If you've coded Swift for a short while, you may have experienced them. They manifest in a few different but ultimately it's just functions as types and values, and it's pretty cool.

A short code sample would be something like:

var closure: () -> ()

This is a very simple uninitiated closure, which would look like this as a function:

func myFunc() 

It doesn't take a parameter, and doesn't return one either. There are quite a few great ways to use closures, the one that come to most peoples minds is the following:

func someFunctionThatTakesAClosure(closure: (String) -> (Int)) {
	print(closure("Hello World")) // prints whatever number the closure returns
}

someFunctionThatTakesAClosure { string in
	print(string) // this prints "Hello World"
	return string.count // This will return 11
}

func printAndCount(string: String) -> Int {
	print(string)
	return string.count
}

someFunctionThatTakesAClosure(closure: printAndCount) // you can also pass in functions as long as it matches the closure, e.g. both `closure` and `printAndReturn` takes a String and return and Int

Notice the signature of closure, it matches a func almost as if it's an func closure(_:String)->String. This is the signature that someFunctionThatTakesAClosure expects, and you can pass in anything as long as it matches. the { string in } captures the parameter, and handles it as the constant string, like if we had the signature (string: String).

A different way to handle functions as types would be in types. We can have the type:

struct Button {
	private(set) var action: () -> ()
	init(action: @escaping () -> ()) {
		self.action = action
	}
}

Note the @escaping in the initialiser, this is because we're saving it so it sticks around after the function returns, which is what is called "escaping" in Swift

This gives us a type Button where each instance has it's own behavior on action(). The private(set) makes it so that we can't accidentally change a given instance's action property. This follow the same rules as with myFunc, so I can actually instanciate the button and use th property like so

let printOne = Button { print(1) }
printOne.action() // prints 1

And if you've played with Vapor for a bit, you might've noticed something. In Vapor we have the function for making get routes look as following

func get<Response>(_ path: PathComponent..., use closure: @escaping (Request) throws -> Response) -> Route where Response : ResponseEncodable

There's a lot in this function to unwrap. Let's start from an end, the <Response> makes a generic type, which'll be used later. the use closure: @escaping (Request) throws -> Response makes it so that the function can be called as: get("index", use: myRoute). This function actually saves the closure to a big array behind the scenes, that gets evaluated on route calls, hence the @escape. The return type of the passed in function has to be Response and conform to ResponseEncodable as seen by the where Response : ResponseEncodable.

So in short, closures is a way to handle functions (and function signatures) as types, and thus pass functions around as you would instances of types. They're powerful, even not used in an asynchronous matter. And if you've used .map you've used closures before.