<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto">Just to get it out of the way and to put the issue down... assuming we could get something like Promises and Futures, how much experience do people have with how async and await + Promises are changing the asynchronous by nature JavaScript? What in there really does not work and should one of the layer we provide be similar to it?<br><br><div id="AppleMailSignature">Sent from my iPhone</div><div><br>On 22 Aug 2017, at 17:32, Joe Groff via swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>> wrote:<br><br></div><blockquote type="cite"><div><meta http-equiv="Content-Type" content="text/html charset=utf-8"><br class=""><div><blockquote type="cite" class=""><div class="">On Aug 21, 2017, at 10:21 PM, David Hart <<a href="mailto:david@hartbit.com" class="">david@hartbit.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Sorry for the shameless bump :-/ but I’d love to get some answers so I can better understand the proposal and participate in the discussions.<div class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On 21 Aug 2017, at 07:58, David Hart via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Hello,<div class=""><br class=""></div><div class="">Thanks for the great work on the async/await proposal! After reading it, I have a few questions and comments about it, so I’m creating this thread to concentrate on that topic (instead of Actors).</div><div class=""><br class=""></div><div style="font-size: 15px;" class=""><b class="">Generators</b></div><div class=""><br class=""></div><div class="">The proposal mentions in <b class="">Problem 6</b> of the <b class="">Motivation</b> how generators can help write sequences:</div><div class=""><br class=""></div><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">In contrast, languages that have generators allow you to write something more close to this:</p><div class="highlight highlight-source-swift" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);"><pre style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.600000381469727px; margin-top: 0px; margin-bottom: 0px; line-height: 1.45; word-wrap: normal; padding: 16px; overflow: auto; background-color: rgb(246, 248, 250); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-break: normal;" class=""><span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">getSequence</span>() <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">-></span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">AnySequence</span><<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">Int</span>> {
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> seq <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> sequence {
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">for</span> i <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">in</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">1</span><span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">...</span><span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">10</span> {
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">yield</span>(i<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">*</span>i)
}
}
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">return</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">AnySequence</span>(seq)
}</pre></div><div class=""><br class=""></div><div class="">This feels very similar to me from C# where the <font face="Menlo" class="">yield</font> keyword is used to support the generator feature. But I fail to see how the coroutines as described in this proposal resolve this problem. Can someone explain?</div></div></div></blockquote></div></div></div></div></blockquote><div><br class=""></div><div>The feature provides general delimited continuations. You could write an IteratorProtocol-conforming interface over a coroutine like this:</div><div><br class=""></div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div><div>class Generator<T>: IteratorProtocol {</div></div><div><div> var next: T? = nil</div></div><div><div> var resume: (() -> ())? = nil</div></div><div><div><br class=""></div></div><div><div> init(_ body: (_ yield: @escaping (T) async -> Void) -> Void) {</div></div><div><div> self.resume = {</div></div><div><div> beginAsync {</div></div><div><div> body(self.yield)</div></div><div><div> }</div></div><div><div> }</div></div><div><div><br class=""></div></div><div><div> func next() -> T? {</div></div><div><div> if let resume = self.resume {</div></div><div><div> resume()</div></div><div><div> return self.next</div></div><div><div> }</div></div><div><div> return nil</div></div><div><div> }</div></div><div><div><br class=""></div></div><div><div> private func yield(_ value: T) async -> Void {</div></div><div><div> self.next = value</div></div><div><div> await suspendAsync { cont in</div></div><div><div> resume = cont</div></div><div><div> }</div></div><div><div> }</div></div><div><div>}</div></div><div><div><br class=""></div></div><div><div>let fibs = Generator { yield in</div></div><div><div> var (a, b) = (0, 1)</div></div><div><div> while a < 1000 {</div></div><div><div> await yield(a)</div></div><div><div> (a, b) = (b, a + b)</div></div><div><div> }</div></div><div><div>}</div></div></blockquote><div><br class=""></div><div>This isn't ideal in a number of ways (awkward, not particularly efficient, and has the gotcha that the generator's `body` could suspend itself with something other than the `yield` operation, doesn't integrate with ownership in the way John proposes in the ownership manifesto), so it may not be a good idea, of course.</div><div><br class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><div class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div style="font-size: 15px;" class=""><b class="">beginAsync</b></div><div class=""><br class=""></div><div class="">The documentation of the <font face="Menlo" class="">beginAsync</font> and <font face="Menlo" class="">suspendAsync</font> functions state:</div><div class=""><br class=""></div><div class=""><pre style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.600000381469727px; margin-top: 0px; margin-bottom: 0px; line-height: 1.45; word-wrap: normal; padding: 16px; overflow: auto; background-color: rgb(246, 248, 250); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-break: normal; color: rgb(36, 41, 46);" class=""><span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);">// NB: Names subject to bikeshedding. These are low-level primitives that most</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> users should not need to interact with directly, so namespacing them</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> and/or giving them verbose names unlikely to collide or pollute code</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> completion (and possibly not even exposing them outside the stdlib to begin</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> with) would be a good idea.