[Pitch] Introducing do-let-catch for Cleaner Error Handling

[1-877-547-7272]

It's mentioned here, under "Motivation":

@Ben_Cohen

The introduction of this keyword also makes stand-alone do expressions more viable. These have two use cases:

1. To produce a value from both the success and failure paths of a do/catch block:

let foo: String = do {
    try bar()
} catch {
    "Error \(error)"
}

2. The ability to initialize a variable when this cannot easily be done with a single expression:

let icon: IconImage = do {
    let image = NSImage(
                    systemSymbolName: "something", 
                    accessibilityDescription: nil)!
    let preferredColor = NSColor(named: "AccentColor")!
    then IconImage(
            image, 
            isSymbol: true, 
            isBackgroundSupressed: true, 
            preferredColor: preferredColor.cgColor)
}

While the above can be composed as a single expression, declaring separate variables and then using them is much clearer.

In other cases, this cannot be done because an API is structured to require you first create a value, then mutate part of it:

let motionManager: CMMotionManager = {
    let manager = CMMotionManager()
    manager.deviceMotionUpdateInterval = 0.05
    return manager
}()

This immediately-executed closure pattern is commonly seen in Swift code. So much so that in some cases, users assume that even single expressions must be surrounded in a closure. do expressions would provide a clearer idiom for grouping these.

And here, under "Detailed Design"

@Ben_Cohen

Additionally do statements will become expressions, with rules matching those of if and switch expressions from SE-0380:

• They can be used to return vales from functions, to assign values to variables, and to declare variables.
• They will not be usable more generally as sub-expressions, arguments to functions etc
• Both the do branch, and each catch branch if present, must either be a single expression, or yield a value using then.
• Further if, switch, and do expressions may be nested inside the do or catch branches, and do expressions can be nested inside if and switch expressions.
• The do and any catch branches must all produce the same type, when type checked independently (see SE-0380 for justification of this).
• If a block either explicitly throws, or terminates the program (e.g. with fatalError), it does not need to produce a value and can have multiple statements before terminating.