<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 09 May 2016, at 09:16, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" class="">xiaodi.wu@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div style="white-space:pre-wrap" class="">One more thought here:<br class=""><br class="">It goes a long way to say "typealiases in protocols are to have the same semantics as aliases outside protocols." I'm inclined to agree on that, but I haven't thought it totally through.<br class=""><br class="">Now, I can have private typealiases outside protocols. Could I have private typealiases inside protocols? They'd be handy for referencing types while implementing default methods in protocol extensions and whatnot without worrying about collisions with typealiases in conforming types…</div></div></blockquote><div><br class=""></div><div>Sounds like it should be allowed. I’ll add something about it in the proposal. Could you give an example of what you mean by "without worrying about collisions with typealiases in conforming types…”?</div><br class=""><blockquote type="cite" class=""><div class=""><div class="gmail_quote"><div dir="ltr" class="">On Mon, May 9, 2016 at 01:52 David Hart <<a href="mailto:david@hartbit.com" class="">david@hartbit.com</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class=""><div class="">I understand that name clashing in those instances is important to discuss, but I still think it is slightly orthogonal to the proposal. Let me try to explain why.</div><div class=""><br class=""></div><div class="">If typealises in protocols are to have the same semantics as alises outside protocols (as I think they should), then they don’t change anything about the rules of collision. For example, the problem already exists today with associated types:</div><div class=""><br class=""></div><div class=""><font face="Menlo" class=""><span style="font-size:12px" class="">protocol Foo {</span><br class=""><span style="font-size:12px" class=""> associatedtype Inner: IntegerType</span><br class=""><span style="font-size:12px" class=""> func foo(inner: Inner)</span><br class=""><span style="font-size:12px" class="">}</span><br class=""><br class=""><span style="font-size:12px" class="">protocol Bar {</span><br class=""><span style="font-size:12px" class=""> associatedtype Inner: FloatingPointType</span><br class=""><span style="font-size:12px" class=""> var inner: Inner { get }</span><br class=""><span style="font-size:12px" class="">}</span><br class=""><br class=""><span style="font-size:12px" class="">struct FooBarImpl: Foo, Bar { // error: Type ‘FooBarImpl’ does not conform to protocol ‘Bar'</span><br class=""><span style="font-size:12px" class=""> func foo(inner: Int) {}</span><br class=""><span style="font-size:12px" class=""> var inner: Float</span><br class=""><span style="font-size:12px" class="">}</span></font></div><div class=""><font face="Menlo" class=""><span style="font-size:12px" class=""><br class=""></span></font></div><div class="">Type aliasing would not change anything about the fact that those collisions already exists in the language and are not very well handled: either they are meant to be forbidden but in that case we need better diagnostics, or we want to have a way to work around them. Perhaps you’d like to start a discussion around fixing that ?</div></div><div style="word-wrap:break-word" class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On 09 May 2016, at 08:06, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" target="_blank" class="">xiaodi.wu@gmail.com</a>> wrote:</div><br class=""><div class=""><div style="white-space:pre-wrap" class="">I see your point that nothing breaks in the stdlib with your proposal alone. It's undeniably true--by construction!--that a purely additive feature, if never used, will not cause problems.<br class=""><br class="">That said, since the time that this feature was outlined in Doug's manifesto, I have been wondering how clashes such as the examples in my previous email are to be handled--i.e. what the rules of the language are to be--which I think is certainly germane to your proposal. Can a conforming type override a protocol typealias? Can a type conform to two protocols with conflicting typealiases if all requirements are otherwise satisfied? Surely, these merit discussion in your proposal.<br class=""></div><br class=""><div class="gmail_quote"><div dir="ltr" class="">On Mon, May 9, 2016 at 12:48 AM David Hart <<a href="mailto:david@hartbit.com" target="_blank" class="">david@hartbit.com</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class=""><div class="">Hello Xiaodi,</div><div class=""><br class=""></div><div class="">What I mean by there is no impact on existing code is that the language change has no impact. Of course, if the Standard Library then declares a typealias Element in Sequence, it will clash with code which has declared an Element typealias in sub-protocols, but that is separate from the proposal.