<html><head><meta http-equiv="Content-Type" content="text/html charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">When dealing with your own types, sure, but if you're talking about standard types or types from other libraries then it would mean extending them with whatever it is you actually need, which I'm not sure is the best way to do it. Type unions are much simpler, and keep all of the code within a single function.</div><div class=""><br class=""></div><div class="">To give a simplistic example, consider a method for adding a property to a property list file. I'm going to trim this down considerably, but it'll give an idea hopefully:</div><div class=""><br class=""></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>func addProperty(key:String, value:Int | String, file:NSFile) {</font></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>var xml = "<key>\(key)</key>\n"</font></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>switch value {</font></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>case value as Int:</font></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>xml += "<number>\(value)</number>\n"</font></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>case value as String:</font></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>xml += "<string>\(value)</string>\n"</font></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>}</font></div><div class=""><font face="Monaco" class=""><br class=""></font></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>// Actually store the xml string in the file here</font></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>}</font></div><div class=""><br class=""></div><div class="">Currently you might instead do this like:</div><div class=""><br class=""></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>func addProperty(key:String, value:Int, file:NSFile) {</font></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>addProperty(key: key, rawValue: "<number>\(value)</number>", file: file)</font></div><div class=""><span style="font-family: Monaco;" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>}</span></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>func addProperty(key:String, value:String, file:NSFile) {</font></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>addProperty(key: key, rawValue: "<string>\(value)</string>", file: file)</font></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>}</font></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>func addProperty(key:String, rawValue:String, file:NSFile) {</font></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>var xml = "<key>\(key)</key>\n\(rawValue\)\n"</font></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>// Actually store the xml string in the file here</font></div><div class=""><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>}</font></div><div class=""><br class=""></div><div class="">(apologies for typos, not at my main computer right now, this is just for illustration anyway)</div><div class=""><br class=""></div><div class="">Of course if I were doing a complex property list bridge I would extract some of this out, but if the above is all I need then IMO the union example is more convenient to work with, and produces less pollution of the addProperty signature, while the compiler can still optimise it out into separate functions (by eliminating the switch for each type).</div><div class=""><br class=""></div><div class="">I could alternatively use a protocol to add a .plistValue computed property to all plist compatible types, but again that seems like overkill, and moves the code out of my single function even though that may be all I really need, it also makes it less clear what all of those types are (without building docs to do so). Or I could use an enum, but again, for small use cases that can be a bit overkill, and isn't as convenient in cases that have similar, but different, union types.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class="">Ultimately it's an issue of choice; not every case will be better using or not using union types, they're just a convenience that can benefit some cases. If you find yourself using the same union types a lot then, yes, probably time to think about a protocol or an enum, but that's a big step in simpler cases.</div><br class=""><div><blockquote type="cite" class=""><div class="">On 19 Aug 2016, at 08:42, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" class="">xiaodi.wu@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">But you can do that already with protocols, can't you?<br class=""><div class="gmail_quote"><div dir="ltr" class="">On Fri, Aug 19, 2016 at 2:24 AM Haravikk via swift-evolution <<a href="mailto:swift-evolution@swift.org" 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=""><div class="">I'm a +1 for union types.</div><div class=""><br class=""></div><div class="">My main reason for wanting it is to eliminate (some) function overloads; behind the scenes the compiler may still produce one compiled function per union type (for performance), but at a high level we only need to worry about one implementation, and one call signature, which I think is a good thing.</div></div><div style="word-wrap:break-word" class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On 11 Aug 2016, at 02:28, Cao Jiannan via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>> wrote:</div><br class=""><div class=""><div style="word-wrap:break-word" class=""><div class="">Hi all,</div><div class=""><br class=""></div><div class="">I want to make a discussion about union type for swift 4.