<div>On Tue, Jul 25, 2017 at 16:35 Robert Bennett <<a href="mailto:rltbennett@icloud.com">rltbennett@icloud.com</a>> wrote:<br><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div>Optional.some(Void) gives me "Expected member name or constructor call after type name”.</div></div></blockquote><div dir="auto"><br></div><div dir="auto">That would appear to be straightforwardly a compiler bug. For instance, the following compiles:</div><div dir="auto"><br></div><div dir="auto">```</div><div dir="auto">let _Void = Void</div><div dir="auto">let foo = Optional.some(_Void)</div><div dir="auto">// I'll discuss the warnings you see in a bit, below.</div><div dir="auto">```</div><div dir="auto"><br></div><div dir="auto">In addition, the following compiles just fine, which is the crux of your use case:</div><div dir="auto"><br></div><div dir="auto">```</div><div dir="auto"><div dir="auto">enum Foo<T> {</div><div dir="auto"> case bar(T)</div><div dir="auto"> case baz</div><div dir="auto">}</div><div dir="auto"><br></div><div dir="auto">let x = Foo.bar(Void)</div></div><div dir="auto">```</div><div dir="auto"><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"><div></div><div>And as I said, the compiler should not be allowed to use the absence of parentheses to infer a Void generic type; only when the generic type is known to be Void may the parentheses be omitted. Under this proposal, Optional.some would not be allowed.</div></div></blockquote><div dir="auto"><br></div><div dir="auto">There is no precedent for such a rule in Swift. Inferring Void is allowed but results in a warning under certain circumstances, but it is not forbidden. This has been discussed on the list before and it is not an oversight that it's a warning and not an error, and that the warning is about storing the initialized result and not about inferring the type.</div><div dir="auto"><br></div><div dir="auto">To be clear, if the rule you propose is permitted, `Optional.some` *would* be inferred to be `Optional.some(Void)` unless type inference rules are revisited (which, I would argue, is entirely overkill in terms of the scope of that undertaking as compared to the magnitude of the issue being addressed). Instead, there would be a warning if you tried to bind `Optional.some` to a variable, but you would absolutely be allowed to pass it to a function: `callSomeOtherFunction(Optional.some!)` would be an alternative spelling for `callSomeOtherFunction(Void)`; effectively, `Optional.some!` would become a synonym for `Void` under certain circumstances but not others.</div><div dir="auto"><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"><div></div><div>The scenario I’m working with, which inspired this thought, is roughly:</div><div><br></div><div>enum Result<T> {</div><div><span class="m_5182128129036007394Apple-tab-span" style="white-space:pre-wrap"> </span>case success(T)</div><div><span class="m_5182128129036007394Apple-tab-span" style="white-space:pre-wrap"> </span>case failure(Error)</div><div>}</div><div><br></div><div>func doSomething(completion: @escaping (Result<Void>)->()) { /* At some point, call completion(.success(())) */ }</div><div><br></div><div>The idea is that in case of success, I merely want to indicate success, whereas in case of an error I want to pass the error through to the completion. This requires calling `completion(.success(()))`.</div></div></blockquote><div dir="auto"><br></div><div dir="auto">This is, as far as I can tell, essentially the same use case that SE-0110 rendered problematic. As the core team wrote in their decision there, it's clear that Swift doesn't have a good story for how to support certain coding styles that involve Void arguments. For the moment, a limited and temporary exception was approved to roll back parts of SE-0110, but the idea is that all of this needs to be revisited.</div><div dir="auto"><br></div><div dir="auto">I'd agree that your use case here is a good example of something that should be taken into account as part of that re-examination, but IMO inventing one or several new rules specific to generic enums cannot be the way to go--especially since it is certain that SE-0110 and related issues *will* be revisited and implicit or explicit tuple splatting will be given great consideration. By that time, implementation of the existing and approved proposal on enums will mean that the exact same issue will be addressed automatically for enum cases.</div><div dir="auto"><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"><div>I don’t think I should have to make another enum with the same cases but no associated value for success (and no generic type) to make this work nicely — that makes things brittle in case I ever want to replace Void with a different type.