<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto">Really like Tony’s suggestion, much cleaner than yet another annotation rammed into the signature. Also the idea of a static factory that could accept previously initialized arguments would be very powerful. <br><br><div id="AppleMailSignature">-- Howard.</div><div><br>On 26 Nov 2017, at 9:25 am, Tony Allevato via swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>> wrote:<br><br></div><blockquote type="cite"><div><div dir="ltr">On Sat, Nov 25, 2017 at 1:16 PM Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com">xiaodi.wu@gmail.com</a>> wrote:<br><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div>On Sat, Nov 25, 2017 at 15:06 Matthew Johnson <<a href="mailto:matthew@anandabits.com" target="_blank">matthew@anandabits.com</a>> wrote:<br></div><div><div class="gmail_quote"><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><blockquote type="cite"><div>On Nov 25, 2017, at 1:28 PM, Tony Allevato via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>> wrote:</div><br class="m_-6940704932475576458m_-1505466801171777419Apple-interchange-newline"><div><div><div class="m_-6940704932475576458m_-1505466801171777419markdown-here-wrapper"><p style="margin:1.2em 0px!important">On Fri, Nov 24, 2017 at 7:18 PM Xiaodi Wu via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>> wrote:</p><div style="margin:1.2em 0px!important"><br class="m_-6940704932475576458m_-1505466801171777419webkit-block-placeholder"></div><div class="m_-6940704932475576458m_-1505466801171777419markdown-here-exclude"><div><br class="m_-6940704932475576458m_-1505466801171777419webkit-block-placeholder"></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div>It's kludgy, but we could have something like:<div><br></div><div>```</div><div>@defaultArgument(configuration = (), where R.Configuration == Void)</div><div>@defaultArgument(actionHandler = { _ in }, where R.Action == Never)</div><div><span style="font-size:12.800000190734863px">func makeResource(with configuration: R.Configuration, actionHandler: @escaping (R.Action) -> Void) -> R { ... }</span><br></div><div>```<br><div><br></div><div>I don't like that we'd be setting a default argument on something lexically before even encountering it in the declaration, but it's serviceable.</div></div></div></blockquote><div><br class="m_-6940704932475576458m_-1505466801171777419webkit-block-placeholder"></div></div><div style="margin:1.2em 0px!important"><br class="m_-6940704932475576458m_-1505466801171777419webkit-block-placeholder"></div><p style="margin:1.2em 0px!important">What if we could take advantage of the fact that you can have non-constant expressions in default arguments? Overload resolution could already do most of the job—what we need on top of that is a way for the author to say that “if no overload matches, then it’s not an error—just don’t have a default argument in that case”. Something like SFINAE in C++, but more explicit.</p><p style="margin:1.2em 0px!important">I’m imagining something like this:</p>
<pre style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;font-size:1em;line-height:1.2em;margin:1.2em 0px"><code class="m_-6940704932475576458m_-1505466801171777419language-swift m_-6940704932475576458m_-1505466801171777419hljs" style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;display:inline;white-space:pre-wrap;overflow:auto;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;border:1px solid rgb(204,204,204);padding:0.5em 0.7em;display:block!important;display:block;overflow-x:auto;padding:0.5em;color:rgb(51,51,51);background-color:rgb(248,248,248);background-position:initial initial;background-repeat:initial initial"><span class="m_-6940704932475576458m_-1505466801171777419hljs-func"><span class="m_-6940704932475576458m_-1505466801171777419hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">func</span> <span class="m_-6940704932475576458m_-1505466801171777419hljs-title" style="color:rgb(153,0,0);font-weight:bold">defaultConfiguration</span><span class="m_-6940704932475576458m_-1505466801171777419hljs-params">()</span> -> <span style="color:rgb(153,0,0);font-weight:bold">Void</span> </span>{
<span class="m_-6940704932475576458m_-1505466801171777419hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">return</span> ()
}
<span class="m_-6940704932475576458m_-1505466801171777419hljs-func"><span class="m_-6940704932475576458m_-1505466801171777419hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">func</span> <span class="m_-6940704932475576458m_-1505466801171777419hljs-title" style="color:rgb(153,0,0);font-weight:bold">defaultActionHandler</span><span class="m_-6940704932475576458m_-1505466801171777419hljs-params">()</span> -> <span class="m_-6940704932475576458m_-1505466801171777419hljs-params">(Never)</span> -> <span class="m_-6940704932475576458m_-1505466801171777419hljs-title" style="color:rgb(153,0,0);font-weight:bold">Void</span> </span>{