</div></div><div style="word-wrap:break-word" class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On 09 May 2016, at 07:28, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" target="_blank" class="">xiaodi.wu@gmail.com</a>> wrote:</div><br class=""><div class="">If the protocol Sequence has typealias Element, does that mean I also have MyConformingSequence.Element?<br class=""><br class="">If so, I think there is a potential impact on existing code not mentioned. Suppose MyConformingSequence already (unwisely) declares typealias Element. Now, what happens when I try to migrate my code to your proposed version of Swift?<br class=""><br class="">This is a toy example, of course. More generally, though, I wonder about this question:<br class=""><br class="">Suppose two protocols A and B each declare typealias Element. These typealiases are, as you proposed, intended to simplify referencing indirect associated types. But are they themselves considered protocol requirements?<br class=""><br class="">I ask because, suppose I want to conform type T to A and B. I implement all the required methods and properties for such conformance. I declare the appropriate typealiases for the associatedtypes declared in both protocols. But, if A.Element and B.Element are incompatible with each other, it is nonetheless impossible to conform T to both A and B? If it's forbidden, isn't that kind of a bummer, since what's getting in the way is a naming clash arising from a facility intended to simplify the naming of things rather than provide for new functionality? If it's permitted, what is T.Element? Some clarity here would be nice.<br class=""><div class="gmail_quote"><div dir="ltr" class="">On Sun, May 8, 2016 at 6:17 PM David Hart via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class="">Hello,<div class=""><br class=""></div><div class="">I’ve come again with another proposal directly from the Generics Manifesto. Please let me know if it needs any modifications before sending the pull request.</div><div class=""><br class=""></div><div class=""><h1 style="font-size:2.25em;margin-right:0px;margin-bottom:16px;margin-left:0px;line-height:1.2;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';background-color:rgb(255,255,255);margin-top:0px!important" class="">Typealiases in protocols and protocol extensions</h1><ul style="padding-left:2em;margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px;background-color:rgb(255,255,255)" class=""><li class="">Proposal: <a href="https://github.com/hartbit/swift-evolution/blob/typealiases-in-protocols/proposals/XXXX-typealiases-in-protocols.md" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none" target="_blank" class="">SE-XXXX</a></li><li class="">Authors: <a href="https://github.com/hartbit" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none" target="_blank" class="">David Hart</a>, <a href="https://github.com/DougGregor" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none" target="_blank" class="">Doug Gregor</a></li><li class="">Status: TBD</li><li class="">Review manager: TBD</li></ul><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';background-color:rgb(255,255,255)" class=""><a href="https://github.com/hartbit/swift-evolution/blob/typealiases-in-protocols/proposals/XXXX-typealiases-in-protocols.md#introduction" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1" target="_blank" class=""><u class=""></u><u class=""></u><u class=""></u><u class=""></u></a>Introduction</h2><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px;background-color:rgb(255,255,255)" class="">This proposal is from the <a href="https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none" target="_blank" class="">Generics Manifesto</a> and brings the <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px" class="">typealias</code> keyword back into protocols for type aliasing.</p><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';background-color:rgb(255,255,255)" class=""><a href="https://github.com/hartbit/swift-evolution/blob/typealiases-in-protocols/proposals/XXXX-typealiases-in-protocols.md#motivation" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1" target="_blank" class=""><u class=""></u><u class=""></u><u class=""></u><u class=""></u></a>Motivation</h2><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px;background-color:rgb(255,255,255)" class="">In Swift versions prior to 2.2, the <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px" class="">typelias</code> keyword was used outside of protocols to declare type aliases and in protocols to declare associated types. Since <a href="https://github.com/apple/swift-evolution/blob/master/proposals/0011-replace-typealias-associated.md" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none" target="_blank" class="">SE-0011</a> and Swift 2.2, associated type now use the <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px" class="">associatedtype</code> keyword and <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px" class="">typelias</code> is available for implementing true associated type