Granted that this whole topic is out of scope currently, I think the enum unwrapping facility is its own subject entirely, separable from the `if let x = x` sugar.<br><br>Approaching this issue from another perspective--namely, improving enums to better enable some use cases prompted by requests for union types--I've developed some ideas for unwrapping that I've been keeping under wraps (hehe) until the next phase of Swift evolution, and it'd be nice to have a dedicated conversation about such ideas when the time is right rather than rolling it into the topic of `if let x = x` sugaring.<br><br><div class="gmail_quote"><div dir="ltr">On Fri, Oct 28, 2016 at 7:42 PM Erica Sadun 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="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg">On Oct 28, 2016, at 6:22 PM, Erica Sadun via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="gmail_msg" target="_blank">swift-evolution@swift.org</a>> wrote:</div></blockquote></div></div><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><div style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg">On Oct 28, 2016, at 5:55 PM, Kevin Nattinger <<a href="mailto:swift@nattinger.net" class="gmail_msg" target="_blank">swift@nattinger.net</a>> wrote:</div></blockquote></div></div></blockquote></div></div><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><div style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><div class="gmail_msg">On Oct 28, 2016, at 4:45 PM, Erica Sadun via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="gmail_msg" target="_blank">swift-evolution@swift.org</a>> wrote:</div></blockquote></div></blockquote></div></div></blockquote></div></div><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><div style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><div class="gmail_msg"><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg">On Oct 28, 2016, at 5:00 PM, Huon Wilson <<a href="mailto:huon@apple.com" class="gmail_msg" target="_blank">huon@apple.com</a>> wrote:</div></blockquote></div></div></div></blockquote></div></blockquote></div></div></blockquote></div></div><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><div style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><div class="gmail_msg"><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg">On Oct 28, 2016, at 15:34, Erica Sadun via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="gmail_msg" target="_blank">swift-evolution@swift.org</a>> wrote:</div></blockquote></div></div></div></blockquote></div></div></div></blockquote></div></blockquote></div></div></blockquote></div></div><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><div style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><div class="gmail_msg"><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><div class="gmail_msg">How does the compiler decide whether to succeed on anycase or succeed on anothercase respectively? In general, the compiler only statically knows that first & second are of type TypeName, not anything about which case (they could be passed in as function parameters, or returned by an opaque function e.g. `let x = OtherLibrary.returnsTypeName(); guard unwrap x else { … }`), and thus the variant to unwrap has to be chosen based only on that piece of information.</div><br class="gmail_msg"></div></div></div></blockquote></div></div></div></blockquote></div></blockquote></div></div></blockquote></div></div><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><div style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><div class="gmail_msg"><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><div style="word-wrap:break-word" class="gmail_msg">It seems to me that doing this either has to be restricted to enums with an “obvious” choice for unwrapping, like Optional, or rely on a sort of forward-looking type inference that Swift doesn’t currently use to deduce the unwrapped type based on how the value is used later (and I’m not sure that works in all cases, e.g. what if T == U for the TypeName example).</div></div></blockquote></div></div></div></blockquote></div></blockquote></div></div></blockquote></div></div><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><div style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><div class="gmail_msg"><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg">It succeeds on any one-item case and fails on any non-item case.</div></div></div></blockquote></div></blockquote></div></div></blockquote></div></div><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><div style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><div style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><div style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">I think he meant this:</div><div style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><div style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class="gmail_msg"><span style="font-variant-ligatures:no-common-ligatures;color:rgb(187,44,162)" class="gmail_msg">enum</span><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg"><span class="m_2894161825494700983Apple-converted-space gmail_msg"> </span>TypeName<T, U> {<span class="m_2894161825494700983Apple-converted-space gmail_msg"> </span></span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(187,44,162)" class="gmail_msg">case</span><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg"><span class="m_2894161825494700983Apple-converted-space gmail_msg"> </span>anycase(</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="gmail_msg">T</span><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg">), anothercase(</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="gmail_msg">U</span><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg">) }</span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class="gmail_msg"><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg"></span><br class="gmail_msg"></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class="gmail_msg"><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg"><span style="font-variant-ligatures:no-common-ligatures;color:rgb(187,44,162)" class="gmail_msg">func</span><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg"><span class="m_2894161825494700983Apple-converted-space gmail_msg"> </span>foo<T, U>(instance:<span class="m_2894161825494700983Apple-converted-space