<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 May 27, 2016, at 5:37 PM, Anders Ha <<a href="mailto:hello@andersio.co" class="">hello@andersio.co</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><blockquote type="cite" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; 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;" class=""><br class="Apple-interchange-newline">On 28 May 2016, at 6:00 AM, Brent Royal-Gordon via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""><br class=""><blockquote type="cite" class=""><blockquote type="cite" class="">I'm not sure how much metadata is necessary beyond name (in the key) and type (discussed soon).<br class=""></blockquote><br class="">In the future we may have user-defined attributes. It could be very useful to expose that.<br class=""></blockquote><br class=""><br class="">This is true.<br class=""><br class="">Actually, I'm wondering if property behaviors should be able to essentially declare alternate PropertyViews and add their properties to them. For instance:<br class=""><br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>var behavior persistent<Value: PersistentValue>: Value where Self: ManagedObject {<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span>belongsto persistentProperties, static persistentInstanceProperties<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span><br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span>name: String<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span>var value: Value<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span><br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span>init() {<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span>value = entity.propertiesByName[name].defaultValue<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span>}<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span><br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span>get { return value }<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span>set { value = newValue }<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>}<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>protocol Managed {<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span>static var persistentProperties: InstancePropertyView<Self> { get }<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span>var persistentProperties: PropertyView<Self> { get set }<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span>...<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>}<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>class Person: Managed {<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span>@persistent var name: String<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span>@persistent var birthDate: Date<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>}<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>print(Person.persistentProperties.keys)<span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span>// => "name", "birthDate"<br class=""><br class="">I think it's probably more likely that you want to access the subset of properties with a particular attribute than that you want to look through all the properties and check whether each one has or doesn't have that attribute.<br class=""><br class="">(Incidentally, this is beginning to look a little like one of the post-@memberwise proposals to have some sort of entity representing a group of properties—only more flexible and with a broader set of use cases. Still, there might be hope for a solution with a `@memberwise` behavior and a macro to look at the resulting property view and generate an initializer from it.)<br class=""><br class=""><blockquote type="cite" class=""><blockquote type="cite" class="">Yes. I would hope that you could downcast a concrete lens:<br class=""><br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>let lens: () -> inout Int = &array.count<span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span>// Assuming this is a "get lens” syntax<br class=""></blockquote><br class="">I think the syntax was actually a little bit different as the lens isn’t bound to the instance and a readonly lens wouldn’t return inout:<br class=""><br class="">let lens: (Array<MyType>) -> Int = &array.count<br class=""></blockquote><br class="">I envision there being both bound lenses (sorry, I forgot that `Array.count` isn't mutating):<br class=""><br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>struct Person { var name: String }<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>var joe = Person(name: "Joe")<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>let lens: () -> inout String = &joe.name<br class=""><br class="">And unbound ones:<br class=""><br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>let unboundLens: (inout Person) -> inout String = Person.name<br class=""><br class="">`properties` on an instance would give you bound lenses; `properties` on a type would give you unbound ones. (Or maybe the latter would be something like `instanceProperties`, since type instances have properties too.)<br class=""><br class="">On the other hand, the idea of bound lenses might not be coherent; the `inout` parameter of an unbound lens helps make sure that mutation affects value-typed instances, so bound lenses might not work. Perhaps `properties` on an instance should be more akin to a `[String: Any]` dictionary, so that `joe.properties["name"] = "Bob"` naturally calls `joe`'s `properties` setter. The problem I see there is that the `openas` might open things too far: If the property is declared to be of protocol type, `openas` would end up giving you the concrete type of its current value instead.<br class=""></blockquote><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; 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;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; 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; float: none; display: inline !important;" class="">In this case, I wonder if it would be less confusing if bound lenses are available only on reference types, or existential with a class bound.</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; 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;" class=""></div></blockquote><div><br class=""></div><div>It could be very useful be able to pass a lens bound to a value type as a nonescaping argument. The implementation would have to guarantee the bound lens can’t escape the effective scope of the value. I don’t know how practical the implementation of that would be though. It might require linear types along the lines of Rust in which case they would have to wait at least until Swift has such a system.</div><br class=""><blockquote type="cite" class=""><div class=""><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; 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;" class=""><blockquote type="cite" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; 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;" class=""><br class=""><blockquote type="cite" class=""><blockquote type="cite" class="">And that you could later open the abstracted lens to get its concrete type:<br class=""><br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>if let concreteLens = abstractLens openas Lens {<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span>print(Lens.ReturnValue)<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span><span class="Apple-tab-span" style="white-space: pre;"> </span>…<br class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>}<br class=""></blockquote><br class="">What about casting like this:<br class=""><br class="">let typedLens = abstractLens as? (Array<MyType>) -> Int<br class=""></blockquote><br class="">If you're specifically expecting an `Int`, sure. But if you have no idea what the type is and want to find out, I think you'd need to (somehow) open the Any lens and see the concrete return type inside.<br class=""><br class="">--<span class="Apple-converted-space"> </span><br class="">Brent Royal-Gordon<br class="">Architechies<br class=""><br class="">_______________________________________________<br class="">swift-evolution mailing list<br class=""><a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a><br class=""><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" class="">https://lists.swift.org/mailman/listinfo/swift-evolution</a></blockquote></div></blockquote></div><br class=""></body></html>