<span class="m_-6940704932475576458m_-1505466801171777419hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">return</span> { <span class="m_-6940704932475576458m_-1505466801171777419hljs-number" style="color:rgb(0,128,128)">_</span> <span class="m_-6940704932475576458m_-1505466801171777419hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">in</span> }
}
<span class="m_-6940704932475576458m_-1505466801171777419hljs-class"><span class="m_-6940704932475576458m_-1505466801171777419hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">struct</span> <span class="m_-6940704932475576458m_-1505466801171777419hljs-title" style="color:rgb(153,0,0);font-weight:bold;color:rgb(68,85,136);font-weight:bold">ResourceDescription</span><<span class="m_-6940704932475576458m_-1505466801171777419hljs-title" style="color:rgb(153,0,0);font-weight:bold;color:rgb(68,85,136);font-weight:bold">R</span>: <span class="m_-6940704932475576458m_-1505466801171777419hljs-title" style="color:rgb(153,0,0);font-weight:bold;color:rgb(68,85,136);font-weight:bold">Resource</span>> </span>{
<span class="m_-6940704932475576458m_-1505466801171777419hljs-func"><span class="m_-6940704932475576458m_-1505466801171777419hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">func</span> <span class="m_-6940704932475576458m_-1505466801171777419hljs-title" style="color:rgb(153,0,0);font-weight:bold">makeResource</span><span class="m_-6940704932475576458m_-1505466801171777419hljs-params">(
with configuration: R.Configuration <b>=?</b> defaultConfiguration<span class="m_-6940704932475576458m_-1505466801171777419hljs-params">()</span>,
actionHandler: @escaping <span class="m_-6940704932475576458m_-1505466801171777419hljs-params">(R.Action)</span> -> Void <b>=?</b> defaultActionHandler<span class="m_-6940704932475576458m_-1505466801171777419hljs-params">()</span>
)</span> -> <span class="m_-6940704932475576458m_-1505466801171777419hljs-title" style="color:rgb(153,0,0);font-weight:bold">R</span> </span>{
<span class="m_-6940704932475576458m_-1505466801171777419hljs-comment" style="color:rgb(153,153,136);font-style:italic">// create a resource using the provided configuration</span>
<span class="m_-6940704932475576458m_-1505466801171777419hljs-comment" style="color:rgb(153,153,136);font-style:italic">// connect the action handler</span>
<span class="m_-6940704932475576458m_-1505466801171777419hljs-comment" style="color:rgb(153,153,136);font-style:italic">// return the resource</span>
}
}
</code></pre><p style="margin:1.2em 0px!important">The main difference here is the strawman <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;display:inline">=?</code> syntax, which would indicate that “the default argument exists if there is a way the RHS can be satisfied for some instances of the generic arguments; otherwise, there is no default”, instead of today’s behavior where it would be an error. There could be multiple overloads of <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;display:inline">defaultConfiguration</code> and <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;display:inline">defaultActionHandler</code> (even ones that are themselves generic) and it would do the right thing when there are matches and when there aren’t.</p><p style="margin:1.2em 0px!important">I like this approach because it mostly takes advantage of existing language features and is fairly lightweight in terms of how it’s expressed in code compared to regular default arguments—we’d just need to design the new operator and type-checker logic around it.</p></div></div></div></blockquote></div></div><div style="word-wrap:break-word"><div><div>This is an interesting approach. One advantage to something in this direction is that it could support defining different defaults for the same argument under different constraints by overloading the default argument factories on their return type.</div><div><br></div><div>One concern I have is that it doesn’t allows us to clearly define under which constraints a default argument is available. I suspect this might be problematic especially for public interfaces where source compatibility is a concern. </div><div></div></div></div></blockquote><div dir="auto"><br></div></div></div><div><div class="gmail_quote"><div dir="auto">It's certainly an interesting idea but it would suggest that the constraints under which a default argument is available can change at runtime. I'm concerned, like you, that this is difficult to reason about. It is still unclear to me how widespread the underlying issue is that requires conditional default arguments, but the conversation thus far has been about compile-time constraints and Tony's design seems to envision much more than that.</div></div></div></blockquote><div><br></div><div></div><div></div><div class="markdown-here-wrapper" style=""><p style="margin:1.2em 0px!important">This runtime/reasoning problem <b>already exists</b> today with default arguments, because you can write something like this:</p>
