I don't know if the core team feels differently now with respect to Swift 4, but union types are listed as a "commonly rejected change":<br><br><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><br>Is there anything in your proposal that goes beyond previous discussions on the topic?<br><div class="gmail_quote"><div dir="ltr">On Wed, Aug 10, 2016 at 21:59 Cao, Jiannan via swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="line-height:1.7;color:#000000;font-size:14px;font-family:Arial"><div><span style="font-family:'PingFang SC'">It is no a mistake. since </span><span style="font-size:10.5pt;font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif;background-color:rgba(255,255,255,0)">fn1: (A</span><span style="font-size:10.5pt;font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif;background-color:rgba(255,255,255,0)">|</span><span style="font-size:10.5pt;font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif;background-color:rgba(255,255,255,0)">B)</span><span style="font-size:10.5pt;font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif;background-color:rgba(255,255,255,0)">-></span><span style="font-size:10.5pt;font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif;background-color:rgba(255,255,255,0)">Void </span><span style="font-size:10.5pt;font-family:'PingFang SC'">is subtype of fn0: </span><span style="font-size:10.5pt;font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif;background-color:rgba(255,255,255,0)">A</span><span style="font-size:10.5pt;font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif;background-color:rgba(255,255,255,0)">-></span><span style="font-size:10.5pt;font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif;background-color:rgba(255,255,255,0)">Void</span><span style="font-size:10.5pt;font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif"> </span></div><div><div style="line-height:1.7;color:#000000;font-size:14px;font-family:Arial"><div><div style="font-family:'PingFang SC'"><br></div><div style="font-family:'PingFang SC'"><span style="font-size:10.5pt;font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif">Detail explain:</span></div><div style="font-family:'PingFang SC'"><br></div><div style="font-family:'PingFang SC'"><div style="font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif"><span style="background-color:rgba(255,255,255,0)">var</span> <span style="background-color:rgba(255,255,255,0)">fn0: A</span><span style="background-color:rgba(255,255,255,0)">-></span><span style="background-color:rgba(255,255,255,0)">Void</span> <span style="background-color:rgba(255,255,255,0)">=</span> <span style="background-color:rgba(255,255,255,0)">{</span><span style="background-color:rgba(255,255,255,0)">print</span><span style="background-color:rgba(255,255,255,0)">($0)} </span></div></div></div></div></div></div><div style="line-height:1.7;color:#000000;font-size:14px;font-family:Arial"><div><div style="line-height:1.7;color:#000000;font-size:14px;font-family:Arial"><div><div style="font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif"><span style="background-color:rgba(255,255,255,0)">var</span> <span style="background-color:rgba(255,255,255,0)">fn1: (A</span><span style="background-color:rgba(255,255,255,0)">|</span><span style="background-color:rgba(255,255,255,0)">B)</span><span style="background-color:rgba(255,255,255,0)">-></span><span style="background-color:rgba(255,255,255,0)">Void</span> <span style="background-color:rgba(255,255,255,0)">=</span> <span style="background-color:rgba(255,255,255,0)">{</span><span style="background-color:rgba(255,255,255,0)">print</span><span style="background-color:rgba(255,255,255,0)">(v0)} </span></div></div></div></div></div><div style="line-height:1.7;color:#000000;font-size:14px;font-family:Arial"><div><div style="line-height:1.7;color:#000000;font-size:14px;font-family:Arial"><div><div style="font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif"><span style="background-color:rgba(255,255,255,0)">let a = A()</span></div><div style="font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif"><span style="background-color:rgba(255,255,255,0)">let b = B()</span></div><div style="font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif"><span style="background-color:rgba(255,255,255,0)"><br></span></div><div style="font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif"><span style="background-color:rgba(255,255,255,0)">So:</span></div><div style="font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif"><br></div><div style="font-family:'PingFang SC'"><span style="font-size:10.5pt;font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif">fn0( a ) // this is OK </span></div><div style="font-family:'PingFang SC'"><span style="font-size:10.5pt;font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif">fn1( a ) // this is also OK</span></div><div style="font-family:'PingFang SC'"><br></div><div style="font-family:'PingFang SC'"><span style="font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif">fn1 is subtype of fn0, because </span><span style="font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif;font-size:10.5pt">fn1 can do anything fn0 do.