And that's why I (and others) are suggesting: func processImageData1a() async -> Image { let dataResource = async loadWebResource("dataprofile.txt") // No future type here... Just another way to call dispatch_async under the hood. let imageResource = async loadWebResource("imagedata.dat")
// ... other stuff can go here to cover load latency... let imageTmp = await decodeImage(dataResource, imageResource) // Compiles force await call here... let imageResult = await dewarpAndCleanupImage(imageTmp) return imageResult } And now we gain all advantages of async/await again without to handle with one more type. Em seg, 28 de ago de 2017 às 17:07, Adam Kemp via swift-evolution < swift-evolution@swift.org> escreveu: > I think the biggest tradeoff is clearer when you look at the examples from > the proposal where futures are built on top of async/await: > > func processImageData1a() async -> Image { > let dataResource = Future { await loadWebResource("dataprofile.txt") } > let imageResource = Future { await loadWebResource("imagedata.dat") } > > // ... other stuff can go here to cover load latency... > > let imageTmp = await decodeImage(dataResource.get(), > imageResource.get()) > let imageResult = await dewarpAndCleanupImage(imageTmp) > return imageResult > } > > > With this approach you have to wrap each call site to create a future. > Compare to this: > > func processImageData1a() -> Future<Image> { > let dataResourceFuture = loadWebResource("dataprofile.txt”); > let imageResourceFuture = loadWebResource("imagedata.dat”); > > // ... other stuff can go here to cover load latency... > > let imageTmp = await decodeImage(await dataResourceFuture, await > imageResourceFuture) > let imageResult = await dewarpAndCleanupImage(imageTmp) > return imageResult > } > > > Here, not only are the explicit wrappers gone, but this function itself > can be used with either await or as a future. You get both options with one > implementation. > > As I’ve mentioned before, C#’s implementation is not tied to any one > particular futures implementation. The Task type is commonly used, but > async/await does not directly depend on Task. Instead it works with any > return type that meets certain requirements (detailed here: > https://blogs.msdn.microsoft.com/pfxteam/2011/01/13/await-anything/). > Swift could do this using a protocol, which can be retroactively applied > using an extension. > > Obviously for this to be useful we would need some kind of existing future > implementation, but at least we wouldn’t be tied to any particular one. > That would mean library maintainers who have already been using their own > futures implementations could quickly adopt async/await in their code > without having to rewrite their futures library or throw wrappers around > every usage of async/await. They could just adopt a protocol (using an > extension, even) and get async/await support for free. > > The downside is that this feature would be specific to the async/await use > case rather than a generic coroutine implementation (i.e., there would have > to be a separate compiler transform for yield return). It’s not clear to me > why it should be a goal to have just one generic coroutine feature. The > real-world usages of async/await and yield return are different enough that > I’m not convinced we could have a single compiler feature that meets the > needs of both cleanly. > > On Aug 27, 2017, at 7:35 PM, Florent Vilmart <flor...@flovilmart.com> > wrote: > > Adam, you’re completely right, languages as c# and JS have been through > the path before, (callback, Promises , async/await) I believe Chris’s goal > it to avoid building a promise implementation and go straight to a > coroutines model, which is more deeply integrated with the compiler. I > don’t see a particular trade off, pursuing that route, and the main benefit > is that coroutines can power any asynchronous metaphor (Signals, Streams, > Futures, Promises etc...) which is not true of Futures so i would tend to > think that for the long run, and to maximize usability, async/await/yield > would probably be the way to go. > > On Aug 27, 2017, 22:22 -0400, Adam Kemp <adam.k...