</div></div></blockquote><div dir="auto"><br></div><div dir="auto">If I understand Swift's error handling rationale correctly, a Result type with no associated value for success would be, _by design_, an Optional<Error>. Why wouldn't you use that in this scenario? (Well, actually, if I understand Swift's error handling rationale, the intended way to spell this in Swift is `throws -> Void` (aka `throws`)).</div><div dir="auto"><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"><div></div></div><div style="word-wrap:break-word"><div><br></div><div><blockquote type="cite"><div>On Jul 25, 2017, at 5:20 PM, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" target="_blank">xiaodi.wu@gmail.com</a>> wrote:</div><br class="m_5182128129036007394Apple-interchange-newline"><div>Yes, I discussed this some time back. It breaks some things with overloads, if I recall, but off the top of my head not recalling what.<br><br>At some point, I also suggested regarding any case without associated values as equivalent to a case with an associated value of type Void. This was never adopted. There _is_ an underlying issue with that idea, as the property “foo” is not the same as the method “foo(_: Void)”, and it is even permitted to have both of these on the same type. The purpose of the most recent Swift proposal on enum cases was to align the syntax with functions as closely as possible; if overloading the base name of a case (as proposed back then to favorable reviews) is ever allowed, then the same will naturally be possible with enums (that is, a case named “foo” and another named “foo(_: Void)” in the same type).<br><br>In the case of concrete enums, a case with associated value of type Void can be given a default value once they are supported (already approved); the trouble here is that in the generic case presented here the same is not possible. It is unclear to me whether this special rule would see broad use, as it appears to come into play *only* for a generic enum. Moreover, in the one scenario I can think of, it reads cryptically: “let foo = Optional.some” would then be allowed (while “let bar = Optional.none” still would not). The upside is that it avoids having to write four or six letters (“Optional.some(Void())” is never necessary, as it’s also just “Optional.some(Void)”, and that reads acceptably in my view).<br><div class="gmail_quote"><div>On Tue, Jul 25, 2017 at 15:26 David Sweeris via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">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 dir="auto"><div><br></div><div>On Jul 25, 2017, at 12:38, Robert Bennett 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>Currently if you have the following enum:<div><br></div><div>enum E<T> {<br><span class="m_5182128129036007394m_7478467993860922014Apple-tab-span" style="white-space:pre-wrap"> </span>case c(T)<br>}</div><div><br></div><div>then if T is Void, you have to write one of the following:</div><div><br></div><div><font face="Inconsolata">let x: E<Void> = .c(Void())</font></div><div><font face="Inconsolata">let y: E<Void> = .c(())</font></div><div><br></div><div>Looks awkward, no? In this case you can omit `<Void>` after `E` because it can be inferred, but if writing a (non-generic) function taking an argument of type `E<Void>`, then the `<Void>` cannot be omitted, and you still have to write `.c(())` for the case name.</div><div><br></div><div>I’m proposing that for enum cases with a single associated value of Void type, or of a generic type that is equal to Void in some instance, you may omit the parentheses altogether and merely write</div><div><br></div><div>let x: E<Void> = .c</div><div><br></div><div>The rationale is twofold: first, double parentheses just looks bad; second, there is only a single value of type Void, which means the associated value of `.c` is trivially inferable, and hence should be omissible.</div><div><br></div><div>I am not proposing that a bare `E.c` imply a type of `E<Void>` — `E.c` should still be illegal in the absence of specification of the generic type — only that when the type is known to be `E<Void>`, `.c` can replace `.c(())`.</div><div><br></div><div>Thoughts?</div></div></blockquote><br></div><div dir="auto"><div>My first response is +1</div><div><br></div><div>My second response is to ask just how much would it break things to expand this and allow omitting the argument anywhere its type is known to be Void? Maybe by implicitly providing a default value of `()` wherever there's a `Void` argument? Like make `func foo(x: Void) {...}` implicitly become `<span style="background-color:rgba(255,255,255,0)">func foo(x: Void = ()) {...}`?</span> I have a sneaking suspicion that this question's already been asked, but I'm not sure.</div><div><br></div><div>- Dave Sweeris</div></div>_______________________________________________<br>
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