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<p dir="auto">Right now, collection types in the standard library (<code style="background-color:#F7F7F7; border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7">Array</code>, <code style="background-color:#F7F7F7; border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7">Dictionary</code>, and <code style="background-color:#F7F7F7; border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7">Set</code>) employ a workaround for the lack of conditional conformance, which would allow us to expose things like <code style="background-color:#F7F7F7; border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7">extension Array : Codable where Element : Codable</code> (i.e. <code style="background-color:#F7F7F7; border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7">Array<T></code> is <code style="background-color:#F7F7F7; border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7">Codable</code> if and only if <code style="background-color:#F7F7F7; border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7">T</code> is <code style="background-color:#F7F7F7; border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7">Codable</code>).<br>
These collections unconditionally conform to <code style="background-color:#F7F7F7; border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7">Codable</code> and trap at runtime if the type is not actually <code style="background-color:#F7F7F7; border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7">Codable</code>. The synthesis complaint is an explicit check in the compiler so we don’t go ahead and happily synthesize code on your behalf that would just trap at runtime.</p>
<p dir="auto">When the conditional conformance feature lands, <code style="background-color:#F7F7F7; border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7">[String : Any]() is Codable</code> should return <code style="background-color:#F7F7F7; border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7">false</code>.</p>
<p dir="auto">On 19 Oct 2017, at 14:15, Kevin Nattinger wrote:</p>
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<blockquote style="border-left:2px solid #777; color:#777; margin:0 0 5px; padding-left:5px"><div id="F696563A-EBB0-4201-870D-60AE9948EAFB"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">So, what I see as the problem here is contradictory compiler statements:<div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 13px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><b class="">MyPlayground.playground:5:9: note: cannot automatically synthesize 'Encodable' because '[String : Any]' does not conform to 'Encodable'</b></span></div><div style="margin: 0px; font-size: 13px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><b class=""> let someDict: [String : Any]</b></span></div><div style="margin: 0px; font-size: 13px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><b class=""> ^</b></span></div><div style="margin: 0px; font-size: 13px; line-height: normal; font-family: Menlo; min-height: 15px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><b class=""></b></span><br class=""></div><div style="margin: 0px; font-size: 13px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><b class="">warning: MyPlayground.playground:8:18: warning: 'is' test is always true</b></span></div><div style="margin: 0px; font-size: 13px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><b class="">[String : Any]() is Codable</b></span></div><div style="margin: 0px; font-size: 13px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><b class=""> ^</b></span></div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><b class=""><br class=""></b></span></div><div>So is it codable or isn't it? You can use a [String: Any] in encode() and decode() directly, so why doesn't it get compiler support? At the least, the message needs to be updated to something that's actually true. </div><div><br class=""><blockquote type="cite" class=""><div class="">On Oct 19, 2017, at 1:38 PM, David Sweeris via swift-users <<a href="mailto:swift-users@swift.org" class="">swift-users@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Oh! Yeah, my bad... You are correct; I'd started thinking like I was on -evolution instead of -users.<div class=""><br class=""><div class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Oct 19, 2017, at 1:29 PM, Itai Ferber <<a href="mailto:iferber@apple.com" class="">iferber@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">
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<div style="font-family:sans-serif" class=""><div style="white-space:normal" class=""><p dir="auto" class="">Mm, the point I’m trying to get at here is that JSON is inherently untyped while Swift is strongly typed, and the two don’t line up.<br class="">
It doesn’t really make sense to ask to decode an existential because there’s not enough type information to influence what you get back.</p><p dir="auto" class="">On 19 Oct 2017, at 13:20, David Sweeris wrote:</p>
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<blockquote style="border-left:2px solid #777; color:#777; margin:0 0 5px; padding-left:5px" class=""><div id="7C1D4686-733E-42FA-902B-BB3B23DA2A7D" class=""><div dir="auto" class=""><div class="">I think if you can figure that out, you’re halfway to letting protocols conform to themselves.</div><div class=""><br class=""></div><div class="">(Syntactically, I would probably say that something like “Codable.Self” would read well, but I think that already means something. Maybe the answer will become clearer when we rework the reflection APIs?)<br class=""><br class=""><div class="">Sent from my iPhone</div><div class=""><br class="">On Oct 19, 2017, at 13:13, Itai Ferber <<a href="mailto:iferber@apple.com" class="">iferber@apple.com</a>> wrote:<br class=""><br class=""></div><blockquote type="cite" class=""><div class="">
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<div style="font-family:sans-serif" class=""><div style="white-space:normal" class=""><p dir="auto" class="">Even then, that wouldn’t necessarily help in the general case. If you decode <code style="background-color:#F7F7F7; border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7" class="">{"key" : 1}</code> as <code style="background-color:#F7F7F7; border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7" class="">[String : Codable]</code>, what concrete type would <code style="background-color:#F7F7F7; border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7" class="">1</code> have? <code style="background-color:#F7F7F7; border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7" class="">Int</code>? <code style="background-color:#F7F7F7; border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7" class="">Double</code>? <code style="background-color:#F7F7F7; border-radius:3px; margin:0; padding:0 0.4em" bgcolor="#F7F7F7" class="">Int8</code>? (Arguments can be made for any one of these, but the key here is that it is inherently ambiguous and there isn’t necessarily a good answer.)</p><p dir="auto" class="">On 19 Oct 2017, at 12:57, David Sweeris wrote:</p>
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<div style="white-space:normal" class=""><blockquote style="border-left:2px solid #777; color:#777; margin:0 0 5px; padding-left:5px" class=""><blockquote style="border-left:2px solid #777; color:#999; margin:0 0 5px; padding-left:5px; border-left-color:#999" class=""><p dir="auto" class="">On Oct 19, 2017, at 12:50 PM, David Baraff via swift-users <<a href="mailto:swift-users@swift.org" class="">swift-users@swift.org</a>> wrote:<br class="">
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Yes; this is a case where anywhere in the code base I want to just say<br class="">
struct MyNewType : Codable {<br class="">
// add codable datatypes<br class="">
}<br class="">
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and don’t want/can’t always go to the centralized place to add it in.<br class="">
Is there some extension-like trick I can pull off that lets me spread the implementation out over different files/libraries?</p>
</blockquote><p dir="auto" class="">Ah, ok.<br class="">
<br class="">
No, I don't think you'll be able to do that until/unless Swift gets more macro/metaprogramming features. Maybe if protocols ever get to conform to themselves? That's a common request, but implementing it is apparently beyond tricky. I'm pretty sure somebody's working on it, but "bigger fish" and all that...<br class="">
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- Dave Sweeris</p>
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