</span></div><div style="font-family:'PingFang SC'"><span style="font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif;font-size:10.5pt">Thus fn0 = fn1 is OK.</span></div><div style="font-family:'PingFang SC'"><span style="font-family:Helvetica,Tahoma,Arial,PingFang,苹方,'Hiragino Sans GB',冬青黑体,'Microsoft YaHei',微软雅黑,SimSun,宋体,STXihei,华文细黑,Heiti,黑体,sans-serif;font-size:10.5pt"><br></span></div><div style="font-family:'PingFang SC'"><font face="Helvetica, Tahoma, Arial, PingFang, 苹方, Hiragino Sans GB, 冬青黑体, Microsoft YaHei, 微软雅黑, SimSun, 宋体, STXihei, 华文细黑, Heiti, 黑体, sans-serif">But:</font></div><div style="font-family:'PingFang SC'"><font face="Helvetica, Tahoma, Arial, PingFang, 苹方, Hiragino Sans GB, 冬青黑体, Microsoft YaHei, 微软雅黑, SimSun, 宋体, STXihei, 华文细黑, Heiti, 黑体, sans-serif"><br></font></div><div style="font-family:'PingFang SC'"><font face="Helvetica, Tahoma, Arial, PingFang, 苹方, Hiragino Sans GB, 冬青黑体, Microsoft YaHei, 微软雅黑, SimSun, 宋体, STXihei, 华文细黑, Heiti, 黑体, sans-serif">fn1( b ) // this is OK</font></div><div style="font-family:'PingFang SC'"><font face="Helvetica, Tahoma, Arial, PingFang, 苹方, Hiragino Sans GB, 冬青黑体, Microsoft YaHei, 微软雅黑, SimSun, 宋体, STXihei, 华文细黑, Heiti, 黑体, sans-serif">fn0( b ) // this is not OK</font></div><div style="font-family:'PingFang SC'"><br></div><div style="font-family:'PingFang SC'"><font face="Helvetica, Tahoma, Arial, PingFang, 苹方, Hiragino Sans GB, 冬青黑体, Microsoft YaHei, 微软雅黑, SimSun, 宋体, STXihei, 华文细黑, Heiti, 黑体, sans-serif">So fn0 is not subtype of fn1</font></div></div></div></div></div><div style="line-height:1.7;color:#000000;font-size:14px;font-family:Arial"><div><div style="line-height:1.7;color:#000000;font-size:14px;font-family:Arial"><div style="zoom:1"></div><div></div><div><br></div>At 2016-08-11 10:41:02, "Step C" <<a href="mailto:schristopher@bignerdranch.com" target="_blank">schristopher@bignerdranch.com</a>> wrote:<br> <blockquote style="PADDING-LEFT:1ex;MARGIN:0px 0px 0px 0.8ex;BORDER-LEFT:#ccc 1px solid"><div></div><div>Shouldn't it be "fn1 = fn0"? Same for the fn2 statements. </div><div><br></div><div>`<span style="background-color:rgba(255,255,255,0)">var</span><span style="background-color:rgba(255,255,255,0)"> fn0: A</span><span style="background-color:rgba(255,255,255,0)">-></span><span style="background-color:rgba(255,255,255,0)">Void</span><span style="background-color:rgba(255,255,255,0)"> </span><span style="background-color:rgba(255,255,255,0)">=</span><span style="background-color:rgba(255,255,255,0)"> {</span><span style="background-color:rgba(255,255,255,0)">print</span><span style="background-color:rgba(255,255,255,0)">(v0)} </span></div><div><span style="background-color:rgba(255,255,255,0)"></span><span style="background-color:rgba(255,255,255,0)">var</span><span style="background-color:rgba(255,255,255,0)"> fn1: (A</span><span style="background-color:rgba(255,255,255,0)">|</span><span style="background-color:rgba(255,255,255,0)">B)</span><span style="background-color:rgba(255,255,255,0)">-></span><span style="background-color:rgba(255,255,255,0)">Void</span><span style="background-color:rgba(255,255,255,0)"> </span><span style="background-color:rgba(255,255,255,0)">=</span><span style="background-color:rgba(255,255,255,0)"> {</span><span style="background-color:rgba(255,255,255,0)">print</span><span style="background-color:rgba(255,255,255,0)">(v0)} </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)"> fn0 </span><span style="background-color:rgba(255,255,255,0)">=</span><span style="background-color:rgba(255,255,255,0)"> fn1 </span><span style="background-color:rgba(255,255,255,0)">// OK, because Original Type and Union Type has a sub-typing relationship</span><span style="background-color:rgba(255,255,255,0)">
