<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body 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 class=""><br class=""></div><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 class=""><br class=""></div><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 class=""><span style="background-color: rgb(255, 255, 255);" class=""><font color="#24292e" class=""><br class=""></font></span></div><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 class=""><font color="#24292e" class=""><br class=""></font></div><div class=""><font color="#24292e" class="">2. Supporting <font face="Menlo" class="">try?</font> and <font face="Menlo" class="">try!</font> adds a lot of confusion:</font></div><div class=""><font color="#24292e" class=""><br class=""></font></div><div class=""><font color="#24292e" class="">As was mentioned by Karim Nassar is another post, if <font face="Menlo" class="">await</font> infers <font face="Menlo" class="">try</font>, then there seems to be no good solution for supporting <font face="Menlo" class="">try?</font> and <font face="Menlo" class="">try!:</font></font></div><div class=""><font color="#24292e" class=""><font face="Menlo" class=""><br class=""></font></font></div><div class=""><ul class="MailOutline"><li class=""><font color="#24292e" class="">Using <font face="Menlo" class="">await?</font> and <font face="Menlo" class="">await!</font> seems slightly conter-intuitive because we are further mixing the concepts of coroutines/asynchronous operations with error handling.</font></li><li class=""><font color="#24292e" class="">Adding <font face="Menlo" class="">try?</font> and <font face="Menlo" class="">try!</font> (like suggested Chris Lattner) feels like it makes point (1) by having both explicit <font face="Menlo" class="">try?/! </font>and implicit <font face="Menlo" class="">try</font> through <font face="Menlo" class="">await</font>.</font></li></ul><div class=""><font color="#24292e" class=""><br class=""></font></div></div><div class=""><font color="#24292e" class="">3. Philosophical discussion</font></div><div class=""><font color="#24292e" class=""><br class=""></font></div><div class=""><font color="#24292e" class="">If async calls don’t return futures because coroutines are a generally useful language features beyond the domain of async/await, doesn’t making async imply throws also muddy the concept of coroutines where failable coroutine operations don’t make much sense?</font></div><div class=""><font color="#24292e" class=""><br class=""></font></div><div class=""><font color="#24292e" class="">David.</font></div></body></html>