</span></pre><div class=""><br class=""></div></div><div class="">But I don’t understand how they can be kept private to the standard library when they are used for the important pattern of spawning off an async operation from a non-async function:</div></div></div></blockquote></div></div></div></div></blockquote><div><br class=""></div><div>beginAsync provides raw material for starting an async process, but I think you'd often want to wrap it up in something more interesting, like a DispatchQueue method that enqueues the coroutine on a specific queue, a Future constructor that captures the eventual result, etc. Nonetheless, I removed this statement from the proposal.</div><br class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><div class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">Despite these problems, it is essential that the model encompasses this pattern, because it is a practical necessity in Cocoa development. With this proposal, it would look like this:</p><div class="highlight highlight-source-swift" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(36, 41, 46); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);"><pre style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.600000381469727px; margin-top: 0px; margin-bottom: 0px; line-height: 1.45; word-wrap: normal; padding: 16px; overflow: auto; background-color: rgb(246, 248, 250); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-break: normal;" class=""><span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">@IBAction</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">buttonDidClick</span>(<span class="pl-smi" style="box-sizing: border-box;"><span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">sender</span></span>:<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">AnyObject</span>) {
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> 1</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"></span> beginAsync {
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> 2</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"></span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">let</span> image <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> await <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">processImage</span>()
imageView.<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">image</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> image
}
<span class="pl-c" style="box-sizing: border-box; color: rgb(106, 115, 125);"><span class="pl-c" style="box-sizing: border-box;">//</span> 3</span></pre></div><div style="font-size: 15px;" class=""><b class="">Futures</b></div></div><div class=""><br class=""></div><div class="">When discussing futures, the proposal states:</div><div class=""><br class=""></div><div class=""><span style="color: rgb(36, 41, 46); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">The exact design for a future type deserves its own proposal, but a proof of concept could look like this:</span></div><div class=""><span style="color: rgb(36, 41, 46); background-color: rgb(255, 255, 255);" class=""><br class=""></span></div><div class=""><span style="background-color: rgb(255, 255, 255);" class=""><font color="#24292e" class="">Does that sentence imply that the Core Team would welcome a Future implementation into the Standard Library?</font></span></div></div></div></blockquote></div></div></div></div></blockquote><div><br class=""></div><div>It's worth discussing. My personal feeling is that a lot of the things people do with futures can potentially be done better with other coordination primitives, but we'll see.</div><br class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><div class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div style="font-size: 15px;" class=""><span style="background-color: rgb(255, 255, 255);" class=""><font color="#24292e" class=""><b class="">async as a subtype of throws instead of orthogonal to it</b></font></span></div><div class=""><span style="background-color: rgb(255, 255, 255);" class=""><font color="#24292e" class=""><br class=""></font></span></div><div class=""><span style="background-color: rgb(255, 255, 255);" class=""><font color="#24292e" class="">I’ve been thinking a lot about this since the proposal came out and I see a few serious </font></span><font color="#24292e" class="">disadvantages at making async a subtype of throws which might benefit from being discussed or/and mentioned in the proposal.</font></div><div class=""><font color="#24292e" class=""><br class=""></font></div><div class=""><font color="#24292e" class="">1. We loose the automatic documentation <font face="Menlo" class="">try</font> provides for signaling failable functions:</font></div><div class=""><br class=""></div><div class=""><font color="#24292e" face="Menlo" class="">let image = await downloadImage()</font></div><div class=""><font color="#24292e" face="Menlo" class="">let processedImage = await processImage(image)</font></div><div class=""><font color="#24292e" face="Menlo" class="">await present(MyViewController(image: image))</font></div><div class=""><font color="#24292e" class=""><br class=""></font></div><div class=""><font color="#24292e" class="">In my example, <font face="Menlo" class="">downloadImage</font> can fail because of network conditions, <font face="Menlo" class="">processImage</font> can not fail, and <font face="Menlo" class="">present</font> is the UIKit function which presents view controllers and it can’t fail either. But that’s not obvious from reading the code. We’ve lost information.</font></div></div></div></blockquote></div></div></div></div></blockquote><div><br class=""></div><div>This seems like a similar pitfall to too narrow exception types that we try to avoid with `throws`. Saying that even a long-lived computation like processImage can't throw is a brittle architectural choice, since you may need to build in support for cancellation at some point, and if you ever decide to offload the computation to a GPU, coprocessor, or out-of-process worker, then it will be able to fail at that point. It's not clear to me why `present` would be async here; it seems to me like a fire-and-forget kind of operation you don't want to wait for.</div></div><br class=""><div class="">-Joe</div></div></blockquote><blockquote type="cite"><div><span>_______________________________________________</span><br><span>swift-evolution mailing list</span><br><span><a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a></span><br><span><a href="https://lists.swift.org/mailman/listinfo/swift-evolution">https://lists.swift.org/mailman/listinfo/swift-evolution</a></span><br></div></blockquote></body></html>