<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 27, 2017, at 6:21 PM, Brent Royal-Gordon <<a href="mailto:brent@architechies.com" class="">brent@architechies.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; line-break: after-white-space;" 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=""><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;">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></blockquote><br class=""></div><div class="">If you're reaching this point. why have a marker protocol at all? Why not treat `subscript(dynamicMember:)` specially on any type that has it, or have an `@dynamicMember subscript(_:)` attribute, or introduce an entire new `dynamicMember(_:)` declaration?</div></div></div></blockquote></div><br class=""><div class="">We’ve had a lot of discussions over the years about how to balance simplicity vs power, implicitness vs explicitness, intentionality vs accidental behavior, etc. For example, in very early discussions about Swift generics, some folks where strong proponents of protocol conformance being fully implicit: satisfying all the requirements of a protocol meant that you conformed to it, even if you didn’t explicitly “inherit” from it.</div><div class=""><br class=""></div><div class="">This is obviously not the design we went with over the long term, and I’m glad we didn’t. That said, if we did, then all of the “ExpressibleBy” protocols wouldn’t need to exist: we’d probably just say that it was enough to implement the requirements to get the behavior and elide the protocol declaration entirely.</div><div class=""><br class=""></div><div class="">I think that DynamicMemberLookup requiring conformance is the same thing: it makes it explicit that the behavior is intentional, and it allows somewhat better error checking (if you conform to the protocol but don’t implement the (implicitly known) requirement, you DO get an error). That said, this is just my opinion. </div><div class=""><br class=""></div><div class="">Do you feel strongly enough about this that you’d like to make a strong argument for changing the behavior?</div><div class=""><br class=""></div><div class="">-Chris</div><div class=""><br class=""></div></body></html>