aliases. </p><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';background-color:rgb(255,255,255)" class=""><a href="https://github.com/hartbit/swift-evolution/blob/typealiases-in-protocols/proposals/XXXX-typealiases-in-protocols.md#proposed-solution" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1" target="_blank" class=""><u class=""></u><u class=""></u><u class=""></u><u class=""></u></a>Proposed solution</h2><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px;background-color:rgb(255,255,255)" class="">The solution allows the creation of associated type aliases. Here is an example from the standard library:</p><div style="margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px;background-color:rgb(255,255,255)" class=""><pre style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;margin-top:0px;margin-bottom:0px;line-height:1.45;padding:16px;overflow:auto;background-color:rgb(247,247,247);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;word-wrap:normal;word-break:normal" class=""><span style="color:rgb(167,29,93)" class="">protocol</span> Sequence {
associatedtype Iterator <span style="color:rgb(167,29,93)" class="">:</span> IteratorProtocol
<span style="color:rgb(167,29,93)" class="">typealias</span> Element <span style="color:rgb(167,29,93)" class="">=</span> Iterator<span style="color:rgb(167,29,93)" class="">.</span>Element
}</pre></div><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px;background-color:rgb(255,255,255)" class="">The example above shows how this simplifies referencing indirect associated types:</p><div style="margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px;background-color:rgb(255,255,255)" class=""><pre style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;margin-top:0px;margin-bottom:0px;line-height:1.45;padding:16px;overflow:auto;background-color:rgb(247,247,247);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;word-wrap:normal;word-break:normal" class=""><span style="color:rgb(167,29,93)" class="">func</span> <span style="color:rgb(121,93,163)" class="">sum</span><T: Sequence <span style="color:rgb(167,29,93)" class="">where</span> T<span style="color:rgb(167,29,93)" class="">.</span>Element <span style="color:rgb(167,29,93)" class="">==</span> <span style="color:rgb(0,134,179)" class="">Int</span>>(sequence: T) <span style="color:rgb(167,29,93)" class="">-></span> <span style="color:rgb(0,134,179)" class="">Int</span> {
<span style="color:rgb(167,29,93)" class="">return</span> sequence<span style="color:rgb(167,29,93)" class="">.</span><span style="color:rgb(0,134,179)" class="">reduce</span>(<span style="color:rgb(0,134,179)" class="">0</span>, combine: <span style="color:rgb(167,29,93)" class="">+</span>)
}</pre></div><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';background-color:rgb(255,255,255)" class=""><a href="https://github.com/hartbit/swift-evolution/blob/typealiases-in-protocols/proposals/XXXX-typealiases-in-protocols.md#detailed-design" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1" target="_blank" class=""><u class=""></u><u class=""></u><u class=""></u><u class=""></u></a>Detailed design</h2><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px;background-color:rgb(255,255,255)" class="">The following grammar rules needs to be added:</p><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px;background-color:rgb(255,255,255)" class=""><em class="">protocol-member-declaration</em> → <em class="">protocol-typealias-declaration</em></p><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px;background-color:rgb(255,255,255)" class=""><em class="">protocol-typealias-declaration</em> → <em class="">typealias-declaration</em></p><h2 style="margin-top:1em;margin-bottom:16px;line-height:1.225;font-size:1.75em;padding-bottom:0.3em;border-bottom-width:1px;border-bottom-style:solid;border-bottom-color:rgb(238,238,238);color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';background-color:rgb(255,255,255)" class=""><a href="https://github.com/hartbit/swift-evolution/blob/typealiases-in-protocols/proposals/XXXX-typealiases-in-protocols.md#impact-on-existing-code" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1" target="_blank" class=""><u class=""></u><u class=""></u><u class=""></u><u class=""></u></a>Impact on existing code</h2><div style="margin-top:0px;color:rgb(51,51,51);font-family:'Helvetica Neue',Helvetica,'Segoe UI',Arial,freesans,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px;background-color:rgb(255,255,255);margin-bottom:0px!important" class="">This will have no impact on existing code, but will probably require improving the Fix-It that was created for migrating <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px" class="">typealias</code> to <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:14px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px" class="">associatedtype</code> in Swift 2.2.</div></div></div>_______________________________________________<br class="">
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