Creating variables that call a function of the same name

Paul Hudson      @twostraws

From Swift 5.4 onwards it’s possible to create a local variable by calling a function of the same name. That might sound obscure, but it’s actually a problem we hit all the time.

For example, this creates a struct with a color(forRow:) method, which gets called and assigned to a local variable called color:

struct Table {
    let count = 10

    func color(forRow row: Int) -> String {
        if row.isMultiple(of: 2) {
            return "red"
        } else {
            return "black"
        }
    }

    func printRows() {
        for i in 0..<count {
            let color = color(forRow: i)
            print("Row \(i): \(color)")
        }
    }
}

let table = Table()
table.printRows()

That kind of usage is only allowed from Swift 5.4 and later. In earlier versions of Swift, it would create a circular reference because Swift couldn’t distinguish between the local color constant and the color(forRow:) method it was calling – you would have seen the error “Variable used within its own initial value”.

This usually resulted in us either using self.color(forRow: 1989) to make it clear we mean the method call, or just naming the local value something else such as colorForRow.

Fortunately Swift 5.4 resolves this and allows us to use the more natural naming.

This change also allows us to make local copies of properties and global variables. For example, we can take a copy of a username property that is also called username, like this:

struct User {
    let username = "Taylor"

    func suggestAlternativeUsername() -> String {
        var username = username
        username += String(Int.random(in: 1000...9999))
        return username
    }
}

let user = User()
user.suggestAlternativeUsername()

Because this also applies to global variables, that same code works just fine even without the struct in place:

let username = "Taytay"

func suggestAlternativeUsername() -> String {
    var username = username
    username += String(Int.random(in: 1000...9999))
    return username
}

suggestAlternativeUsername()

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Other changes in Swift 5.4…

• Improved implicit member syntax
• Multiple variadic parameters in functions
• Local functions now support overloading
• Result builders
• Property wrappers are now supported for local variables
• Packages can now declare executable targets

Download all Swift 5.4 changes as a playground Link to Swift 5.4 changes

Browse changes in all Swift versions