<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 Mar 2, 2016, at 5:22 PM, Douglas Gregor 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=""><div class=""><span class="" style="font-size: 14px;"><i class="">Private conformances </i></span></div><div class=""><br class=""></div><div class="">Right now, a protocol conformance can be no less visible than the minimum of the conforming type’s access and the protocol’s access. Therefore, a public type conforming to a public protocol must provide the conformance publicly. One could imagine removing that restriction, so that one could introduce a private conformance:</div></div><div class=""><br class=""></div><blockquote class="" style="margin: 0px 0px 0px 40px; border: none; padding: 0px;"><div class=""><div class=""><font face="Menlo" class="">public protocol P { }</font></div><div class=""><font face="Menlo" class="">public struct X { }</font></div></div><div class=""><div class=""><font face="Menlo" class="">extension X :<span class="Apple-converted-space"> </span><b class="">internal P</b><span class="Apple-converted-space"> </span>{ … } // X conforms to P, but only within this module</font></div></div></blockquote><div class=""><div class=""><br class=""></div><div class="">The main problem with private conformances is the interaction with dynamic casting. If I have this code:</div><div class=""><br class=""></div></div><blockquote class="" style="margin: 0px 0px 0px 40px; border: none; padding: 0px;"><div class=""><div class=""><font face="Menlo" class="">func foo(value: Any) {</font></div></div><div class=""><div class=""><font face="Menlo" class=""> if let x = value as? P { print(“P”) }</font></div></div><div class=""><div class=""><font face="Menlo" class="">}</font></div></div><div class=""><div class=""><font face="Menlo" class=""><br class=""></font></div></div><div class=""><div class=""><font face="Menlo" class="">foo(X())</font></div></div></blockquote><div class=""><div class="" 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;"><br class=""></div><div class="" 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;">Under what circumstances should it print “P”? If foo() is defined within the same module as the conformance of X to P? If the call is defined within the same module as the conformance of X to P? Never? Either of the first two answers requires significant complications in the dynamic casting infrastructure to take into account the module in which a particular dynamic cast occurred (the first option) or where an existential was formed (the second option), while the third answer breaks the link between the static and dynamic type systems—none of which is an acceptable result.</div></div></div></blockquote></div><br class=""><div class="">You don't need private conformances to introduce these coherence problems with dynamic casting. You only need two modules that independently extend a common type to conform to a common protocol. As Jordan discussed in his resilience manifesto, a publicly-subclassable base class that adopts a new protocol has the potential to create a conflicting conformance with external subclasses that may have already adopted that protocol. This seems to me like poor grounds for rejecting the ability to have private conformances. I think they're a really useful feature.</div><div class=""><br class=""></div><div class="">-Joe</div></body></html>