</span><span style="background-color:rgba(255,255,255,0)">var</span><span style="background-color:rgba(255,255,255,0)"> </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)">fn2: (A</span><span style="background-color:rgba(255,255,255,0)">|</span><span style="background-color:rgba(255,255,255,0)">B</span><span style="background-color:rgba(255,255,255,0)">|</span><span style="background-color:rgba(255,255,255,0)">C)</span><span style="background-color:rgba(255,255,255,0)">-></span><span style="background-color:rgba(255,255,255,0)">Void</span><span style="background-color:rgba(255,255,255,0)"> </span><span style="background-color:rgba(255,255,255,0)">=</span><span style="background-color:rgba(255,255,255,0)"> {</span><span style="background-color:rgba(255,255,255,0)">print</span><span style="background-color:rgba(255,255,255,0)">($</span><span style="background-color:rgba(255,255,255,0)">0</span><span style="background-color:rgba(255,255,255,0)">)} </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)"> fn0 </span><span style="background-color:rgba(255,255,255,0)">=</span><span style="background-color:rgba(255,255,255,0)"> fn2 </span><span style="background-color:rgba(255,255,255,0)">// OK</span><span style="background-color:rgba(255,255,255,0)"> </span></div><div><span style="background-color:rgba(255,255,255,0)"> fn1 </span><span style="background-color:rgba(255,255,255,0)">=</span><span style="background-color:rgba(255,255,255,0)"> fn2 </span><span style="background-color:rgba(255,255,255,0)">// OK`</span></div><div><br>On Aug 10, 2016, at 9:28 PM, Cao Jiannan via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>> wrote:<br><br></div><blockquote type="cite"><div><div>Hi all,</div><div><br></div><div>I want to make a discussion about union type for swift 4.</div><div>See <a href="https://github.com/frogcjn/swift-evolution/blob/master/proposals/xxxx-union-type.md" target="_blank">https://github.com/frogcjn/swift-evolution/blob/master/proposals/xxxx-union-type.md</a></div><div><br></div><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)">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)"><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"><span style="color:rgb(167,29,93)">var</span> stringOrURL: <span style="color:rgb(0,134,179)">String</span> <span style="color:rgb(167,29,93)">|</span> URL <span style="color:rgb(167,29,93)">=</span> <span style="color:rgb(24,54,145)"><span>"</span><a href="https://www.apple.com" target="_blank">https://www.apple.com</a><span>"</span></span></pre></div><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)">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)"><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"><span style="color:rgb(167,29,93)">func</span> <span style="color:rgb(121,93,163)">input</span>(value: A <span style="color:rgb(167,29,93)">|</span> B <span style="color:rgb(167,29,93)">|</span> C) {
<span style="color:rgb(0,134,179)">print</span>(value<span style="color:rgb(167,29,93)">.</span>commonProperty) <span style="color:rgb(150,152,150)">// type checker will calculate the common interface, developer just use it out of box</span>
<span style="color:rgb(167,29,93)">switch</span> value {
<span style="color:rgb(167,29,93)">case</span> <span style="color:rgb(167,29,93)">let</span> value <span style="color:rgb(167,29,93)">as</span> A:
<span style="color:rgb(150,152,150)">// value is type A</span>
<span style="color:rgb(0,134,179)">print</span>(value<span style="color:rgb(167,29,93)">.</span>propertyInA)
<span style="color:rgb(167,29,93)">case</span> <span style="color:rgb(167,29,93)">let</span> value <span style="color:rgb(167,29,93)">as</span> B:
<span style="color:rgb(150,152,150)">// value is type B</span>
<span style="color:rgb(0,134,179)">print</span>(value<span style="color:rgb(167,29,93)">.</span>propertyInB)
<span style="color:rgb(167,29,93)">case</span> <span style="color:rgb(167,29,93)">let</span> value <span style="color:rgb(167,29,93)">as</span> C:
<span style="color:rgb(150,152,150)">// value is type C</span>
<span style="color:rgb(0,134,179)">print</span>(value<span style="color:rgb(167,29,93)">.</span>propertyInC)
}
<span style="color:rgb(150,152,150)">// 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)">Note: A, B, C can be either class or protocol, or any other types. This leaves developer more freedom.</p><div><br></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)">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"><li>This is a new feature, developer who need declare common type will alter to this new grammar.</li><li style="margin-top:0.25em">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><br></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)"><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"></a></h2></div></div></div></blockquote><blockquote type="cite"><div><span>_______________________________________________</span><br><span>swift-evolution mailing list</span><br><span><a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a></span><br><span><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" target="_blank">https://lists.swift.org/mailman/listinfo/swift-evolution</a></span><br></div></blockquote></blockquote></div></div></div><br><br><span title="neteasefooter"><p> </p></span>_______________________________________________<br>
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