What is the syntax for a closure argument in swift

Question

In Swift headers, the isSeparator: argument accepts a closure

public func split(maxSplit: Int = default, allowEmptySlices: Bool = default, @noescape isSeparator: (Self.Generator.Element) throws -> Bool) rethrows -> [Self.SubSequence]

But in the documentation, it lists closure syntax differently

{ (parameters) -> return type in statements }

How are you supposed to know that (Self.Generator.Element) throws -> Bool rethrows refers to a closure / requires a closure? Are there other ways that the headers/docs might list argument as meaning a closure?

In a header (parameters) -> return type within the parentheses of a function is always a closure. — vadian
– vadian, Commented Jan 7, 2016 at 22:36

luk2302 · Accepted Answer · 2016-01-07 23:00:27Z

The "thing" giving away that this is a closure is the ->. The full type is

(Self.Generator.Element) throws -> Bool

It means that the closure takes a variable of type Self.Generator.Element and has to return a Bool upon some calculation based on the input. It may additionally throw some error while doing so - that is what the throws is for.

What you then write

{ (parameters) -> return type in statements }

would be an actual implementation, a value of some generic closure type.

The type of a closure is for example (someInt:Int, someDouble:Double) -> String:

var a : ((someInt:Int, someDouble:Double) -> String)

Once again the thing giving away that a is actually a closure is the -> in the type declaration.

Then you assign something to a via some code snippet following your second code block:

a = { (integer, floating) -> String in return "\(integer) \(floating)" }

Aaron Rasmussen · Accepted Answer · 2016-01-07 22:42:55Z

You can tell by the argument's type. Everything in Swift has a type, including functions and closures.

For example, this function...

func add(a: Int, to b: Int) -> Int { return a + b }

...has type (Int, Int) -> Int. (It takes two Ints as parameters, and returns an Int.)

And this closure...

let identity: Int -> Int = { $0 }

...has type Int -> Int.

Every function and closure has a type, and in the type signature there is always a -> that separates the parameters from the return value. So anytime you see a parameter (like isSeparator) that has a -> in it, you know that the parameter expects a closure.

aimak · Accepted Answer · 2016-01-07 22:39:39Z

the isSeparator definition means (Self.Generator.Element) throws -> Bool that you will be given an Element and you should return a Bool. When you will call split, you then can do the following :

 [1,2,3].split(…, isSeparator : { element -> Bool in return false })

This is a pure silly example but that illustrates the second part of your question

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