[swift-evolution] [Concurrency] Fixing race conditions in async/await example
Howard Lovatt
howard.lovatt at gmail.com
Tue Aug 29 19:43:23 CDT 2017
@Vladimir,
Default values are a problem for await/async when combined with parallel
running because await returns a value and not an optional (unlike future's
get). Below is a more realistic code for parallel running and a default
value using async/await:
func updateImage() async {
let image: Image
async do { // Runs in parallel (async)
image = try async preprocessImage(downloadImage())
} catch {
image = defaultImage
}
let text: String
async do { // Runs in parallel (async)
text = try async translate(downloadText())
} catch {
text = defaultText
}
// This line is complicated! We want render not to block (async), but have
to await for image and text.
// Render does not throw because it always has valid input.
// If async were allowed to prevent blocking then await could not
async render(image: await image, text: await text)
}
Which I don't think reads as well as the Future version:
func updateImage() -> Future<Void> {
return AsynchronousFuture { _ -> Void in
let image = preprocessImage(downloadImage()) // Parallel, Futures are
queued on creation
let text = translate(downloadText()) // Parallel, Futures are queued on
creation
// Does not block (Futures are queued on creation), but has to wait for its
inputs (get).
render(image: image.get ?? defaultImage, text: text.get ?? defaultText)
}
}
In addition the async/await version does not have timeout; unlike the
Future version.
Suppose that downloadImage doesn't fail, it just takes forever to download
the image. The Future version will timeout automatically and the default
image will be used. With async/await the code for downloadImage and
downloadText will have to start timers and throw if the timers timeout.
Nothing to do in the Future version, it handles timeout for you.
Neither or the above versions have cancel or control over the queue they
execute on, but both would be much easier to add to the Future version,
like timeout is much easier to add, since Futures support cancel and queue
control directly.
-- Howard.
On 30 August 2017 at 02:45, Vladimir.S via swift-evolution <
swift-evolution at swift.org> wrote:
> On 29.08.2017 19:02, Wallacy via swift-evolution wrote:
>
>> In this example i think we lose clarity, just looking for the code we
>> cant know if this two line will run on parallel or not!
>> Also, image.get blocks the thread, in this case we need the await anyway!
>> And `async` can throws too... So the error handler can be pretty similar.
>>
>> let image= asyncpreprocessImage(downloadImage()) // These first two
>> lines run in parallel and I can "see" the async keyword.
>> let text= asynctranslate(downloadText())
>> await render(image: image ?? defaultImage,text: text ?? defaultText) //
>> No blocking!
>>
>>
> FWIW: I'm following the whole discussion from the start, and do support
> the opinion that async/await is much clear solution that proposed Futures,
> especially for beginners.
> We need a low-level building blocks which can be used to implement
> Futures/Promises in libraries.
> Also I really like the idea of 'async' on the caller side to have code
> running in parallel.
>
> The 'async' version of func declaration is clearly saying what type it
> *want* to return, and 'async' modifier just saying *how* it will/can return
> that type('Image' in examples). So on both sides, on declaration and on
> caller side, we are clear what types we are working with.
> Future<Type> - is mixing of what is returning and how this will be
> returned. Code is saying that we preprocessesImage, but actually we have
> Future<Image> type, no 'markers' of asynchronous code.
>
> Also, I wonder(if I missed that in proposal/discussion, please let me
> know), if I have async function like
>
> func foo() async -> Type {}
>
> , may I want to call it synchronously? If so, what would be a solution
> here? I can think about something like 'sync' modifier on caller side:
> let x = sync foo() // calling asynchronous function synchronously
>
> I believe that is what Future.get is doing, no?
> let future = ...
> future.get() // blocks the execution, waits for the result.
>
> Probably it is reasonable to allow just call foo() to get blocking result,
> just like any other 'simple' blocking funcs that we call, but this can lead
> to unexpected behavior as user can expect async execution.
