<div dir="ltr"><br><br><div class="gmail_quote"><div dir="ltr">On Mon, Mar 13, 2017 at 4:41 PM Tony Allevato <<a href="mailto:tony.allevato@gmail.com">tony.allevato@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr" class="gmail_msg"><div class="gmail_quote gmail_msg"><div dir="ltr" class="gmail_msg">On Mon, Mar 13, 2017 at 4:41 PM Tony Allevato <<a href="mailto:tony.allevato@gmail.com" class="gmail_msg" target="_blank">tony.allevato@gmail.com</a>> wrote:<br class="gmail_msg"></div><blockquote class="gmail_quote gmail_msg" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr" class="gmail_msg">This strikes me as fairly equivalent to "union types", which are on the frequently rejected proposals list:</div></blockquote><div class="gmail_msg"><br class="gmail_msg"></div></div></div><div dir="ltr" class="gmail_msg"><div class="gmail_quote gmail_msg"><div class="gmail_msg">(hit Send too soon)</div></div></div></blockquote><div><br></div><div>(My e-mail client hates me today; apologies for the spam)</div><div><br></div><div><a href="https://github.com/apple/swift-evolution/blob/master/commonly_proposed.md">https://github.com/apple/swift-evolution/blob/master/commonly_proposed.md</a><br></div><div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr" class="gmail_msg"><div class="gmail_quote gmail_msg"><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg"> </div><blockquote class="gmail_quote gmail_msg" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr" class="gmail_msg"><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg"><br class="gmail_msg"><br class="gmail_msg"><div class="gmail_quote gmail_msg"><div dir="ltr" class="gmail_msg">On Mon, Mar 13, 2017 at 4:14 PM Elijah Johnson via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="gmail_msg" target="_blank">swift-evolution@swift.org</a>> wrote:<br class="gmail_msg"></div><blockquote class="gmail_quote gmail_msg" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class="gmail_msg">I don’t know if this feature has yet existed in any languague, but its pretty simple.<div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">Let say you are preparing data for a JSON encoder. Right now, you would add to an Dictionary or Array like this one:</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">let data = [String:Any]();</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">but what one is really saying is that “Any” is from a number of distinct types:</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">group typedef JSONValueType : String, Double, Int, [String:JSONValueType], [JSONValueType], Bool</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">So from the compiler’s point of view, this group typedef would be just like the “Any” type, only it would refuse to compile unless the proper type was passed. So the result would be, as an example, that there could be created an (almost) fail-safe JSON encoder:</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">let data = [String:JSONValueType]();</div><div class="gmail_msg">data[“key] = “value”;</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">I’m sure this has plenty of uses outside of JSON encoding and would be pretty simple to do. One problem that it would solve it the fact that in Swift, everything is included by Any, including optionals, ex:</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg"><p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class="gmail_msg"><span style="font-variant-ligatures:no-common-ligatures;color:#ba2da2" class="gmail_msg">let</span><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg"> b : </span><span style="font-variant-ligatures:no-common-ligatures;color:#703daa" class="gmail_msg">String</span><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg">? = </span><span style="font-variant-ligatures:no-common-ligatures;color:#ba2da2" class="gmail_msg">nil</span><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg">;</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class="gmail_msg"><span style="font-variant-ligatures:no-common-ligatures;color:#ba2da2" class="gmail_msg">let</span><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg"> a : </span><span style="font-variant-ligatures:no-common-ligatures;color:#ba2da2" class="gmail_msg">Any</span><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg"> = b; // result is warning: “Expression implicitly coerced from String? to Any"</span></p></div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">and so there is no way to include value types and objects without also including optionals, though this “Any” (like Object and void* in Java/C) syntax makes for poorly documented code when the type isn’t really “Any”, but a collection of known types.</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">There already exist protocols, but they are not useful for types that one has not defined (like the basic system types), or for mixing particular values, structs, and classes.</div></div></blockquote></div></div></div></blockquote></div></div></blockquote><div><br></div><div>You can retroactively extend any type to support a protocol, so that's one way that you could achieve this. The catch is that you don't want arbitrary types outside the set that you bless to be able to conform to that protocol. For that case, I think being able to define a "closed" protocol would be nice and achieve what you want,</div><div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr" class="gmail_msg"><div class="gmail_quote gmail_msg"><blockquote class="gmail_quote gmail_msg" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr" class="gmail_msg"><div class="gmail_msg"><div class="gmail_quote gmail_msg"><blockquote class="gmail_quote gmail_msg" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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