<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div><br></div><div>On Jan 9, 2017, at 02:13, Charlie Monroe via swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>> wrote:<br><br></div><blockquote type="cite"><div><meta http-equiv="Content-Type" content="text/html charset=us-ascii"><div class="">I came across something that I'm not sure it's a bug or by design and if it's by design, whether this should be discussed here.</div><div class=""><br class=""></div><div class="">Example:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">class</span><span style="font-variant-ligatures: no-common-ligatures" class=""> Foo {</span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">init</span><span style="font-variant-ligatures: no-common-ligatures" class="">(number: </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span><span style="font-variant-ligatures: no-common-ligatures" class="">) { </span><span style="font-variant-ligatures: no-common-ligatures; color: #008400" class="">/* ... */</span><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo; min-height: 10px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><br class=""></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">let</span><span style="font-variant-ligatures: no-common-ligatures" class=""> closure = </span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">Foo</span><span style="font-variant-ligatures: no-common-ligatures" class="">.</span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">init</span><span style="font-variant-ligatures: no-common-ligatures" class="">(number:) </span><span style="font-variant-ligatures: no-common-ligatures; color: #008400" class="">// (Int) -> Foo</span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">[</span><span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">1</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">, </span><span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">2</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">, </span><span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">3</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">].</span><span style="font-variant-ligatures: no-common-ligatures; color: #3e1e81" class="">map</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">(</span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">closure</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">) </span><span style="font-variant-ligatures: no-common-ligatures" class="">// [Foo, Foo, Foo]</span></div></div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><br class=""></span></div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class="">This works great until the initializer gets a default argument:</span></div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><br class=""></span></div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">class</span><span style="font-variant-ligatures: no-common-ligatures" class=""> Foo {</span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">init</span><span style="font-variant-ligatures: no-common-ligatures" class="">(number: </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span><span style="font-variant-ligatures: no-common-ligatures" class="">, string: </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">String</span><span style="font-variant-ligatures: no-common-ligatures" class=""> = </span><span style="font-variant-ligatures: no-common-ligatures; color: #d12f1b" class="">""</span><span style="font-variant-ligatures: no-common-ligatures" class="">) { </span><span style="font-variant-ligatures: no-common-ligatures; color: #008400" class="">/* ... */</span><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo; min-height: 10px;" class=""><span style="color: rgb(0, 132, 0);" class=""><br class=""></span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo; min-height: 10px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""></span><span style="color: rgb(0, 132, 0);" class="">// Error: Foo has no member init(number:)</span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">let</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> closure = </span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">Foo</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">.</span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">init</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">(number:) </span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""><br class=""></span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""><span style="font-family: Helvetica; font-size: 10px;" class="">I was wondering if we could get closures to methods without the default arguments. Currently, this needs to be worked around by e.g. creating a second closure that invokes the method without the default arguments:</span></span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""><span style="font-family: Helvetica; font-size: 10px;" class=""><br class=""></span></span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">let</span> closure: (<span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Int</span>) -> <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">Foo</span> = { <span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">Foo</span>(number: $0) }</span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""><br class=""></span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""><span style="font-family: Helvetica; font-size: 10px;" class="">But to me it seems like something that should work "out of the box".</span></span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""><span style="font-family: Helvetica; font-size: 10px;" class=""><br class=""></span></span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""><span style="font-family: Helvetica; font-size: 10px;" class="">Thoughts?</span></span></div></span></div></div></blockquote><br><div><div><span style="background-color: rgba(255, 255, 255, 0);">IIRC, this issue was raised a while ago, and as best as I recall the gist of the answer was that default arguments are implemented at the call site, and because of that you can't pass a function with default arguments to something expecting a function with fewer arguments even though the two calls look identical in the source code.</span></div><span style="background-color: rgba(255, 255, 255, 0);"><br></span><div><span style="background-color: rgba(255, 255, 255, 0);">It causes other issues, too. For instance, if we have</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"> protocol Initable { init() }</span></div><div><span style="background-color: rgba(255, 255, 255, 0);">And</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"> struct Foo { init(_ x: Int = 0) {} }</span></div><div><span style="background-color: rgba(255, 255, 255, 0);">We're left in an odd situation where `Foo` can't meaningfully conform to `Initable` because while "init(_: Int = 0)" is not the same as "init()", if you add a "init()" to `Foo`</span></div><div><span style="background-color: rgba(255, 255, 255, 0);">you'll get an ambiguous somethingerather error because there's no mechanism for the compiler to know whether you want the actual "0 argument" function or the "1 argument with 1 default value" function.</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"><br></span></div><div><span style="background-color: rgba(255, 255, 255, 0);">Aside from re-architecting the default argument system (which I'm not even sure is possible, let alone a good idea), I think I see couple ways forward for the protocol conformance issue. Both have downsides, though.</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"><br></span></div><div><span style="background-color: rgba(255, 255, 255, 0);">1) Require any potentially conflicting protocol functions to be in an extension so the compiler knows what's going on, have "Foo()" call the one defined in the type, and use "(Foo as Initable)()" for the protocol version defined in an extension. This could get real confusing real fast if people don't realize there's two functions with, as far as they can tell, the same signature.</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"><br></span></div><div><span style="background-color: rgba(255, 255, 255, 0);">2) Add default argument support to protocols. The syntax that makes sense to me would be something like</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"> protocol Bar {</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"> func baz(_: Int = _)</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"> }</span></div><div><span style="background-color: rgba(255, 255, 255, 0);">On the downside, I suspect this would necessarily add a phantom "Self or associated type requirement" so that the compiler could have a way to get at each implementation's default value. It's not ideal... You'd get an error kinda out of the blue if you tried to use the function non-generically, but at least you couldn't have a function change out from under you.</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"><br></span></div><div><span style="background-color: rgba(255, 255, 255, 0);">- Dave Sweeris </span></div></div></body></html>