@apple.com>, wrote: > > As has been explained, futures can be built on top of async/await (or the > other way around). You can have the best of both worlds. We are not losing > anything by having this feature. It would be a huge improvement to have > this as an option. > > However, using futures correctly requires more nested closures than you > have shown in your examples to avoid blocking any threads. That's why > you're not seeing the advantage to async/await. You're comparing examples > that have very different behaviors. > > That said, I have also expressed my opinion that it is better to build > async/await on top of futures rather than the other way around. I believe > it is more powerful and cleaner to make async/await work with any arbitrary > future type (via a protocol). The alternative (building futures on top of > async/await) requires more code when the two are mixed. I very much prefer > how it's done in C#, where you can freely mix the two models without having > to resort to ad-hoc wrappers, and you can use async/await with any futures > implementation you might already be using. > > I really think we should be having more discussion about the tradeoffs > between those two approaches, and I'm concerned that some of the opinions > about how C# does it are not based on a clear and accurate understanding of > how it actually works in that language. > > -- > Adam Kemp > > On Aug 27, 2017, at 6:02 PM, Howard Lovatt <howard.lov...@gmail.com> > wrote: > > The async/await is very similar to the proposed Future (as I posed > earlier) with regard to completion-handler code, they both re-write the > imported completion-handler function using a closure, the relevant sentence > from the Async Proposal is: > > "Under the hood, the compiler rewrites this code using nested closures ..." > > > Unlike the proposed future code the async code is not naturally parallel, > in the running example the following lines from the async code are run in > series, i.e. await blocks: > > let dataResource = await loadWebResource("dataprofile.txt") > let imageResource = await loadWebResource("imagedata.dat") > > The equivalent lines using the proposed Future: > > let dataResource = loadWebResource("dataprofile.txt") > let imageResource = loadWebResource("imagedata.dat") > > Run in parallel and therefore are potentially faster assuming that > resources, like cores and IO, are available. > > Therefore you would be better using a Future than an async, so why provide > an async unless you can make a convincing argument that it allows you to > write a better future? > > -- Howard. > > On 28 August 2017 at 09:59, Adam Kemp <adam.k...@apple.com> wrote: > >> This example still has nested closures (to create a Future), and still >> relies on a synchronous get method that will block a thread. Async/await >> does not require blocking any threads. >> >> I’m definitely a fan of futures, but this example isn’t even a good >> example of using futures. If you’re using a synchronous get method then >> you’re not using futures properly. They’re supposed to make it easy to >> avoid writing blocking code. This example just does the blocking call on >> some other thread. >> >> Doing it properly would show the benefits of async/await because it would >> require more nesting and more complex error handling. By simplifying the >> code you’ve made a comparison between proper asynchronous code (with >> async/await) and improper asynchronous code (your example). >> >> That tendency to want to just block a thread to make it easier is exactly >> why async/await is so valuable. You get simple code while still doing it >> correctly. >> >> -- >> Adam Kemp >> >> On Aug 27, 2017, at 4:00 PM, Howard Lovatt via swift-evolution < >> swift-evolution@swift.org> wrote: >> >> The running example used in the white paper coded using a Future is: >> >> func processImageData1() -> Future<Image> { >> return AsynchronousFuture { _ -> Image in >> let dataResource = loadWebResource("dataprofile.txt") // >> dataResource and imageResource run in parallel. >> let imageResource = loadWebResource("imagedata.dat") >> let imageTmp = decodeImage(dataResource.get ?? >> Resource(path: "Default data resource or prompt user"), imageResource.get >> ?? Resource(path: "Default image resource or prompt user")) >> let imageResult = dewarpAndCleanupImage(imageTmp.get ?? >> Image(dataPath: "Default image or prompt user", imagePath: "Default image >> or prompt user")) >> return imageResult.get ?? Image(dataPath: "Default image or >> prompt user", imagePath: "Default image or prompt user") >> } >> } >> >> This also avoids the pyramid of doom; the pyramid is avoided by >> converting continuation-handlers into either a sync or future, i.e. it is >> the importer that eliminates the nesting by translating the code >> automatically. >> >> This example using Future also demonstrates three advantages of Future: >> they are naturally parallel (dataResource and imageResource lines run in >> parallel), they timeout automatically (get returns nil if the Future has >> taken too long), and if there is a failure (for any reason including >> timeout) it provides a method of either detecting the failure or providing >> a default (get returns nil on failure). >> >> There are a three of other advantages a Future has that this example >> doesn’t show: control over which thread the Future runs on, Futures can be >> cancelled, and debugging information is available. >> >> You could imagine `async` as