@David, Using the `Future` library based on GCD that I have previously posted your example would be:
let image = preprocessImage(downloadImage()) // These first two lines run in parallellet text = translate(downloadText())render(image: image.get ?? defaultImage, text: text.get ?? defaultText) The main difference, and I would argue an improvement, is that the `Future` version handles errors. So what advantage does async/await have over a `Future` library we can write today? -- Howard. On 29 August 2017 at 15:28, David Hart via swift-evolution < [email protected]> wrote: > > On 29 Aug 2017, at 02:22, Xiaodi Wu via swift-evolution < > [email protected]> wrote: > > On Mon, Aug 28, 2017 at 16:10 Adam Kemp via swift-evolution < > [email protected]> wrote: > >> I know what the proposal said. I’m making a case that there is value in >> doing it differently. >> >> The composability of futures is valuable. Mixing and matching async/await >> with futures is also valuable. The queue-returning behavior that you can >> get from futures is also valuable, and building async/await on top of >> futures means async/await can get that for free. >> > > Why couldn't you mix and match async/await and futures and get the > queue-return behavior of futures if futures are built on top of async/await > instead off the other way around? > > > We could, but the syntax is much worse. Contrast: > > *async/await built on top of Futures* > > let image = preprocessImage(downloadImage())let text = > translate(downloadText()) > await render(image: image, text: text) > > > *Futures built on top of async/await* > > let image = Future(downloadImage).then({ preprocessImage($0) })let text = > Future(downloadText).then({ translate($0) }) > await render(image: image.get(), text: text.get()) > > > Maybe you don’t value those things, which is fine. But I do, and maybe >> other people do too. That’s why we’re having a discussion about it. >> >> It can also be valuable having a minimal implementation, but we have to >> acknowledge that it comes with a downside as well. The problem with doing a >> minimal implementation is that you can be stuck with the consequences for a >> long time. I want to make sure that we’re not stuck with the consequences >> of a minimal implementation that doesn’t adequately address the problems >> that async/await should be addressing. I’d hate for Swift to get an >> async/await that is so weak that it has to be augmented by tedious >> boilerplate code before it’s useful. >> >> >> On Aug 28, 2017, at 1:54 PM, Wallacy <[email protected]> wrote: >> >> We don't need to this now! >> >> Again: (Using proposal words) >> >> "It is important to understand that this is proposing compiler support >> that is completely concurrency runtime-agnostic. This proposal does not >> include a new runtime model (like "actors") - it works just as well with >> GCD as with pthreads or another API. Furthermore, unlike designs in other >> languages, it is independent of specific coordination mechanisms, such as >> futures or channels, allowing these to be built as library feature" >> >> and >> >> "This proposal does not formally propose a Future type, or any other >> coordination abstractions. There are many rational designs for futures, and >> a lot of experience working with them. On the other hand, there are also >> completely different coordination primitives that can be used with this >> coroutine design, and incorporating them into this proposal only makes it >> larger." >> >> and >> >> We focus on task-based concurrency abstractions commonly encountered in >> client and server applications, particularly those that are highly event >> driven (e.g. responding to UI events or requests from clients). This does >> not attempt to be a comprehensive survey of all possible options, nor does >> it attempt to solve all possible problems in the space of concurrency. >> Instead, it outlines a single coherent design thread that can be built over >> the span of years to incrementally drive Swift to further greatness. >> >> and >> >> This proposal has been kept intentionally minimal, but there are many >> possible ways to expand this in the future. >> >> .... >> >> The point is: No Future type is indeed proposed yet! >> >> The proposal try to include de "minimum" required to implement a basic >> async/await to solve the problem created by the GCD! (Pyramid of doom) >> >> The question is: How do you do the same using dispatch_async ? >> dispatch_async also does not return nothing to do what you are intentend do >> do! >> >> Algo, by Swift 5 manifesto, there's no compromise to make a "complete" >> concurrency model by this time! >> >> My intention is only make parity to dispatch_async, but also make the >> ground free to make more complex implementation like Futures in another >> round on top of this one. >> >> This 'async T' can be a real type in the future? Maybe will... But >> doesn't matter now! Now we only need to is some kind of type which need to >> be unwrapped using await before use. Maybe this intermediary/virtual type >> can be a real thing and gain some abilities at some point! Maybe a full >> Future type, why not? >> >> Em seg, 28 de ago de 2017 às 17:33, Adam Kemp <[email protected]> >> escreveu: >> >>> How would these anonymous types get composed? If I wanted to implement a >>> function that takes a collection of futures and wait on it, how would I do >>> that? That is, how would I implement the equivalent of C#’s Task.WhenAll >>> and Task.WhenAny methods? >>> >>> More generally, how do you pass one of these typeless futures to some >>> other function so that we can do the waiting there? >>> >>> >>> On Aug 28, 2017, at 1:23 PM, Wallacy <[email protected]> wrote: >>> >>> 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 < >>> [email protected]> 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 <[email protected]> >>>> 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 <[email protected]>, 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 <[email protected]> >>>> 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 <[email protected]> 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 < >>>>> [email protected]> 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 < >>>>> [email protected]> 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 < >>>>>> [email protected]>, 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 < >>>>>> [email protected]> 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 <[email protected]> wrote: >>>>>> >>>>>>> >>>>>>> On Aug 25, 2017, at 12:34 AM, Howard Lovatt <[email protected]> >>>>>>> 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 >>>>>> [email protected] >>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution >>>>>> >>>>>> -- >>>>> -- Howard. >>>>> >>>>> _______________________________________________ >>>>> swift-evolution mailing list >>>>> [email protected] >>>>> https://lists.swift.org/mailman/listinfo/swift-evolution >>>>> >>>>> >>>> >>>> _______________________________________________ >>>> swift-evolution mailing list >>>> [email protected] >>>> https://lists.swift.org/mailman/listinfo/swift-evolution >>>> >>> >>> >> _______________________________________________ >> swift-evolution mailing list >> [email protected] >> https://lists.swift.org/mailman/listinfo/swift-evolution >> > _______________________________________________ > swift-evolution mailing list > [email protected] > https://lists.swift.org/mailman/listinfo/swift-evolution > > > > _______________________________________________ > swift-evolution mailing list > [email protected] > https://lists.swift.org/mailman/listinfo/swift-evolution > >
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