</div><div class="">See <a href="https://github.com/frogcjn/swift-evolution/blob/master/proposals/xxxx-union-type.md" target="_blank" class="">https://github.com/frogcjn/swift-evolution/blob/master/proposals/xxxx-union-type.md</a></div><div class=""><br class=""></div><div class=""><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:-apple-system,BlinkMacSystemFont,'Segoe UI',Roboto,Helvetica,Arial,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px;background-color:rgb(255,255,255)" class="">Add union type grammar, represents the type which is one of other types.</p><div style="margin-bottom:16px;color:rgb(51,51,51);font-family:-apple-system,BlinkMacSystemFont,'Segoe UI',Roboto,Helvetica,Arial,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="">var</span> stringOrURL: <span style="color:rgb(0,134,179)" class="">String</span> <span style="color:rgb(167,29,93)" class="">|</span> URL <span style="color:rgb(167,29,93)" class="">=</span> <span style="color:rgb(24,54,145)" class=""><span class="">"</span><a href="https://www.apple.com/" target="_blank" class="">https://www.apple.com</a><span class="">"</span></span></pre></div><div class=""><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:-apple-system,BlinkMacSystemFont,'Segoe UI',Roboto,Helvetica,Arial,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px;background-color:rgb(255,255,255)" class="">Now, if we using the new union type feature, we can declare type conveniently, No other type declaration, and compiler will automatically calculate the common interface.</p><div style="margin-bottom:16px;color:rgb(51,51,51);font-family:-apple-system,BlinkMacSystemFont,'Segoe UI',Roboto,Helvetica,Arial,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="">input</span>(value: A <span style="color:rgb(167,29,93)" class="">|</span> B <span style="color:rgb(167,29,93)" class="">|</span> C) {
<span style="color:rgb(0,134,179)" class="">print</span>(value<span style="color:rgb(167,29,93)" class="">.</span>commonProperty) <span style="color:rgb(150,152,150)" class="">// type checker will calculate the common interface, developer just use it out of box</span>
<span style="color:rgb(167,29,93)" class="">switch</span> value {
<span style="color:rgb(167,29,93)" class="">case</span> <span style="color:rgb(167,29,93)" class="">let</span> value <span style="color:rgb(167,29,93)" class="">as</span> A:
<span style="color:rgb(150,152,150)" class="">// value is type A</span>
<span style="color:rgb(0,134,179)" class="">print</span>(value<span style="color:rgb(167,29,93)" class="">.</span>propertyInA)
<span style="color:rgb(167,29,93)" class="">case</span> <span style="color:rgb(167,29,93)" class="">let</span> value <span style="color:rgb(167,29,93)" class="">as</span> B:
<span style="color:rgb(150,152,150)" class="">// value is type B</span>
<span style="color:rgb(0,134,179)" class="">print</span>(value<span style="color:rgb(167,29,93)" class="">.</span>propertyInB)
<span style="color:rgb(167,29,93)" class="">case</span> <span style="color:rgb(167,29,93)" class="">let</span> value <span style="color:rgb(167,29,93)" class="">as</span> C:
<span style="color:rgb(150,152,150)" class="">// value is type C</span>
<span style="color:rgb(0,134,179)" class="">print</span>(value<span style="color:rgb(167,29,93)" class="">.</span>propertyInC)
}
<span style="color:rgb(150,152,150)" class="">// there is no default case other than A, B or C. we already declared that.</span>
}</pre></div><p style="margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:-apple-system,BlinkMacSystemFont,'Segoe UI',Roboto,Helvetica,Arial,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px;background-color:rgb(255,255,255)" class="">Note: A, B, C can be either class or protocol, or any other types. This leaves developer more freedom.</p><div class=""><br class=""></div><div class=""><h2 style="margin-top:24px;margin-bottom:16px;line-height:1.25;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:-apple-system,BlinkMacSystemFont,'Segoe UI',Roboto,Helvetica,Arial,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';background-color:rgb(255,255,255)" class="">Impact on existing code</h2><ul style="padding-left:2em;margin-top:0px;color:rgb(51,51,51);font-family:-apple-system,BlinkMacSystemFont,'Segoe UI',Roboto,Helvetica,Arial,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=""><li class="">This is a new feature, developer who need declare common type will alter to this new grammar.</li><li style="margin-top:0.25em" class="">Enum based version optional or IUO will be replaced by Union-based ones. Any optional type will automatically replaced by union type</li></ul><div class=""><br class=""></div></div><h2 style="margin-top:24px;margin-bottom:16px;line-height:1.25;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:-apple-system,BlinkMacSystemFont,'Segoe UI',Roboto,Helvetica,Arial,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';background-color:rgb(255,255,255)" class=""><a href="https://github.com/frogcjn/swift-evolution/blob/master/proposals/xxxx-union-type.md#detailed-design" style="background-color:transparent;color:rgb(64,120,192);text-decoration:none;float:left;padding-right:4px;line-height:1" target="_blank" class=""><u class=""></u><u class=""></u><u class=""></u><u class=""></u></a></h2></div></div></div>_______________________________________________<br class="">swift-evolution mailing list<br class=""><a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a><br class=""><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" target="_blank" class="">https://lists.swift.org/mailman/listinfo/swift-evolution</a><br class=""></div></blockquote></div><br class=""></div>_______________________________________________<br class="">
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