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How do you return a value from a closure that takes no parameters?

Paul Hudson    @twostraws   

Updated for Xcode 12.0

Closures in Swift have a distinct syntax that really separates them from simple functions, and one place that can cause confusion is how we accept and return parameters.

First, here’s a closure that accepts one parameter and returns nothing:

let payment = { (user: String) in
    print("Paying \(user)…")
}

Now here’s a closure that accepts one parameter and returns a Boolean:

let payment = { (user: String) -> Bool in
    print("Paying \(user)…")
    return true
}

If you want to return a value without accepting any parameters, you can’t just write -> Bool in – Swift won’t understand what you mean. Instead, you should use empty parentheses for your parameter list, like this:

let payment = { () -> Bool in
    print("Paying an anonymous person…")
    return true
}

If you think about it, that works just the same as a standard function where’d write func payment() -> Bool.

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