<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 Nov 29, 2017, at 2:21 AM, Jonathan Hull <<a href="mailto:jhull@gbis.com" class="">jhull@gbis.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">I noticed all of the examples return the same type as they are defined on (JSON has a subscript that returns JSON). Is there an example of where this is not the case?</div></div></blockquote><div><br class=""></div><div>Yes, they exist, there is one in the Python interop layer that I’m working on: it is a separate type that vends all the Python builtins. It returns them as PyVal's.</div><div><br class=""></div><div>-Chris</div><div><br class=""></div><div><br class=""></div><br class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><br class=""></div><div class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Nov 25, 2017, at 3:16 PM, Chris Lattner 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=""><meta http-equiv="Content-Type" content="text/html; charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">On Nov 20, 2017, at 10:36 PM, Chris Lattner <<a href="mailto:clattner@nondot.org" class="">clattner@nondot.org</a>> wrote:<div class=""><blockquote type="cite" class=""><div class=""><div class="">Hi all,<br class=""><br class="">I’ve significantly revised the ‘dynamic member lookup’ pitch, here’s the second edition:<br class=""><a href="https://gist.github.com/lattner/b016e1cf86c43732c8d82f90e5ae5438" class="">https://gist.github.com/lattner/b016e1cf86c43732c8d82f90e5ae5438</a><br class=""><br class="">I’ve incorporated some minor changes to it:<br class="">- I’ve made it possible to provide read-only dynamic members.<br class="">- I’ve added an example JSON use-case which uses read-only dynamic members.<br class="">- Minor wording changes.<br class=""></div></div></blockquote></div><br class=""><div class="">Just to talk to myself a bit here, but I’ve come to realize that the right design really is to have a simple empty marker protocol like this:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0); background-color: rgb(255, 255, 255);" class="">/// Types type conform to this protocol have the behavior that member lookup -</div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0); background-color: rgb(255, 255, 255);" class="">/// accessing `someval.member` will always succeed. Failures to find normally</div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0); background-color: rgb(255, 255, 255);" class="">/// declared members of `member` will be turned into subscript references using</div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0); background-color: rgb(255, 255, 255);" class="">/// the `someval[dynamicMember: member]` member.</div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0); background-color: rgb(255, 255, 255);" class="">///</div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""><span style="color: #ba2da2" class="">public</span> <span style="color: #ba2da2" class="">protocol</span> DynamicMemberLookupProtocol {</div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0); background-color: rgb(255, 255, 255);" class=""><span style="" class=""> </span>// Implementations of this protocol must have a subscript(dynamicMember:)</div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0); background-color: rgb(255, 255, 255);" class=""><span style="" class=""> </span>// implementation where the keyword type is some type that is</div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0); background-color: rgb(255, 255, 255);" class=""><span style="" class=""> </span>// ExpressibleByStringLiteral. It can be get-only or get/set which defines</div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0); background-color: rgb(255, 255, 255);" class=""><span style="" class=""> </span>// the mutability of the resultant dynamic properties.</div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255); min-height: 13px;" class=""> <br class="webkit-block-placeholder"></div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0); background-color: rgb(255, 255, 255);" class=""><span style="" class=""> </span>// subscript<KeywordType: ExpressibleByStringLiteral, LookupValue></div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0); background-color: rgb(255, 255, 255);" class=""><span style="" class=""> </span>// (dynamicMember name: KeywordType) -> LookupValue { get }</div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class="">}</div></div><div class=""><br class=""></div><div class=""><br class=""></div><div class="">A design like this can almost work:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""><span style="color: #ba2da2" class="">public</span> <span style="color: #ba2da2" class="">protocol</span> DynamicMemberLookupProtocol {</div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""> <span style="color: #ba2da2" class="">associatedtype</span> DynamicMemberLookupKeyword : <span style="color: #703daa" class="">ExpressibleByStringLiteral</span></div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""> <span style="color: #ba2da2" class="">associatedtype</span> DynamicMemberLookupValue</div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255); min-height: 13px;" class=""> <br class="webkit-block-placeholder"></div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(112, 61, 170); background-color: rgb(255, 255, 255);" class=""><span style="" class=""> </span><span style="color: #ba2da2" class="">subscript</span><span style="" class="">(dynamicMember name: </span>DynamicMemberLookupKeyword<span style="" class="">)</span></div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(112, 61, 170); background-color: rgb(255, 255, 255);" class=""><span style="" class=""> -> </span>DynamicMemberLookupValue<span style="" class=""> { </span><span style="color: #ba2da2" class="">get</span><span style="" class=""> }</span></div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0); background-color: rgb(255, 255, 255);" class=""><div style="font-family: Helvetica; font-size: 12px;" class=""><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo;" class="">}</div><div class=""><br class=""></div></div></div></div><div class="">The problem is that now everything that conforms to DynamicMemberLookupProtocol is a PAT, so it doesn’t work with existentials. We could almost make due with a generic subscript:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""><span style="color: rgb(186, 45, 162);" class="">public</span> <span style="color: rgb(186, 45, 162);" class="">protocol</span> DynamicMemberLookupProtocol {</div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""> <span style="color: #ba2da2" class="">subscript</span><KeywordType: ExpressibleByStringLiteral, LookupValue></div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""> (dynamicMember name: <span style="color: #703daa" class="">KeywordType</span>) -> <span style="color: #703daa" class="">LookupValue</span> { <span style="color: #ba2da2" class="">get</span> }</div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class="">}</div><div style="margin: 0px; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0); background-color: rgb(255, 255, 255);" class=""><div style="font-family: Helvetica; font-size: 12px;" class=""><div class=""><br class=""></div><div class="">but it turns out that while you can declare that, nothing can actually conform to it with concrete types (I filed SR-6473, but it isn’t clear that it ever can work given how our generics system works).</div><div class=""><br class=""></div><div class="">Defining this as an empty marker protocol has several advantages:</div><div class=""> - Only one protocol is required</div><div class=""> - Suddenly you can define mutating getters and nonmutating setters</div><div class=""> - Existentials work as well as concrete types.</div><div class=""><br class=""></div><div class="">-Chris</div><div class=""><br class=""></div></div></div></div></div>_______________________________________________<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><br class=""></div></blockquote></div><br class=""></div></div></div></blockquote></div><br class=""></body></html>