<pre style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;font-size:1em;line-height:1.2em;margin:1.2em 0px"><code class="hljs language-swift" style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;display:inline;white-space:pre;overflow:auto;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;border:1px solid rgb(204,204,204);padding:0.5em 0.7em;display:block!important;display:block;overflow-x:auto;padding:0.5em;color:rgb(51,51,51);background-color:rgb(248,248,248);background-position:initial initial;background-repeat:initial initial"><span class="hljs-class"><span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">struct</span> <span class="hljs-title" style="color:rgb(153,0,0);font-weight:bold;color:rgb(68,85,136);font-weight:bold">Foo</span> </span>{
<span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">static</span> <span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">var</span> defaultExponent = <span class="hljs-number" style="color:rgb(0,128,128)">2.0</span>
<span class="hljs-func"><span class="hljs-keyword" style="color:rgb(51,51,51);font-weight:bold">func</span> <span class="hljs-title" style="color:rgb(153,0,0);font-weight:bold">raise</span><span class="hljs-params">(<span class="hljs-number" style="color:rgb(0,128,128)">_</span> x: Double, to exponent: Double = defaultExponent)</span> </span>{
<span class="hljs-built_in" style="color:rgb(0,134,179)">print</span>(pow(x, exponent))
}
}
<span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">Foo</span>().raise(<span class="hljs-number" style="color:rgb(0,128,128)">4</span>) <span class="hljs-comment" style="color:rgb(153,153,136);font-style:italic">// "16.0"</span>
<span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">Foo</span>.defaultExponent = <span class="hljs-number" style="color:rgb(0,128,128)">3.0</span>
<span class="hljs-type" style="color:rgb(68,85,136);font-weight:bold">Foo</span>().raise(<span class="hljs-number" style="color:rgb(0,128,128)">4</span>) <span class="hljs-comment" style="color:rgb(153,153,136);font-style:italic">// "64.0"</span>
</code></pre>
<p style="margin:1.2em 0px!important">Swift lets you write a default value expression that references static (but not instance) vars of the enclosing type, as well as anything else that’s visible from that expression’s scope. Should people do this? Probably not, for the reasons that you described.</p>
<p style="margin:1.2em 0px!important">But the point is that my example is no more harmful or difficult to reason about than default arguments in the language today. My proposed solution <b>in no way</b> changes the runtime behavior of default argument expressions. I’m not envisioning anything more than what default arguments can already do except for adding a way to choose different default factories (or choose none without error) based on the <em>static</em> types of the generic arguments that are bound at a particular call site.</p>
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PC9kaXY+" style="height:0;width:0;max-height:0;max-width:0;overflow:hidden;font-size:0em;padding:0;margin:0"></div></div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div class="gmail_quote"><div dir="auto"><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 dir="auto">I think I prefer Xiaodi’s suggestion for that reason. His approach could also support multiple defaults for the same parameter as long as the constraints are not allowed to overlap (overlapping constraints would result in ambiguity similar to ambiguous overload resolution) or an explicit argument is required if they do.<br></div></div><div style="word-wrap:break-word"><div><br><blockquote type="cite"><div><div><div class="m_-6940704932475576458m_-1505466801171777419markdown-here-wrapper"><p style="margin:1.2em 0px!important"><br></p><div style="margin:1.2em 0px!important"><br class="m_-6940704932475576458m_-1505466801171777419webkit-block-placeholder"></div><div class="m_-6940704932475576458m_-1505466801171777419markdown-here-exclude"><div><br class="m_-6940704932475576458m_-1505466801171777419webkit-block-placeholder"></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div><div><br></div><div><div class="gmail_extra"><br><div class="gmail_quote">On Fri, Nov 24, 2017 at 8:36 PM, T.J. Usiyan via swift-evolution <span><<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><div>I am all for this. are many types where there is an obvious 'zero' or 'default' value and the ability to express "use that when possible" without an overload is welcome.