="cite" class=""><blockquote type="cite" class=""></blockquote></blockquote>func insert(_ element: Element) { ... }<br class=""><blockquote type="cite" class=""><blockquote type="cite" class=""></blockquote></blockquote><font color="#00afcd" class=""><br class=""></font><blockquote type="cite" class=""><blockquote type="cite" class=""></blockquote></blockquote>Note: I also removed some redundancies - we could just say that the parameters are a list of things the purity of which `insert`’s purity depends.<br class=""><blockquote type="cite" class=""><blockquote type="cite" class=""></blockquote></blockquote><font color="#00afcd" class=""><br class=""></font><blockquote type="cite" class=""><blockquote type="cite" class=""></blockquote></blockquote>For function arguments it would look something like this:<br class=""><blockquote type="cite" class=""><blockquote type="cite" class=""></blockquote></blockquote><font color="#00afcd" class=""><br class=""></font><blockquote type="cite" class=""><blockquote type="cite" class=""></blockquote></blockquote>pure(transform)<br class="">func map<T>(transform: Element -> T)</div><br class=""><blockquote type="cite" class=""><div class=""><div class=""><br class="">-- <br class="">Michel Fortin<br class=""><a href="https://michelf.ca" class="">https://michelf.ca</a><br class=""><br class="">_______________________________________________<br class="">swift-evolution mailing list<br class="">swift-evolution@swift.org<br class="">https://lists.swift.org/mailman/listinfo/swift-evolution<br class=""></div></div></blockquote></div><br class=""></body></html>