a syntax sugar for Future, e.g. the above >> Future example could be: >> >> func processImageData1() async -> Image { >> let dataResource = loadWebResource("dataprofile.txt") // >> dataResource and imageResource run in parallel. >> let imageResource = loadWebResource("imagedata.dat") >> let imageTmp = decodeImage(dataResource.get ?? Resource(path: >> "Default data resource or prompt user"), imageResource.get ?? >> Resource(path: "Default image resource or prompt user")) >> let imageResult = dewarpAndCleanupImage(imageTmp.get ?? >> Image(dataPath: "Default image or prompt user", imagePath: "Default image >> or prompt user")) >> return imageResult.get ?? Image(dataPath: "Default image or prompt >> user", imagePath: "Default image or prompt user") >> } >> >> Since an async is sugar for Future the async runs as soon as it is >> created (as soon as the underlying Future is created) and get returns an >> optional (also cancel and status would be still be present). Then if you >> want control over threads and timeout they could be arguments to async: >> >> func processImageData1() async(queue: DispatchQueue.main, timeout: >> .seconds(5)) -> Image { ... } >> >> On Sat, 26 Aug 2017 at 11:00 pm, Florent Vilmart <flor...@flovilmart.com> >> wrote: >> >>> Howard, with async / await, the code is flat and you don’t have to >>> unowned/weak self to prevent hideous cycles in the callbacks. >>> Futures can’t do that >>> >>> On Aug 26, 2017, 04:37 -0400, Goffredo Marocchi via swift-evolution < >>> swift-evolution@swift.org>, wrote: >>> >>> With both he now built in promises in Node8 as well as libraries like >>> Bluebird there was ample time to evaluate them and convert/auto convert at >>> times libraries that loved callback pyramids of doom when the flow grows >>> complex into promise based chains. Converting to Promises seems magical for >>> the simple case, but can quickly descend in hard to follow flows and hard >>> to debug errors when you move to non trivial multi path scenarios. JS is >>> now solving it with their implementation of async/await, but the point is >>> that without the full picture any single solution would break horribly in >>> real life scenarios. >>> >>> Sent from my iPhone >>> >>> On 26 Aug 2017, at 06:27, Howard Lovatt via swift-evolution < >>> swift-evolution@swift.org> wrote: >>> >>> My argument goes like this: >>> >>> 1. You don't need async/await to write a powerful future type; you can >>> use the underlying threads just as well, i.e. future with async/await is no >>> better than future without. >>> >>> 2. Since future is more powerful, thread control, cancel, and timeout, >>> people should be encouraged to use this; instead because async/await are >>> language features they will be presumed, incorrectly, to be the best way, >>> consequently people will get into trouble with deadlocks because they don't >>> have control. >>> >>> 3. async/await will require some engineering work and will at best >>> make a mild syntax improvement and at worst lead to deadlocks, therefore >>> they just don't carry their weight in terms of useful additions to Swift. >>> >>> Therefore, save some engineering effort and just provide a future >>> library. >>> >>> To turn the question round another way, in two forms: >>> >>> 1. What can async/wait do that a future can't? >>> >>> 2. How will future be improved if async/await is added? >>> >>> >>> -- Howard. >>> >>> On 26 August 2017 at 02:23, Joe Groff <jgr...@apple.com> wrote: >>> >>>> >>>> On Aug 25, 2017, at 12:34 AM, Howard Lovatt <howard.lov...@gmail.com> >>>> wrote: >>>> >>>> In particular a future that is cancellable is more powerful that the >>>> proposed async/await. >>>> >>>> >>>> It's not more powerful; the features are to some degree disjoint. You >>>> can build a Future abstraction and then use async/await to sugar code that >>>> threads computation through futures. Getting back to Jakob's example, >>>> someone (maybe the Clang importer, maybe Apple's framework developers in an >>>> overlay) will still need to build infrastructure on top of IBActions and >>>> other currently ad-hoc signalling mechanisms to integrate them into a more >>>> expressive coordination framework. >>>> >>>> -Joe >>>> >>> >>> _______________________________________________ >>> swift-evolution mailing list >>> swift-evolution@swift.org >>> https://lists.swift.org/mailman/listinfo/swift-evolution >>> >>> -- >> -- Howard. >> >> _______________________________________________ >> swift-evolution mailing list >> swift-evolution@swift.org >> https://lists.swift.org/mailman/listinfo/swift-evolution >> >> > > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution >
_______________________________________________ swift-evolution mailing list swift-evolution@swift.org https://lists.swift.org/mailman/listinfo/swift-evolution