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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