SWIFT: In the example below why must I place (let one) next to .Success for the code to work?

Question

// In the example below why must I place (let failure) next to .Success for the code to work? Why not (let = success) or (success)?
enum Status {
    case Success(String)
    case Failure(String)
}
let downloadStatus = Status.Failure("Network connection unavailable")
switch downloadStatus {
case .Success(let success):
    println(success)
case .Failure(let failure):
    println(failure)
}

Cauli Tomaz · Accepted Answer

When you created your enum you've decided to use a String to describe the Status even further than just Success or Failure
Swift
enum Status {
    case Success(String), Failure(String)
}

Because of that, you were obliged to declare your status with an accompanying String, a message that further describes your status:
Swift
let downloadStatus = Status.Failure("Network connection unavailable")

Following this idea, when you want to retrieve your status, you can retrieve your String. This is your '(let failure)' and '(let success)', but you could name it anything you like.
Swift
// prints "Network connection unavailable"
switch downloadStatus {
    case .Success(let success): println(success)
    case .Failure(let failure): println(failure)
}

But you are not obliged to retrieve the parameter. Take a look:
Swift
// prints "Failure"
switch downloadStatus {
    case .Success: println("Success")
    case .Failure: println("Failure")
}

Full playground code:
```Swift
enum Status {
    case Success(String), Failure(String)
}
let downloadStatus = Status.Failure("Network connection unavailable")
switch downloadStatus {
    case .Success(let success): println(success)
    case .Failure(let failure): println(failure)
}
switch downloadStatus {
    case .Success: println("Success")
    case .Failure: println("Failure")
}
```

Steven Andrew · Answer

Thanks !

Stone Preston · Answer

that is the syntax for accessing en enumeration member's associated value within a switch statement.
from the Swift eBook (https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Enumerations.html#//apple_ref/doc/uid/TP40014097-CH12-XID_225):
You extract each associated value as a constant (with the let prefix) or a variable (with the var prefix) for use within the switch case’s body
swift
case .QRCode(let productCode):
    println("QR code: \(productCode).")
}

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Steven Andrew

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SWIFT: In the example below why must I place (let one) next to .Success for the code to work?

3 Answers

Cauli Tomaz

Cauli Tomaz

Stone Preston

Stone Preston

Steven Andrew

Steven Andrew