<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=""><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><br class=""><div><blockquote type="cite" class=""><div class="">On 09 May 2016, at 07:28, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" class="">xiaodi.wu@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><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" 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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