Shorthand parameter names

We just made a travel() function. It accepts one parameter, which is a closure that itself accepts one parameter and returns a string. That closure is then run between two calls to print().

Here’s that in code:

func travel(action: (String) -> String) {
    print("I'm getting ready to go.")
    let description = action("London")
    print(description)
    print("I arrived!")
}

We can call travel() using something like this:

travel { (place: String) -> String in
    return "I'm going to \(place) in my car"
}

However, Swift knows the parameter to that closure must be a string, so we can remove it:

travel { place -> String in
    return "I'm going to \(place) in my car"
}

It also knows the closure must return a string, so we can remove that:

travel { place in
    return "I'm going to \(place) in my car"
}

As the closure only has one line of code that must be the one that returns the value, so Swift lets us remove the return keyword too:

travel { place in
    "I'm going to \(place) in my car"
}

Swift has a shorthand syntax that lets you go even shorter. Rather than writing place in we can let Swift provide automatic names for the closure’s parameters. These are named with a dollar sign, then a number counting from 0.

travel {
    "I'm going to \($0) in my car"
}