<div dir="ltr">I agree `guard let y = optional where z == 2` reads quite strangely.<div><br><div>To me, though, it could make sense if variables bound in a guard statement are restricted to a boolean assertion only after a `where`, but unrelated boolean assertions should definitely not require it (i.e. `guard let y = optional where y == 2` does seem more sensible than `guard let y = optional, y == 2`, but `guard let y = optional where z == 2` is unpleasant.)<br></div><div class="gmail_extra"><br></div><div class="gmail_extra"><br><div class="gmail_quote">On Fri, May 20, 2016 at 12:07 PM, Erica Sadun via swift-evolution <span dir="ltr"><<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div><b>Earlier on Swift Evolution:</b></div><div><br></div><div>Me: "<i>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><br></div><div>Joe Groff: "<i>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>There's no technical reason we couldn't accept either 'where' or ',' consistently."</i></div><div><br></div><div><font face="Menlo"><span style="white-space:pre-wrap"> </span>guard x == 0,</font></div><div><font face="Menlo"><span style="white-space:pre-wrap"> </span> let y = optional where</font></div><div><font face="Menlo"><span style="white-space:pre-wrap"> </span> z == 2 {</font></div><div><font face="Menlo"><span style="white-space:pre-wrap"> </span>}</font></div><div><br></div><div><b>Pitch: </b></div><div><br></div><div>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><br></div><div><div><font face="Menlo">guard</font></div><div><font face="Menlo"> let fileContents = fileContents,</font></div><div><font face="Menlo"> let jsonDict = try NSJSONSerialization.JSONObjectWithData(fileContents, options: []) as? NSDictionary,</font></div><div><font face="Menlo"> let featuresArray = jsonDict["features"] as? NSArray <b>where </b></font><b><font face="Menlo">featuresArray.count > 0,</font></b></div><div><font face="Menlo"> let featuresDict = featuresArray[0] as? NSDictionary,</font></div><div><font face="Menlo"> let coordinatesArray = featuresDict["geometry"] <b>where </b></font><b><font face="Menlo">coordinatesArray.count > 0,</font></b></div><div><font face="Menlo"> let coordinateArray = coordinatesArray[0] as? NSArray <b>where </b></font><b style="font-family:Menlo">coordinateArray.count > 3</b></div><div><font face="Menlo"> else { fatalError("Reason") }</font></div></div><div><br></div><div>Each `where` test is a separate test. While there <i>are</i> semantic ties between the conditional binding and the count tests, there <i>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><br></div><div>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><br></div><div><span style="font-family:Menlo">guard</span></div><div><div><span style="font-family:Menlo"> let fileContents = fileContents,</span></div><div><span style="font-family:Menlo"> let jsonDict = try NSJSONSerialization.JSONObjectWithData(fileContents, options: []) as? NSDictionary,</span></div><div><span style="font-family:Menlo"> let featuresArray = jsonDict["features"] as? NSArray,</span></div><div><b><font face="Menlo"> featuresArray.count > 0,</font></b></div><div><span style="font-family:Menlo"> let featuresDict = featuresArray.firstObject as? NSDictionary,</span></div><div><span style="font-family:Menlo"> let coordinatesArray = featuresDict["geometry"],</span></div><div><b><font face="Menlo"> coordinatesArray.count > 0,</font></b></div><div><span style="font-family:Menlo"> let coordinateArray = coordinatesArray.firstObject as? NSArray,</span></div><div><span style="font-family:Menlo"> </span><b style="font-family:Menlo">coordinateArray.count > 3</b></div><div><span style="font-family:Menlo"> else { fatalError("Reason") }</span></div></div><div><br></div><div>The motivation for this approach becomes more compelling when the Boolean tests are disjoint from binding or pattern matches.</div><div><br></div><div><div><font face="Menlo">guard</font></div><div><font face="Menlo"> minimumShapeCount > 4,</font></div><div><font face="Menlo"> let shapes = decompose(map, minimum: minimumShapeCount),</font></div><div><font face="Menlo"> availableArea > minimumArea,</font></div><div><font face="Menlo"> let map = placeShapes(shapes, availableArea) else {</font></div><div><font face="Menlo"> fatalError()</font></div><div><font face="Menlo">}</font></div></div><div><br></div><div>would be allowed compared to current Swift which mandates where between the second and third tests:</div><div><br></div><div><div><font face="Menlo"> let shapes = decompose(map, minimum: minimumShapeCount) where </font><span style="font-family:Menlo">availableArea > minimumArea,</span></div></div><div><br></div><div>In my vision, Swift would continue to allow where clauses and expand to allow disjoint Boolean entries.</div><div><br></div><div>Thoughts?</div><span class="HOEnZb"><font color="#888888"><div><br></div><div>-- E </div><div><br></div></font></span></div><br>_______________________________________________<br>
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