<div style="white-space:pre-wrap">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><br>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></div><br><div class="gmail_quote"><div dir="ltr">On Mon, May 9, 2016 at 12:48 AM David Hart <<a href="mailto:david@hartbit.com">david@hartbit.com</a>> wrote:<br></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"><div>Hello Xiaodi,</div><div><br></div><div>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"><br><div><blockquote type="cite"><div>On 09 May 2016, at 07:28, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" target="_blank">xiaodi.wu@gmail.com</a>> wrote:</div><br><div>If the protocol Sequence has typealias Element, does that mean I also have MyConformingSequence.Element?<br><br>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><br>This is a toy example, of course. More generally, though, I wonder about this question:<br><br>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><br>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><div class="gmail_quote"><div dir="ltr">On Sun, May 8, 2016 at 6:17 PM David Hart via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>> wrote:<br></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">Hello,<div><br></div><div>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><br></div><div><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">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)"><li>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">SE-XXXX</a></li><li>Authors: <a href="https://github.com/hartbit" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none" target="_blank">David Hart</a>, <a href="https://github.com/DougGregor" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none" target="_blank">Doug Gregor</a></li><li>Status: TBD</li><li>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)"><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"><u></u><u></u><u></u><u></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)">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">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">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)"><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"><u></u><u></u><u></u><u></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)">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">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">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">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">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)"><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"><u></u><u></u><u></u><u></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)">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)"><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"><span style="color:rgb(167,29,93)">protocol</span> Sequence {
associatedtype Iterator <span style="color:rgb(167,29,93)">:</span> IteratorProtocol
<span style="color:rgb(167,29,93)">typealias</span> Element <span style="color:rgb(167,29,93)">=</span> Iterator<span style="color:rgb(167,29,93)">.</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)">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)"><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"><span style="color:rgb(167,29,93)">func</span> <span style="color:rgb(121,93,163)">sum</span><T: Sequence <span style="color:rgb(167,29,93)">where</span> T<span style="color:rgb(167,29,93)">.</span>Element <span style="color:rgb(167,29,93)">==</span> <span style="color:rgb(0,134,179)">Int</span>>(sequence: T) <span style="color:rgb(167,29,93)">-></span> <span style="color:rgb(0,134,179)">Int</span> {
<span style="color:rgb(167,29,93)">return</span> sequence<span style="color:rgb(167,29,93)">.</span><span style="color:rgb(0,134,179)">reduce</span>(<span style="color:rgb(0,134,179)">0</span>, combine: <span style="color:rgb(167,29,93)">+</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)"><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"><u></u><u></u><u></u><u></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)">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)"><em>protocol-member-declaration</em> → <em>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)"><em>protocol-typealias-declaration</em> → <em>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)"><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"><u></u><u></u><u></u><u></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">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">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">associatedtype</code> in Swift 2.2.</div></div></div>_______________________________________________<br>
swift-evolution mailing list<br>
<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a><br>
<a href="https://lists.swift.org/mailman/listinfo/swift-evolution" rel="noreferrer" target="_blank">https://lists.swift.org/mailman/listinfo/swift-evolution</a><br>
</blockquote></div>
</div></blockquote></div><br></div></blockquote></div>