[swift-evolution] [Pitch] making where and , interchangeable in guard conditions.

[Erica Sadun]

Or you can make sure every expression is boolean, which would take care of this as well...

-- E


> On May 24, 2016, at 12:47 PM, Jacob Bandes-Storch <jtbandes at gmail.com> wrote:
> 
> Could we just require "let" (or var) to introduce every binding, rather than allowing the combination "if let x = y, z = q, ..."? I always use "let" anyway; I think it's easier to read.
> 
> On Tue, May 24, 2016 at 11:42 AM, Erica Sadun via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> Okay, and here is where the problem is (thanks Chris L)
> 
> `z = q` is an expression. It returns Void.
> 
> For example:
> 
> let q = 5
> var z = 0
> let foo = z = q // foo inferred to have type `()` which may be unexpected
> 
> So if you have the following statement:
> 
> guard let x = optional, z = q else {...}
> 
> where q is non-optional, there's issues in that `q` is not an optional and `z = q` is an expression.
> 
> -- E
> 
> 
>> On May 24, 2016, at 11:59 AM, Austin Zheng <austinzheng at gmail.com <mailto:austinzheng at gmail.com>> wrote:
>> 
>> I like the idea in principle.
>> 
>> However, right now you can write something like:
>> 
>> if let a = optionalA, frob = fooBarBaz() { ... }
>> 
>> It's clear that both clauses are optional binding clauses.
>> 
>> With this change, it's not clear anymore whether the second clause is an optional binding clause, or a logic test erroneously using '=' instead of '=='.
>> 
>> To be fair, though, since assignment in Swift doesn't return the new value as it does in C, there is far less room for disastrous bugs caused by this sort of mistake.
>> 
>> Austin
>> 
>> 
>> 
>> On Fri, May 20, 2016 at 10:07 AM, Erica Sadun via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> Earlier on Swift Evolution:
>> 
>> Me: "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?"
>> 
>> Joe Groff: "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. There's no technical reason we couldn't accept either 'where' or ',' consistently."
>> 
>> 	guard x == 0,
>> 	  let y = optional where
>> 	  z == 2 {
>> 	}
>> 
>> Pitch: 
>> 
>> 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.
>> 
>> guard
>>     let fileContents = fileContents,
>>     let jsonDict = try NSJSONSerialization.JSONObjectWithData(fileContents, options: []) as? NSDictionary,
>>     let featuresArray = jsonDict["features"] as? NSArray where featuresArray.count > 0,
>>     let featuresDict = featuresArray[0] as? NSDictionary,
>>     let coordinatesArray = featuresDict["geometry"] where coordinatesArray.count > 0,
>>     let coordinateArray = coordinatesArray[0] as? NSArray where coordinateArray.count > 3
>>     else { fatalError("Reason") }
>> 
>> Each `where` test is a separate test. While there are semantic ties between the conditional binding and the count tests, there doesn't have to be. 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.
>> 
>> 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`:
>> 
>> guard
>>     let fileContents = fileContents,
>>     let jsonDict = try NSJSONSerialization.JSONObjectWithData(fileContents, options: []) as? NSDictionary,
>>     let featuresArray = jsonDict["features"] as? NSArray,
>>     featuresArray.count > 0,
>>     let featuresDict = featuresArray.firstObject as? NSDictionary,
>>     let coordinatesArray = featuresDict["geometry"],
>>     coordinatesArray.count > 0,
>>     let coordinateArray = coordinatesArray.firstObject as? NSArray,
>>     coordinateArray.count > 3
>>     else { fatalError("Reason") }
>> 
>> The motivation for this approach becomes more compelling when the Boolean tests are disjoint from binding or pattern matches.
>> 
>> guard
>>     minimumShapeCount > 4,
>>     let shapes = decompose(map, minimum: minimumShapeCount),
>>     availableArea > minimumArea,
>>     let map = placeShapes(shapes, availableArea) else {
>>         fatalError()
>> }
>> 
>> would be allowed compared to current Swift which mandates where between the second and third tests:
>> 
>>     let shapes = decompose(map, minimum: minimumShapeCount) where availableArea > minimumArea,
>> 
>> In my vision, Swift would continue to allow where clauses and expand to allow disjoint Boolean entries.
>> 
>> Thoughts?
>> 
>> -- E 
>> 
>> 
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>> 
> 
> 
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