<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div><span></span></div><div><div></div><div>Hi Cao,</div><div><br></div><div>I would be in favor until I find another approach to this problem:</div><div><br></div><div>Consider you have a geometry framework and two types: Point and Line</div><div><br></div><div>An intersection between two lines can be either none, a point or a line (if both are identical).</div><div><br></div><div>The return type would probably be (Point | Line)?</div><div><br></div><div>I've modeled it with an empty protocol "GeometryType". However this has a major disadvantage:</div><div>If you have a general "GeometryType?" you have to cast it in a switch to the specific type.</div><div>In case of (Point| Line)? the switch statement can be checked for exhaustiveness.</div><div><br></div><div><b>For future directions:</b></div><div><br></div><div>There should also be a subtype relationship:</div><div><br></div><div>let tu: (T | U) = T()</div><div>let tuv: (T | U | V) = tu // works</div><div><br></div><div><br></div><div>Overloaded functions/operators could also take Union types based on their overloads:</div><div><br></div><div>func take(_ i: Int) -> String { ... }</div><div><br></div><div>func take(_ s: String) -> Int? { ... }</div><div><br></div><div>let value: (Int | String) = "1234"</div><div>let value2 = take(value) // returns (String | Int?)</div><div><br></div><div>Best regards</div><div>Maximilian</div><div><br>Am 11.08.2016 um 03:28 schrieb Cao Jiannan via swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>>:<br><br></div><blockquote type="cite"><div><meta http-equiv="Content-Type" content="text/html charset=us-ascii"><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" 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="box-sizing: border-box; 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 class="highlight highlight-source-swift" style="box-sizing: border-box; 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="box-sizing: border-box; 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 class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">var</span> stringOrURL: <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">String</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">|</span> URL <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">=</span> <span class="pl-s" style="box-sizing: border-box; color: rgb(24, 54, 145);"><span class="pl-pds" style="box-sizing: border-box;">"</span><a href="https://www.apple.com" class="">https://www.apple.com</a><span class="pl-pds" style="box-sizing: border-box;">"</span></span></pre></div><div class=""><p style="box-sizing: border-box; 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 class="highlight highlight-source-swift" style="box-sizing: border-box; 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="box-sizing: border-box; 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 class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">input</span>(value: A <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">|</span> B <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">|</span> C) {
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">print</span>(value<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">.</span>commonProperty) <span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);">// type checker will calculate the common interface, developer just use it out of box</span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">switch</span> value {
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">case</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">let</span> value <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">as</span> A:
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);">// value is type A</span>
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">print</span>(value<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">.</span>propertyInA)
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">case</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">let</span> value <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">as</span> B:
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);">// value is type B</span>
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">print</span>(value<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">.</span>propertyInB)
<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">case</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">let</span> value <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">as</span> C:
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);">// value is type C</span>
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">print</span>(value<span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">.</span>propertyInC)
}
<span class="pl-c" style="box-sizing: border-box; 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="box-sizing: border-box; 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="box-sizing: border-box; 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="box-sizing: border-box; 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 style="box-sizing: border-box;" class="">This is a new feature, developer who need declare common type will alter to this new grammar.</li><li style="box-sizing: border-box; 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="box-sizing: border-box; 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 id="user-content-detailed-design" class="anchor" href="https://github.com/frogcjn/swift-evolution/blob/master/proposals/xxxx-union-type.md#detailed-design" aria-hidden="true" style="box-sizing: border-box; background-color: transparent; -webkit-text-decoration-skip: objects; color: rgb(64, 120, 192); text-decoration: none; float: left; padding-right: 4px; margin-left: -20px; line-height: 1;"><svg aria-hidden="true" class="octicon octicon-link" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></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">swift-evolution@swift.org</a></span><br><span><a href="https://lists.swift.org/mailman/listinfo/swift-evolution">https://lists.swift.org/mailman/listinfo/swift-evolution</a></span><br></div></blockquote></div></body></html>