<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div><br></div><div><br>On May 20, 2016, at 7:07 PM, Erica Sadun 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=us-ascii"><div class=""><b class="">Earlier on Swift Evolution:</b></div><div class=""><br class=""></div><div class="">Me: "<i class="">Is there a technical reason that Swift cannot be expanded to allow arbitrary mixes of conditional binding and boolean assertions within a single compound guard statement?</i>"</div><div class=""><br class=""></div><div class="">Joe Groff: "<i class="">No. You already can, we just have the somewhat strange rule that to separate `guard` conditions uses `,` before optional or pattern conditions, but `where` before Boolean conditions. </i><i class="">There's no technical reason we couldn't accept either 'where' or ',' consistently."</i></div><div class=""><br class=""></div><div class=""><font face="Menlo" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>guard x == 0,</font></div><div class=""><font face="Menlo" class=""><span class="Apple-tab-span" style="white-space:pre"> </span> let y = optional where</font></div><div class=""><font face="Menlo" class=""><span class="Apple-tab-span" style="white-space:pre"> </span> z == 2 {</font></div><div class=""><font face="Menlo" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>}</font></div><div class=""><br class=""></div><div class=""><b class="">Pitch: </b></div><div class=""><br class=""></div><div class="">I'd like to update Swift's grammar to interchangeably and consistently accept `where` or `,` to separate guard conditions. This would allow a more consistent approach that supports intermingling conditional binding and boolean assertions. Here's a real-world bit of code I was helping someone with a few evenings ago. It's attempting to navigate through some JSON, using optional conditions with where clauses.</div><div class=""><br class=""></div><div class=""><div class=""><font face="Menlo" class="">guard</font></div><div class=""><font face="Menlo" class=""> let fileContents = fileContents,</font></div><div class=""><font face="Menlo" class=""> let jsonDict = try NSJSONSerialization.JSONObjectWithData(fileContents, options: []) as? NSDictionary,</font></div><div class=""><font face="Menlo" class=""> let featuresArray = jsonDict["features"] as? NSArray <b class="">where </b></font><b class=""><font face="Menlo" class="">featuresArray.count > 0,</font></b></div><div class=""><font face="Menlo" class=""> let featuresDict = featuresArray[0] as? NSDictionary,</font></div><div class=""><font face="Menlo" class=""> let coordinatesArray = featuresDict["geometry"] <b class="">where </b></font><b class=""><font face="Menlo" class="">coordinatesArray.count > 0,</font></b></div><div class=""><font face="Menlo" class=""> let coordinateArray = coordinatesArray[0] as? NSArray <b class="">where </b></font><b style="font-family: Menlo;" class="">coordinateArray.count > 3</b></div><div class=""><font face="Menlo" class=""> else { fatalError("Reason") }</font></div></div></div></blockquote><blockquote type="cite"><div><div class=""><br class=""></div><div class="">Each `where` test is a separate test. While there <i class="">are</i> semantic ties between the conditional binding and the count tests, there <i class="">doesn't have to be</i>. Under Swift's current rules, you must use the `where` keyword to introduce a Boolean test after a binding or pattern, regardless of whether or not there's an underlying semantic link between the two.</div><div class=""><br class=""></div><div class="">By removing this requirement and allowing interchangeability between `where` and `,`, you're given the option of tying the boolean to the binding/pattern match or introducing a boolean statement with no connection to previous steps. Here's what this example looks like after excluding `where`:</div><div class=""><br class=""></div><div class=""><span style="font-family: Menlo;" class="">guard</span></div><div class=""><div class=""><span style="font-family: Menlo;" class=""> let fileContents = fileContents,</span></div><div class=""><span style="font-family: Menlo;" class=""> let jsonDict = try NSJSONSerialization.JSONObjectWithData(fileContents, options: []) as? NSDictionary,</span></div><div class=""><span style="font-family: Menlo;" class=""> let featuresArray = jsonDict["features"] as? NSArray,</span></div><div class=""><b class=""><font face="Menlo" class=""> featuresArray.count > 0,</font></b></div><div class=""><span style="font-family: Menlo;" class=""> let featuresDict = featuresArray.firstObject as? NSDictionary,</span></div><div class=""><span style="font-family: Menlo;" class=""> let coordinatesArray = featuresDict["geometry"],</span></div><div class=""><b class=""><font face="Menlo" class=""> coordinatesArray.count > 0,</font></b></div><div class=""><span style="font-family: Menlo;" class=""> let coordinateArray = coordinatesArray.firstObject as? NSArray,</span></div><div class=""><span style="font-family: Menlo;" class=""> </span><b style="font-family: Menlo;" class="">coordinateArray.count > 3</b></div><div class=""><span style="font-family: Menlo;" class=""> else { fatalError("Reason") }</span></div></div><div class=""><br class=""></div></div></blockquote><div><br></div><div>I have mixed thoughts about it. The former perl swchartzian-transform fan loves it, and the more responsible high speed trading systems developer finds it the<span style="background-color: rgba(255, 255, 255, 0);"> kind of look-it-works-without-the-hands code i am not typically eager to see people do in my teams. </span></div><div><span style="background-color: rgba(255, 255, 255, 0);">The apache spark project is a great example of a widely popular scala project that succeeded because it banned the kind of back-breaking areobatics that compilers can make sense of and normal (i.e not overly expensive and reasonably productive) developers typically get tangled in.</span></div><br><blockquote type="cite"><div><div class="">The motivation for this approach becomes more compelling when the Boolean tests are disjoint from binding or pattern matches.</div><div class=""><br class=""></div><div class=""><div class=""><font face="Menlo" class="">guard</font></div><div class=""><font face="Menlo" class=""> minimumShapeCount > 4,</font></div><div class=""><font face="Menlo" class=""> let shapes = decompose(map, minimum: minimumShapeCount),</font></div><div class=""><font face="Menlo" class=""> availableArea > minimumArea,</font></div><div class=""><font face="Menlo" class=""> let map = placeShapes(shapes, availableArea) else {</font></div><div class=""><font face="Menlo" class=""> fatalError()</font></div><div class=""><font face="Menlo" class="">}</font></div></div><div class=""><br class=""></div><div class="">would be allowed compared to current Swift which mandates where between the second and third tests:</div><div class=""><br class=""></div><div class=""><div class=""><font face="Menlo" class=""> let shapes = decompose(map, minimum: minimumShapeCount) where </font><span style="font-family: Menlo;" class="">availableArea > minimumArea,</span></div></div><div class=""><br class=""></div><div class="">In my vision, Swift would continue to allow where clauses and expand to allow disjoint Boolean entries.</div><div class=""><br class=""></div><div class="">Thoughts?</div><div class=""><br class=""></div><div class="">-- E </div><div class=""><br class=""></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>