<div><br></div><div><br></div><div>The best thing that I can think of right now, in terms of syntax, is actually using @overload</div><div><br></div><div>```</div><div><span class="m_-6940704932475576458m_-1505466801171777419m_2686493406202021523m_7361162569456919605gmail-"><div style="font-size:12.800000190734863px">struct ResourceDescription<R: Resource> {</div><div style="font-size:12.800000190734863px"><br></div><div style="font-size:12.800000190734863px"> func makeResource(with configuration: R.Configuration, actionHandler: @escaping (R.Action) -> Void) -> R </div></span><div style="font-size:12.800000190734863px"> @overload(R.Configuration == Void) func makeResource(actionHandler: @escaping (R.Action) -> Void) -> R</div><div style="font-size:12.800000190734863px">@overload(R.Action == Never) func makeResource(with configuration: R.Configuration) -> R</div><span class="m_-6940704932475576458m_-1505466801171777419m_2686493406202021523m_7361162569456919605gmail-"><div style="font-size:12.800000190734863px">{</div><div style="font-size:12.800000190734863px"> // create a resource using the provided configuration</div><div style="font-size:12.800000190734863px"> // connect the action handler</div><div style="font-size:12.800000190734863px"> // return the resource</div><div style="font-size:12.800000190734863px"> }</div><div style="font-size:12.800000190734863px">}</div></span></div><div>```</div><div><br></div><div><br></div><div>This isn't great though…</div></div><div class="gmail_extra"><br><div class="gmail_quote"><div><div class="m_-6940704932475576458m_-1505466801171777419m_2686493406202021523m_7361162569456919605gmail-h5">On Fri, Nov 24, 2017 at 6:11 PM, Matthew Johnson via swift-evolution <span><<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>></span> wrote:<br></div></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><div><div class="m_-6940704932475576458m_-1505466801171777419m_2686493406202021523m_7361162569456919605gmail-h5"><div style="word-wrap:break-word"><div>As mentioned in my prior message, I currently have a PR open to update the generics manifesto (<a href="https://github.com/apple/swift/pull/13012" target="_blank">https://github.com/apple/swift/pull/13012</a>). I removed one topic from that update at Doug Gregor’s request that it be discussed on the list first. </div><div><br></div><div>The idea is to add the ability to make default arguments conditional (i.e. depend on generic constraints). It is currently possible to emulate conditional default arguments using an overload set. This is verbose, especially when several arguments are involved. Here is an example use case using the overload method to emulate this feature:</div><div><br></div><div>```swift</div><div>protocol Resource {</div><div> associatedtype Configuration</div><div> associatedtype Action</div><div>}</div><div>struct ResourceDescription<R: Resource> {</div><div> func makeResource(with configuration: R.Configuration, actionHandler: @escaping (R.Action) -> Void) -> R {</div><div> // create a resource using the provided configuration</div><div> // connect the action handler</div><div> // return the resource</div><div> }</div><div>}</div><div><br></div><div>extension ResourceDescription where R.Configuration == Void {</div><div> func makeResource(actionHandler: @escaping (R.Action) -> Void) -> R {</div><div> return makeResource(with: (), actionHandler: actionHandler)</div><div> }</div><div>}</div><div><br></div><div>extension ResourceDescription where R.Action == Never {</div><div> func makeResource(with configuration: R.Configuration) -> R {</div><div> return makeResource(with: configuration, actionHandler: { _ in })</div><div> }</div><div>}</div><div><br></div><div>extension ResourceDescription where R.Configuration == Void, R.Action == Never {</div><div> func makeResource() -> R {</div><div> return makeResource(with: (), actionHandler: { _ in })</div><div> }</div><div>}</div><div><br></div><div>```</div><div><br></div><div>Adding language support for defining these more directly would eliminate a lot of boilerplate and reduce the need for overloads. Doug mentioned that it may also help simplify associated type inference (<a href="https://github.com/apple/swift/pull/13012#discussion_r152124535" target="_blank">https://github.com/apple/swift/pull/13012#discussion_r152124535</a>).</div><div><br></div><div>The reason that I call this a pre-pitch and one reason Doug requested it be discussed on list is that I haven’t thought of a good way to express this syntactically. I am interested in hearing general feedback on the idea. I am also looking for syntax suggestions.</div><span class="m_-6940704932475576458m_-1505466801171777419m_2686493406202021523m_7361162569456919605gmail-m_-7331325194590092274HOEnZb"><font color="#888888"><div><br></div><div>Matthew</div></font></span></div><br></div></div>_______________________________________________<br>
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<br></blockquote></div><br></div>
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