I kind of like that idea. I’d rather have a Task/Future/Promise/whatever, but if that’s not going to happen then Void vs. Never seems like a reasonable way of distinguishing these two cases.
> On Nov 12, 2017, at 9:55 AM, Xiaodi Wu <xiaodi...@gmail.com> wrote: > > Sorry, I'm just getting into this conversation late and am by no means > experienced in the area, but why can't the one where you *don't* want the > caller to wait for the result be spelled `async -> Never`? Theoretically, > `async -> Void` means you're awaiting a result with only one possible value, > but if you're not waiting at all, then there is truly no result, yes? > > > On Sun, Nov 12, 2017 at 9:27 AM, Yuta Koshizawa via swift-evolution > <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: > Sorry, I had got some confusion. Please let me retry to explain. > > As you said, C# provides three kinds of async functions: `async Void`, > `async Task` and `async Task<Foo>`. All of them are necessary and > Swift should provide same functionalities. > > When we think about `async/await` in Swift, because we have already > had `throws/try`, it is desired that `async/await` in Swift is > consistent with `throws/try`. So it is better to have `async/await` > without introducing a type like `Task` (or `Promise`). > > Even if we employ `async/await` without `Task`, Swift has to provides > functionalities to implement "three kinds of async functions" in C#. > However if `async -> Void` in Swift works similarly to `async Void` in > C#, how can we express ones like `async Task` in C#? I think there are > two possibilities: > > 1. Calling `async -> Void` functions without `await` in Swift works > like `async Void` in C# and calling them *with* `await` works like > `async Task` in C#. > 2. Calling `async -> Void` functions without `await` in Swift works > like `async Void` in C# and never support something like `async Task` > in C#. > > I think 2 is impermissible. For example, handling completion events of > asynchronous operations without result values needs something like > `async Task` in C#. However, with 1, we lose the benefit of static > checks by the compiler. Because both of `fooAsync()` without `await` > and `await fooAsync()` are allowed, even if we want it to work like > `async Task` in C# and forget to mark `await`, the compiler tell us > nothing and it works like `async Void` in C#. It causes unexpected > behaviors. It is hard to fix such kinds of bugs. So I think > introducing `beginAsync` is better. > > -- > Yuta > > > 2017-11-12 10:23 GMT+09:00 Yuta Koshizawa via swift-evolution > <swift-evolution@swift.org <mailto:swift-evolution@swift.org>>: > > 2017-11-12 2:57 GMT+09:00 Adam Kemp <adam.k...@apple.com > > <mailto:adam.k...@apple.com>>: > >> > >> > >>> On Nov 11, 2017, at 6:24 AM, Yuta Koshizawa <ko...@koherent.org > >>> <mailto:ko...@koherent.org>> wrote: > >>> > >>> If you replace `async` with `throws`, you can get answers. > >>> > >>> > >>>> Can you declare an async closure variable? > >>> > >>> Yes. Like `let throwingClosure:() throws -> Void = { ... }`. > >>> > >>> > >>>> Can a non-async closure be passed to a function expecting a async > >>>> closure? > >>> > >>> Yes. Like we can pass `() -> Void` to a function expecting a throwing > >>> closure `() throws -> Void`. > >>> > >>> It is possible because `(Foo) throws -> Bar` is a supertype of `(Foo) > >>> -> Bar`. `(Foo) async -> Bar` is a supertype of `(Foo) -> Bar` in the > >>> same way. > >>> > >>> To treat an async function as a sync function is legal. It is similar > >>> to make a `Promise` by `Promise(value)` which is completed > >>> immediately. > >>> > >>> > >>>> Can an async closure be passed to a function expecting a non-async > >>>> closure? > >>> > >>> No. `() -> Void` is a subtype of `() async -> Void`. It is same as > >>> passing `() throws -> Void` to a function expecting `() -> Void` is > >>> not allowed. > >> > >> But why not? Just asserting that it must work the same as throws > >> is not a convincing argument. You have to justify why it must work > >> that way. I think there is good reason to allow it, which I have described. > >> What reason is there to disallow it? > > > > `() async -> Void` needs to be called with `await` because it prevents > > us from forgetting handling asynchronous operations. > > > > If we use callbacks to handle asynchronous operations, it is shown to > > us by a compiler as a compilation error. > > > > ``` > > func fooAsync(_ handler: () -> Void) -> Void { ... } > > > > fooAsync() // compilation error > > > > fooAsync { > > // handles a completion event here > > } > > ``` > > > > With proposed `async/await`, it is realized similarly like below. > > > > ``` > > func fooAsync() async -> Void { ... } > > > > fooAsync() // compilation error > > > > await fooAsync() > > // handles a completion event here > > ``` > > > > However, if async void functions work like `beginAsync`, we can easily > > forget it and it can cause unexpected behaviors. > > > > ``` > > func fooAsync() async -> Void { ... } > > > > fooAsync() // OK > > // hard to know this line is executed asynchronously > > ``` > > > > Readability also suffers seriously. If we don't know `bar` in the > > following code is a async function, it is impossible to expect lines > > after `baz()` are executed asynchronously. > > > > ``` > > foo() > > bar() > > baz() > > qux() > > ``` > > > > > >>>> It’s weird to me that we would allow you to have async void closures but > >>>> not async void functions > >>> > >>> I am not sure what you mean. "async void closures" and "async void > >>> functions" have a same type. Following two are almost same. > >>> > >>> ``` > >>> func foo() async -> Void { ... } > >>> let foo: () async -> Void = { ... } > >>> ``` > >> > >> What started this thread is my suggestion that you should be able to write > >> an async void function. The current proposal doesn’t allow that. That’s why > >> you have to use beginAsync. > >> > >> I don’t think that makes sense. It sounds like you also think that would > >> be strange, > >> hence your assumption that you could. > > > > By the reasons I wrote above, we need `await` even for async void > > functions for checks by compilers. Then it is required to provide a > > way to write entry points of async functions. That is `beginAsync`. > > > > -- > > Yuta > > _______________________________________________ > > swift-evolution mailing list > > swift-evolution@swift.org <mailto:swift-evolution@swift.org> > > https://lists.swift.org/mailman/listinfo/swift-evolution > > <https://lists.swift.org/mailman/listinfo/swift-evolution> > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org <mailto:swift-evolution@swift.org> > https://lists.swift.org/mailman/listinfo/swift-evolution > <https://lists.swift.org/mailman/listinfo/swift-evolution> >
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