<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 Jun 23, 2016, at 1:39 PM, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" class="">xiaodi.wu@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">Good to know. I absolutely agree that the gains to be had here wouldn't be worth a one-off hack.<br class=""></div></blockquote><div><br class=""></div><div>If people are strongly (or even mildly) opposed to removing this rule, we can give some thought to a more general solution. I don’t feel very strongly about this proposal one way or another, it’s just a bit of ugly code in TypeCheckType.cpp that would be nice to remove, and a developer on Twitter recently noticed this behavior and found it surprising.</div><div><br class=""></div><blockquote type="cite" class=""><div class=""><div class="gmail_quote"><div dir="ltr" class="">On Thu, Jun 23, 2016 at 15:36 Slava Pestov <<a href="mailto:spestov@apple.com" class="">spestov@apple.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=""><blockquote type="cite" class=""><div class="">On Jun 23, 2016, at 1:34 PM, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" target="_blank" class="">xiaodi.wu@gmail.com</a>> wrote:</div><br class=""><div class="">Sorry, it's I who is saying things all wrong. I meant to ask, is it feasible to keep both behaviors but have #2 "win" over #1, instead of getting rid of behavior #1 entirely?<br class=""></div></blockquote><div class=""><br class=""></div></div></div><div style="word-wrap:break-word" class=""><div class="">I suspect there might be some way, but I think it would have to be some kind of one-off hack, which is not in line with our long-term goal of making the type checker more maintainable and correct ‘by construction’.</div></div><div style="word-wrap:break-word" class=""><div class=""><br class=""><blockquote type="cite" class=""><div class=""><br class=""><div class="gmail_quote"><div dir="ltr" class="">On Thu, Jun 23, 2016 at 15:30 Slava Pestov <<a href="mailto:spestov@apple.com" target="_blank" class="">spestov@apple.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=""><blockquote type="cite" class=""><div class="">On Jun 23, 2016, at 1:27 PM, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" target="_blank" class="">xiaodi.wu@gmail.com</a>> wrote:</div><br class=""><div class="">When you mention the difficulty of an alternative, is that to say that it's not feasible for the GenericBox in the last example to be resolved as GenericBox<T>? From an end-user point of view, that seems to be the most sensible behavior.<br class=""></div></blockquote><div class=""><br class=""></div></div></div><div style="word-wrap:break-word" class=""><div class="">With my proposed change, GenericBox would be resolved as GenericBox<T> in the last example. Right now it fails to type check.</div><div class=""><br class=""></div><div class="">Here is an example that works right now, but would not work with my proposed change:</div><div class=""><br class=""></div><div class="">struct GenericBox<Contents> {</div><div class=""><span style="white-space:pre-wrap" class=""> </span>// Currently Swift resolves this as ‘GenericBox<Contents>’</div><div class=""><span style="white-space:pre-wrap" class=""> </span>// With the new rule, we cannot infer the parameter, because there’s no expression to infer it from</div><div class=""><span style="white-space:pre-wrap" class=""> </span>func combine(other: GenericBox) {</div><div class=""><span style="white-space:pre-wrap" class=""> </span>…</div><div class=""><span style="white-space:pre-wrap" class=""> </span>}</div><div class="">}</div><div class=""><br class=""></div><div class="">Basically the meaning of ‘GenericBox’ right now depends on whether it appears inside its own definition or extension thereof, or not. The behavior when it appears elsewhere is more general — we infer the parameters from the surrounding expression, instead of assuming they’re equal to the context parameters.</div><div class=""><br class=""></div><div class="">This is a subtle change — definitely let me know if I’m not explaining it well.</div></div><div style="word-wrap:break-word" class=""><div class=""><br class=""></div><div class="">Slava</div></div><div style="word-wrap:break-word" class=""><div class=""><br class=""><blockquote type="cite" class=""><div class=""><div class="gmail_quote"><div dir="ltr" class="">On Thu, Jun 23, 2016 at 15:14 Slava Pestov 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=""><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="">Simpler interpretation of a reference to a generic type with no arguments</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/slavapestov/swift-evolution/blob/silly-proposals/proposals/9999-simplify-unbound-generic-type.md" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none" target="_blank" class="">SE-9999</a></li><li class="">Author: <a href="https://github.com/slavapestov" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none" target="_blank" class="">Slava Pestov</a></li><li class="">Status: <span class="">Awaiting review</span></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/slavapestov/swift-evolution/tree/silly-proposals/proposals#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 cleans up the semantics of a reference to a generic type when no generic arguments are applied.</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="">Swift-evolution thread: <a href="http://news.gmane.org/gmane.comp.lang.swift.evolution" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none" target="_blank" class="">Discussion thread topic for that proposal</a></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/slavapestov/swift-evolution/tree/silly-proposals/proposals#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="">Right now, we allow a generic type to be referenced with no generic arguments applied in a handful of special cases. The two primary rules here are the following:</p><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=""><p style="margin-top:16px;margin-bottom:16px" class="">If the scope from which the reference is made is nested inside the definition of the type or an extension thereof, omitting generic arguments just means to implicitly apply the arguments from context.</p><p style="margin-top:16px;margin-bottom:16px" class="">For example,</p></li></ul><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:13.600000381469727px;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" class=""><span style="color:rgb(167,29,93)" class="">struct</span> GenericBox<span style="color:rgb(167,29,93)" class=""><</span>Contents<span style="color:rgb(167,29,93)" class="">></span> {
