Function currying translates a single function with multiple arguments into a series of functions each with one argument. Let's examine an example. Suppose that we have a function that combines firstName and lastName to return the full name:

func extractFullUserName(firstName: String, lastName: String) -> String {
    return "(firstName) (lastName)"
}

This function can be translated into a curried function as follows:

func curriedExtractFullUserName(firstName: String)(lastName:
  String) -> String {
    return "(firstName) (lastName)"
}

As seen from this example, we replace the comma with ) ( parentheses.

So now we can use this function as follows:

let fnIncludingFirstName = curriedExtractFullUserName("John")
let extractedFullName = fnIncludingFirstName(lastName: "Doe")

Here, fnIncludingFirstName will have firstName in it so that, when we use it, we can provide lastName and extract the full name. We will use this technique in upcoming chapters.

Starting with Swift 2.2, Apple has deprecated function currying and removed it from Swift 3.0. It is proposed to convert the function currying to returning a closure explicitly:

// Before:
func curried(x: Int)(y: String) -> Float {
    return Float(x) + Float(y)!
}

// Swift 3.0 syntax:
func curried(x: Int) -> (String) -> Float {
    return {(y: String) -> Float in
        return Float(x) + Float(y)!
    }
}

Let's convert our curried function to return the closure version explicitly:

func explicityRetunClosure(firstName: String) -> (String) -> String {
    return { (lastName: String) -> String in
        return "(firstName) (lastName)"
    }
}

We can use this function as follows and the result is going to be identical:

let fnIncludingFirstName = explicityRetunClosure(firstName: "John")
let extractedFullName = fnIncludingFirstName("Doe")