<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Thu, May 26, 2016 at 12:47 PM, L. Mihalkovic via swift-evolution <span dir="ltr"><<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="auto"><span class=""><div><br><br></div><div>On May 25, 2016, at 11:43 PM, David Hart via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>> wrote:<br><br></div><blockquote type="cite"><div>Here’s a pitch for removing associated type inference as per the Generics Manifesto. If we want to do it, we’d better do it before Swift 3:<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">Remove associated type inference</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/apple/swift-evolution/blob/master/proposals/XXXX-remove-associated-type-inference.md" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none" target="_blank">SE-XXXX</a></li><li>Author: <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">Douglas 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/remove-associated-type-inference/proposals/XXXX-remove-associated-type-inference.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 seeks to remove the inference of associated types in types conforming to protocols.</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/remove-associated-type-inference/proposals/XXXX-remove-associated-type-inference.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)">Even if associated types inference in a useful feature, it is also a big source of bugs in the compiler. This proposal argues that the usefulness does not outweight its architectural complexity. As per 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>:</p><blockquote style="margin:0px 0px 16px;padding:0px 15px;color:rgb(119,119,119);border-left-width:4px;border-left-style:solid;border-left-color:rgb(221,221,221);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)"><div style="margin-top:0px;margin-bottom:0px">associated type inference is the only place in Swift where we have a global type inference problem: it has historically been a major source of bugs, and implementing it fully and correctly requires a drastically different architecture to the type checker.</div></blockquote><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)">Because this is a breaking change, it would be beneficial to implement it for Swift 3. </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/remove-associated-type-inference/proposals/XXXX-remove-associated-type-inference.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 proposal would remove associated type inference and make code which relied on it invalid:</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;word-wrap:normal;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-break:normal"><span style="color:rgb(167,29,93)">protocol</span> IteratorProtocol {
associatedtype Element
<span style="color:rgb(167,29,93)">mutating</span> <span style="color:rgb(167,29,93)">func</span> <span style="color:rgb(121,93,163)">next</span>() <span style="color:rgb(167,29,93)">-></span> Element?
}
<span style="color:rgb(167,29,93)">struct</span> IntIterator <span style="color:rgb(167,29,93)">:</span> IteratorProtocol {
<span style="color:rgb(167,29,93)">mutating</span> <span style="color:rgb(167,29,93)">func</span> <span style="color:rgb(121,93,163)">next</span>() <span style="color:rgb(167,29,93)">-></span> <span style="color:rgb(0,134,179)">Int</span>? { <span style="color:rgb(167,29,93)">...</span> } <span style="color:rgb(150,152,150)">// used to infer Element = Int</span>
}</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 compiler would generate an error message stating: <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">error: IntIterator is missing its Element associated type declaration</code>. The code would have to be modified as follows to fix the error:</p></div></div></blockquote><div><br></div></span><div>From a purely english syntax viewpoint, i would prefer the error to be along the lines of</div><div><br></div><div><blockquote type="cite"><div><p style="margin-top:0px;margin-bottom:16px"><code style="padding:0.2em 0px;margin:0px;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgba(255,255,255,0)"><font color="#000000" face="UICTFontTextStyleTallBody">IntIterator is missing a declaration for the Element associated type</font></code></p></div></blockquote>Or </div><div><br><blockquote type="cite"><div><p style="margin-top:0px;margin-bottom:16px"><code style="padding:0.2em 0px;margin:0px;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;background-color:rgba(255,255,255,0)"><font color="#000000" face="UICTFontTextStyleTallBody">IntIterator is missing a declaration for its Element associated type</font></code></p></div></blockquote></div></div></blockquote><div>Clearer still:</div><div>"IntIterator is missing a declaration for its associated type Element"</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="auto"><span class=""><br><blockquote type="cite"><div><div><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;word-wrap:normal;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-break:normal"><span style="color:rgb(167,29,93)">struct</span> IntIterator <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> <span style="color:rgb(0,134,179)">Int</span>
<span style="color:rgb(167,29,93)">mutating</span> <span style="color:rgb(167,29,93)">func</span> <span style="color:rgb(121,93,163)">next</span>() <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> <span style="color:rgb(0,134,179)">nil</span> } <span style="color:rgb(150,152,150)">// used to infer Element = Int</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/remove-associated-type-inference/proposals/XXXX-remove-associated-type-inference.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><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 is a breaking change that will require conforming types which relied on the inference, including in the Standard Library, to explicitly declare associated types. A Fix-It could be introduced to add 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> and leave the type to be filled in. That way, all the type inference could be removed from the compiler.</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/remove-associated-type-inference/proposals/XXXX-remove-associated-type-inference.md#alternatives-considered" 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>Alternatives Considered</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">The only alternative is to keep the inference with the known consequences on the compiler.</div></div></div></blockquote></span><blockquote type="cite"><div><span>_______________________________________________</span><span class=""><br><span>swift-evolution mailing list</span><br><span><a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a></span><br><span><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" target="_blank">https://lists.swift.org/mailman/listinfo/swift-evolution</a></span><br></span></div></blockquote></div><br>_______________________________________________<br>
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