Apple's Own Code In It's Documentation Has Errors?

I'm afraid you might be missing the fact that the part you are referring to is written in the context where this function is defined:

func greet(person: String) -> String {
    let greeting = "Hello, " + person + "!"
    return greeting
}

(See. Defining and Calling Functions)

Documentations (or books) of Apple may have some errors and you can send a bug report using the link Report Bugs shown below.

But you should better check if something is really an error before reporting it as error.

I does work perfectly.

Of course, you need to read completely the chapter, and insert

func greet(person: String) -> String {
    let greeting = "Hello, " + person + "!"
    return greeting
}

By the way, that is a good learning exercise to learn to analyze error messages and try to figure out why greet(person:) is not recognized.

Read the book to fully understand the signature concept.

A function is not defined by its name ; it is defined by the complete signature.

Hence

func greetAgain(person: String) -> String { }

is different from

func greetAgain(person: String, again: Bool) -> String { }

It is even different from (see the _ )

func greetAgain(_ person: String) -> String {
    return "Hello you, " + person + "!"
}

let name2: String = greetAgain("Tim")

will give

"Hello you, Tim!"

Changing the return type also defines another func; consider the following example

func greetAgain(person: String) -> String {
    return "Hello again, " + person + "!"
}

func greetAgain(person: String) -> Int {
    return 10
}

These are 2 different func.

When you call, need to tell what type you expect:

let x: Int = greetAgain(person: "Tim")
let name: String = greetAgain(person: "Tim")

if you just try

let z = greetAgain(person: "Tim")

you get an error

Ambiguous use of 'greetAgain(person:)'

Because compiler (or playground interpreter) cannot guess which to use

Also, how can the this block of code (or is this called a code snippet? What's the difference, by the way?) be fixed, if the arguments/input value, 'alreadyGreeted' is changed to 'false' in line 14?

What is your point ? Just try

print(greet(person: "Tim", alreadyGreeted: false))

and see the result.