<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div><blockquote type="cite" class=""><div class="">On Jun 30, 2017, at 5:38 AM, Matthew Johnson <<a href="mailto:matthew@anandabits.com" class="">matthew@anandabits.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="auto" class=""><div class=""><blockquote type="cite" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;" class=""><div class=""><div class="">3. If we later change `throw` from being a statement to being a `Never`-returning expression, you could use `throw` on the right-hand side of `??`.</div></div></blockquote><div style="font-family: Helvetica; font-size: 12px; 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; -webkit-text-stroke-width: 0px;" class=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px; 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; -webkit-text-stroke-width: 0px;" class="">What do you have in mind here? <span style="background-color: rgba(255, 255, 255, 0);" class="">I don't recall any discussion of `throw` return Never. It seems like a novel use of a bottom type that might preclude the possibility of ever having a Result type that seamlessly bridges to Swift's error handling. </span></div></div></div></div></blockquote></div><div class=""><br class=""></div><div class="">`throw` is currently a statement. Imagine, for sake of illustration, that it was instead a function. This function would take an `Error` as a parameter and throw it. It would never return normally—it would only return by throwing—so its return type would be `Never`:</div><div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>@_implicitlyTry</div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>func throw(_ error: Error) throws -> Never {</div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>try Builtin.throw(error)</div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>unreachable()</div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>}</div><div class=""><br class=""></div><div class="">What I'm suggesting is that `throw` should remain a keyword, but should have the semantics of this `throw(_:)` function. The parser should allow it in expression context, the `try` checker should treat it as though it was already marked `try`, and the type checker should treat it as an expression that returns `Never` but can throw.</div><div class=""><br class=""></div><div class="">That would then allow you to say things like:</div><div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>let lastItem = array.last ?? throw MyError.arrayEmpty</div><div class=""><br class=""></div><div class="">It would not have any negative effect I can think of on `Result`. In fact, trying to directly wrap a `throw SomeError.foo` statement in a `Result` would produce a `Result<Never, SomeError>`, correctly expressing the fact that the result of that particular expression can never be successful.</div><br class=""><div class="">
<span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div class=""><div style="font-size: 12px; " class="">-- </div><div style="font-size: 12px; " class="">Brent Royal-Gordon</div><div style="font-size: 12px; " class="">Architechies</div></div></span>
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