<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="">What exactly is the difference to just returning a sequence?<div class=""><br class=""></div><div class=""><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><font size="1" class=""><span style="color: rgb(53, 86, 138);" class="">func</span> helloGenerator(name : <span style="color: rgb(195, 89, 0);" class="">String</span>?) -&gt; [<span style="color: rgb(195, 89, 0);" class="">String</span>] {</font></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><font size="1" class="">&nbsp; &nbsp; <span style="font-variant-ligatures: no-common-ligatures; color: #35568a" class="">return</span> [</font></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><font size="1" class="">&nbsp; &nbsp; &nbsp; &nbsp; <span style="font-variant-ligatures: no-common-ligatures; color: #e82300" class="">"Hello"</span>,</font></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><font size="1" class="">&nbsp; &nbsp; &nbsp; &nbsp; name ?? <span style="font-variant-ligatures: no-common-ligatures; color: #e82300" class="">"World"</span></font></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><font size="1" class="">&nbsp; &nbsp; ]</font></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><font size="1" class="">}</font></div><div style="margin: 0px; line-height: normal; font-family: Menlo; min-height: 16px;" class=""><font size="1" class=""><br class=""></font></div><div style="margin: 0px; line-height: normal; font-family: Menlo; color: rgb(88, 126, 168);" class=""><font size="1" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #35568a" class="">for</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> str </span><span style="font-variant-ligatures: no-common-ligatures; color: #35568a" class="">in</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span>helloGenerator<span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">(</span><span style="font-variant-ligatures: no-common-ligatures; color: #e82300" class="">"David"</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">) {</span></font></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><font size="1" class="">&nbsp; &nbsp; <span style="font-variant-ligatures: no-common-ligatures; color: #587ea8" class="">print</span>(str)</font></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><font size="1" class="">}</font></div><div><br class=""></div><div>And if you want if lazy:</div><div><br class=""></div><div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><font size="1" class=""><span style="color: rgb(53, 86, 138);" class="">func</span> helloGenerator(name : <span style="color: rgb(195, 89, 0);" class="">String</span>?) -&gt; <span style="color: rgb(195, 89, 0);" class="">LazyCollection</span>&lt;[<span style="color: rgb(195, 89, 0);" class="">String</span>]&gt; {</font></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><font size="1" class="">&nbsp; &nbsp; <span style="font-variant-ligatures: no-common-ligatures; color: #35568a" class="">return</span> [</font></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><font size="1" class="">&nbsp; &nbsp; &nbsp; &nbsp; <span style="font-variant-ligatures: no-common-ligatures; color: #e82300" class="">"Hello"</span>,</font></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><font size="1" class="">&nbsp; &nbsp; &nbsp; &nbsp; name ?? <span style="font-variant-ligatures: no-common-ligatures; color: #e82300" class="">"World"</span></font></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><font size="1" class="">&nbsp; &nbsp; ].<span style="font-variant-ligatures: no-common-ligatures; color: #587ea8" class="">lazy</span></font></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><font size="1" class="">}</font></div></div><div><br class=""></div><div>Seems like exactly the same, except it doesn’t need two new keywords nor revamping of SequenceType/GeneratorType</div><div><br class=""><blockquote type="cite" class=""><div class="">On 12 Dec 2015, at 01:21, David Waite via swift-evolution &lt;<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>&gt; wrote:</div><br class="Apple-interchange-newline"><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Looking for feedback on crafting a proposal adding generator functions to Swift. I understand this will likely be a very involved proposal at the language level, although I actually don’t know the complexity of the change within the Swift compiler itself.<div class=""><div class=""><br class=""></div><div class="">This would be a function which returns multiple values, which is converted by the compiler to a function returning a SequenceType<div class=""><br class=""></div><div class="">A very basic syntax would be to add generator as a modifier to func, and likely involve a new keyword ‘yield’ to differentiate from the flow control behavior of ‘return’.</div><div class=""><br class=""></div><div class="">So for example:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class="">generator func helloGenerator(name:String?) -&gt; String {</div><div class="">&nbsp; &nbsp; yield “Hello”</div><div class="">&nbsp; &nbsp; yield name ?? “World”</div><div class="">}</div></blockquote><div class=""><br class=""></div><div class="">Would have the following expected usage:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class="">for str in helloGenerator(“David") {</div><div class="">&nbsp; &nbsp;print str</div>// prints:</blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class="">// &nbsp; &nbsp;Hello<br class=""><div class="">// &nbsp; &nbsp;David</div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class="">}</div></blockquote><br class=""><div class=""><div class="">And for those unfamiliar to these sorts of simple cases, would have equivalent behavior to the following code:</div><div class=""><br class=""></div><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;45&gt; </span>func helloGenerator(name:String?) -&gt; HelloGenerator {&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;46. </span>&nbsp; return HelloGenerator(name)&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(169, 169, 169);" class="">&nbsp;47. <span style="" class="">}&nbsp;</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(169, 169, 169);" class="">&nbsp;48. <span style="" class="">&nbsp;</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;49. </span>struct HelloGenerator : GeneratorType, SequenceType {&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;50.&nbsp;</span> &nbsp; var position:Int = 0&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;51.&nbsp;</span> &nbsp; let name:String?&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(169, 169, 169);" class="">&nbsp;52. <span style="" class="">&nbsp;</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;53.&nbsp;</span> &nbsp; private init(_ name:String?) {&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;54. </span>&nbsp; &nbsp; &nbsp; self.name = name&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;55.&nbsp;</span> &nbsp; }&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(169, 169, 169);" class="">&nbsp;56.&nbsp;<span style="" class=""> &nbsp; &nbsp;</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;57.&nbsp;</span> &nbsp; func generator() -&gt; HelloGenerator {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;58.&nbsp;</span> &nbsp; &nbsp; &nbsp; return self&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;59.&nbsp;</span> &nbsp; }&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;60. </span>&nbsp; &nbsp; &nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;61.&nbsp;</span> &nbsp; mutating func next() -&gt; String? {&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;62. </span>&nbsp; &nbsp; &nbsp; switch position {&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;63. </span>&nbsp; &nbsp; &nbsp; case 0:&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;64. </span>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; position = 1&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;65. </span>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return "Hello"&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;66. </span>&nbsp; &nbsp; &nbsp; case 1:&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;67. </span>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; position = 2&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;68. </span>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return name ?? "World"&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;69. </span>&nbsp; &nbsp; &nbsp; default:&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;70. </span>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return nil&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;71. </span>&nbsp; &nbsp; &nbsp; }&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #a9a9a9" class="">&nbsp;72.&nbsp;</span> &nbsp; }&nbsp;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(169, 169, 169);" class="">&nbsp;73. <span style="" class="">}&nbsp;</span></div></div><div class=""><br class=""></div></blockquote>This syntax has at a bare minimum issues with generator closures and for a terse syntax for yielding over another sequence type within a generator function vs. using a loop. (possibly “yield in sequenceName”)</div><div class=""><br class=""></div><div class="">The interaction with the error system might involve disallowing throws from generator functions, or having the Element type be a Result&lt;T&gt; rather than T, as the GeneratorType next() method is not declared as throwing.</div><div class=""><br class=""></div><div class=""><div class="">This could pair well to make for-in loops more comprehensive, especially if C-style for loops are eliminated.</div></div><div class=""><br class=""></div><div class="">This would possibly be a first step toward a coroutine-based concurrency system, although I am not proposing that sort of usage or scope here. The goal would be to emit an object compatible with SequenceType</div></div><div class=""><br class=""></div><div class="">-David Waite (DW)</div></div>
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