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iOS Swift Enums and Structs Enums Associated Values

Blaine Fallis
Blaine Fallis
2,449 Points

what's the colon doing? .Failure(let success):

here's the code in question:

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)
}

I get that downloadStatus is of type Status so you can shortcut that. What I'm unsure of is the syntax of (let failure): and also I need to review capitalization. Status is an enum type, downloadStatus a variable of type Status. what is failure with small f?

2 Answers

Chris Shaw
Chris Shaw
26,650 Points

Hi Blaine,

This can be a bit tricky since we learned that let checks for optionals and also defines a constant.

So what is it doing in the case of the enum and the switch?

Well it's assigning a value to a new variable which is something we've done many times in the past through previous Swift courses, just in this case it doesn't look right as we've never use it within a case statement before.

Let's break this down.

  1. You open your case statement and assign a value from the Status enum
  2. next you open your parenthesis (optional)
  3. now you use a constant assignment to get the String value assigned to the enum
  4. you now have access to that String value within the case statement
Still confusing?

It will become clearer over time, essentially the constant assignment can be called anything you want so in the case of failure we could call that wantWentWrong and print that out using our println statement.

Also as I said above enum values are optional so we can for instance remove (String) from our Success case and then just check for Success in our switch case, in doing this we would need to change our println statement to reference either a static string or a variable defined elsewhere but the overall code would work the same way.

enum Status {
    case Success
    case Failure(String)
}

let downloadStatus = Status.Success

switch downloadStatus {
case .Success:
    println("The download was successful")
case .Failure(let failure):
    println(failure)
}

Hope that helps, let me know if it's still confusing.

One more thing.

If we assign the type Status to our downloadStatus constant we can then remove the static reference to Status in the value assignment.

let downloadStatus: Status = .Success
Blaine Fallis
Blaine Fallis
2,449 Points

actually, I do read here that the colon is just built into the grammar of a switch statement: switch control expression { case pattern 1: statements case pattern 2 where condition: statements case pattern 3 where condition, pattern 4 where condition: statements default: statements }

but with all the new types of expressions I'm learning you start to wonder, is the colon strictly part of the enums and associated values? But actually, it's just part of the switch statement grammar, as expected. We just happen to be using it here as part of an enum associated value. So it helps to review the basic syntax to narrow down why statements look the way they do. I was just trying to get down to the colon and why it's there, and discovered it is built into a switch statement. simple

But the let statement is also a cool topic, and optionals.