gmail_msg"> </span></span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="gmail_msg">TypeName</span><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg"><</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="gmail_msg">T</span><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg">,<span class="m_2894161825494700983Apple-converted-space gmail_msg"> </span></span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="gmail_msg">U</span><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg">>)</span> {</span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class="gmail_msg"><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg"><span class="m_2894161825494700983Apple-tab-span gmail_msg" style="white-space:pre-wrap"> </span></span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(187,44,162)" class="gmail_msg">guard</span><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg"><span class="m_2894161825494700983Apple-converted-space gmail_msg"> </span>unwrap instance<span class="m_2894161825494700983Apple-converted-space gmail_msg"> </span></span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(187,44,162)" class="gmail_msg">else</span><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg"><span class="m_2894161825494700983Apple-converted-space gmail_msg"> </span>{ ... }</span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class="gmail_msg"><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg">}</span></div><div class="gmail_msg"><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg"><br class="gmail_msg"></span></div><div class="gmail_msg"><span style="font-variant-ligatures:no-common-ligatures" class="gmail_msg">What type does instance have? </span></div></div></div></blockquote></div></div></blockquote></div></div><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><br style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><div style="font-family:Palatino-Roman;font-size:14px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">Fair enough. How about: An unwrappable enumeration must have at most one generic type so the compiler can guarantee at compile time that the type is unambiguous if the unwrapping succeeds</div></div></blockquote></div></div><div style="word-wrap:break-word" class="gmail_msg"><div class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"></div></blockquote><br class="gmail_msg"></div><div class="gmail_msg">And updated gist:</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg"><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',Helvetica,Arial,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';background-color:rgb(255,255,255)" class="gmail_msg">Detailed Design</h2><p style="box-sizing:border-box;margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:-apple-system,BlinkMacSystemFont,'Segoe UI',Helvetica,Arial,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px;background-color:rgb(255,255,255)" class="gmail_msg">The <code style="box-sizing:border-box;font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:13.600000381469727px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px" class="gmail_msg">unwrap</code> statement shadows an enumeration variable to an unwrapped version of the same type. Using <code style="box-sizing:border-box;font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:13.600000381469727px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px" class="gmail_msg">unwrap</code> is limited to enumerations that satisfy the following conditions:</p><ul style="box-sizing:border-box;padding-left:2em;margin-top:0px;margin-bottom:16px;color:rgb(51,51,51);font-family:-apple-system,BlinkMacSystemFont,'Segoe UI',Helvetica,Arial,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';font-size:16px;background-color:rgb(255,255,255)" class="gmail_msg"><li style="box-sizing:border-box" class="gmail_msg">An unwrappable enumeration must specify the unwrap type, probably through an <code style="box-sizing:border-box;font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:13.600000381469727px;padding:0.2em 0px;margin:0px;background-color:rgba(0,0,0,0.0392157);border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px" class="gmail_msg">@unwrappable</code> attribute.</li><li style="box-sizing:border-box;margin-top:0.25em" class="gmail_msg">Unwrapping is limited to one-item cases with that type.</li><li style="box-sizing:border-box;margin-top:0.25em" class="gmail_msg">No-item or multi-item cases cannot unwrap.</li><li style="box-sizing:border-box;margin-top:0.25em" class="gmail_msg">Cases using a different associated value type cannot unwrap.</li></ul><pre style="box-sizing:border-box;font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;font-size:13.600000381469727px;margin-top:0px;margin-bottom:16px;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;color:rgb(51,51,51)" class="gmail_msg"><code style="box-sizing:border-box;font-family:Consolas,'Liberation Mono',Menlo,Courier,monospace;padding:0px;margin:0px;background-color:transparent;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;word-break:normal;border:0px;display:inline;overflow:visible;line-height:inherit;word-wrap:normal" class="gmail_msg">@unwrappable(type: Wrapped)
public enum Optional<Wrapped> {
case none
case some(Wrapped)
}
@unwrappable(type: ValueType)
public enum Result<ValueType> {
case value(ValueType)
case error(Error)
}
@unwrappable(type: Int)
public enum Count {
case none // not unwrappable
case many // not unwrappable
case some(Int)
case owed(Int)
case something(String) // not unwrappable
case multiple(Int, Int) // not unwrappable
}
</code></pre><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',Helvetica,Arial,sans-serif,'Apple Color Emoji','Segoe UI Emoji','Segoe UI Symbol';background-color:rgb(255,255,255)" class="gmail_msg"><a id="m_2894161825494700983user-content-impact-on-existing-code" class="m_2894161825494700983anchor gmail_msg" href="https://gist.github.com/erica/db9ce92b3d23cb20799460f603c0ae7c#impact-on-existing-code" style="box-sizing:border-box;background-color:transparent;color:rgb(64,120,192);text-decoration:none;float:left;padding-right:4px;line-height:1" target="_blank"><u class="gmail_msg"></u><u class="gmail_msg"></u><u class="gmail_msg"></u><u class="gmail_msg"></u></a></h2></div><br class="gmail_msg"></div>_______________________________________________<br class="gmail_msg">
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