>
> With Futures, it seems like we can't "just" call such function and need to
> call .get() later:
> let future = someFuncReturnsFuture() // already returns Future<Type> type
>
> Vladimir.
>
>
>> Like i said before! Today's, the proposal only lack two things over the
>> `Future`....
>> Parallel computing: Can be implemented by a third party library or a
>> personal one, but i don't think this is a good approach to the first
>> version.
>> Coordination: This we can wait! And why? Because coordination, can be
>> made in different ways, maybe is more suitable to a standard library
>> class/function, not a language level resource.
>>
>> Also, coordination cant be applied to all variants of the runtimes in the
>> same way! async/await as language level works just as well with GCD as
>> with pthreads or another API. And coordination is a compromise that we can
>> make after that one.
>>
>> Em ter, 29 de ago de 2017 às 05:23, Howard Lovatt via swift-evolution <
>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>> escreveu:
>>
>> @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 parallel
>> let 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
>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>
>>
>> On 29 Aug 2017, at 02:22, Xiaodi Wu via swift-evolution
>>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>
>>> wrote:
>>>
>>> On Mon, Aug 28, 2017 at 16:10 Adam Kemp via swift-evolution
>>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>
>>> 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())
>> awaitrender(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) })
>> awaitrender(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 <wallacyf at gmail.com
>>>> <mailto:wallacyf at gmail.com>> 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 <
>>>> adam.kemp at apple.com
>>>> <mailto:adam.kemp at apple.com>> 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 <
>>>>> wallacyf at gmail.com
>>>>> <mailto:wallacyf at gmail.com>> 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
>>>>> <swift-evolution at swift.org <mailto:
>>>>> swift-evolution at 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.c
>>>>> om/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
>>>>>> <florent at flovilmart.com <mailto:
>>>>>> florent at 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.kemp at apple.com
>>>>>> <mailto:adam.kemp at 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.lovatt at gmail.com <mailto:
>>>>>>> howard.lovatt at 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=
>>>>>>>> awaitloadWebResource("dataprofile.txt")
>>>>>>>> let imageResource=
>>>>>>>> awaitloadWebResource("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.kemp at apple.com
>>>>>>>> <mailto:adam.kemp at 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 at swift.org
>>>>>>>> <mailto:swift-evolution at 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
>>>>>>>>> <florent at flovilmart.com
>>>>>>>>> <mailto:florent at 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 at swift.org
>>>>>>>>> <mailto:swift-evolution at 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 at swift.org
>>>>>>>>>> <mailto:swift-evolution at 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
>>>>>>>>>>> <jgroff at apple.com <mailto:
>>>>>>>>>>> jgroff at apple.com>> wrote:
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> On Aug 25, 2017, at 12:34 AM,
>>>>>>>>>>>> Howard
>>>>>>>>>>>> Lovatt <howard.lovatt at gmail.com
>>>>>>>>>>>> <mailto:howard.lovatt at 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 at swift.org
>>>>>>>>>>> <mailto:swift-evolution at swift.org>
>>>>>>>>>>> https://lists.swift.org/mailma
>>>>>>>>>>> n/listinfo/swift-evolution
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>> --
>>>>>>>>> -- Howard.
>>>>>>>>> ______________________________
>>>>>>>>> _________________
>>>>>>>>> swift-evolution mailing list
>>>>>>>>> swift-evolution at swift.org
>>>>>>>>> <mailto:swift-evolution at swift.org>
>>>>>>>>> https://lists.swift.org/mailma
>>>>>>>>> n/listinfo/swift-evolution
>>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>> _______________________________________________
>>>>> swift-evolution mailing list
>>>>> swift-evolution at swift.org <mailto:
>>>>> swift-evolution at swift.org>
>>>>> https://lists.swift.org/mailma
>>>>> n/listinfo/swift-evolution
>>>>>
>>>>>
>>>>
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
>>
>>
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
>>
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
>>
>>
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
>> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20170830/853bd38f/attachment.html>
More information about the swift-evolution
mailing list