<span style="color:rgb(167,29,93)" class="">let</span> contents: Contents
<span style="color:rgb(150,152,150)" class="">// Equivalent to: func clone() -> GenericBox<Contents></span>
<span style="color:rgb(167,29,93)" class="">func</span> <span style="color:rgb(121,93,163)" class="">clone</span>() <span style="color:rgb(167,29,93)" class="">-></span> GenericBox {
<span style="color:rgb(167,29,93)" class="">return</span> GenericBox(contents: contents)
}
}
<span style="color:rgb(167,29,93)" class="">extension</span> GenericBox {
<span style="color:rgb(167,29,93)" class="">func</span> <span style="color:rgb(121,93,163)" class="">print</span>() {
<span style="color:rgb(150,152,150)" class="">// Equivalent to: let cloned: GenericBox<Contents></span>
<span style="color:rgb(167,29,93)" class="">let</span> cloned: GenericBox <span style="color:rgb(167,29,93)" class="">=</span> clone()
<span style="color:rgb(0,134,179)" class="">print</span>(cloned<span style="color:rgb(167,29,93)" class="">.</span>contents)
}
}</pre></div><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=""><p style="margin-top:16px;margin-bottom:16px" class="">If the type is referenced from an unrelated scope, we attempt to infer the generic parameters.</p><p style="margin-top:16px;margin-bottom:16px" class="">For example,</p></li></ul><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:13.600000381469727px;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" class=""><span style="color:rgb(167,29,93)" class="">func</span> <span style="color:rgb(121,93,163)" class="">makeABox</span>() <span style="color:rgb(167,29,93)" class="">-></span> GenericBox<<span style="color:rgb(0,134,179)" class="">Int</span>> {
<span style="color:rgb(150,152,150)" class="">// Equivalent to: GenericBox<Int>(contents: 123)</span>
<span style="color:rgb(167,29,93)" class="">return</span> GenericBox(contents: <span style="color:rgb(0,134,179)" class="">123</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)" class="">The problem appears when the user expects the second behavior, but instead encounters the first. For example, the following does not type check:</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:13.600000381469727px;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" class=""><span style="color:rgb(167,29,93)" class="">extension</span> GenericBox {
<span style="color:rgb(167,29,93)" class="">func</span> <span style="color:rgb(121,93,163)" class="">transform</span><T>(f: Contents <span style="color:rgb(167,29,93)" class="">-></span> T) <span style="color:rgb(167,29,93)" class="">-></span> GenericBox<T> {
<span style="color:rgb(150,152,150)" class="">// We resolve 'GenericBox' as 'GenericBox<Contents>', rather than</span>
<span style="color:rgb(150,152,150)" class="">// inferring the type parameter</span>
<span style="color:rgb(167,29,93)" class="">return</span> GenericBox(contents: f(contents))
}
}</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/slavapestov/swift-evolution/tree/silly-proposals/proposals#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 proposed solution is to remove the first rule altogether. If the generic parameters cannot be inferred from context, they must be specified explicitly with the usual <code style="font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:13.600000381469727px;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="">Type<Args...></code> syntax.</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/slavapestov/swift-evolution/tree/silly-proposals/proposals#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="">This really just involves removing an existing piece of logic from the type resolver code.</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/slavapestov/swift-evolution/tree/silly-proposals/proposals#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><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 will have a small impact on existing code that uses a pattern similar to the above.</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/slavapestov/swift-evolution/tree/silly-proposals/proposals#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" class=""><u class=""></u><u class=""></u><u class=""></u><u class=""></u></a>Alternatives considered</h2><h3 style="margin-top:1em;margin-bottom:16px;line-height:1.43;font-size:1.5em;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/slavapestov/swift-evolution/tree/silly-proposals/proposals#status-quo" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1.2" target="_blank" class=""><u class=""></u><u class=""></u><u class=""></u><u class=""></u></a>Status quo</h3><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="">We could keep the current behavior, but one can argue it is not very useful, and adds a special case where one is not needed.</p><h3 style="margin-top:1em;margin-bottom:16px;line-height:1.43;font-size:1.5em;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/slavapestov/swift-evolution/tree/silly-proposals/proposals#more-complex-inference-of-generic-parameters" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none;display:inline-block;padding-right:2px;line-height:1.2" target="_blank" class=""><u class=""></u><u class=""></u><u class=""></u><u class=""></u></a>More complex inference of generic parameters</h3><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="">We could attempt to unify the two rules for resolving a reference to a generic type with no arguments, however this presents theoretical difficulties with our constraint solver design. Even if it were easy to implement, it would increase type checking type by creating new possibilities to consider, with very little actual benefit.</div></div>_______________________________________________<br class="">
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