Error Handling

Error handling allows your program to safely respond to and recover from unexpected failures.

Representing Errors

In Swift, errors are represented using types that conform to the Error protocol. Enums are the most common and effective way to group related error conditions.

enum PrinterError: Error {
    case outOfPaper
    case noToner
    case onFire
}

Throwing Errors

Use the throws keyword in a function's declaration to indicate it can throw an error.

func send(job: Int, toPrinter printerName: String) throws -> String {
    if printerName == "Broken" {
        throw PrinterError.onFire
    }
    return "Job sent"
}

Handling Errors (do-catch)

Wrap your throwing functions inside a do block and use the try keyword to call them. You can then handle specific failures in the following catch blocks.

do {
    let response = try send(job: 10, toPrinter: "Broken")
    print(response)
} catch PrinterError.onFire {
    print("Call the fire department!")
} catch {
    print("Error: \(error)")
}

Optional Errors (try?)

Use try? to handle an error by converting it to an optional value. If an error is thrown, the result is nil.

let response = try? send(job: 10, toPrinter: "Broken")
// response is nil

Disabling Error Propagation (try!)

Use try! if you are absolutely certain a function will not throw an error at runtime. If it does throw, your app will immediately crash.

let response = try! send(job: 10, toPrinter: "Office")