Ⅰ.What is () -> Void in Swift?

It represents a closure that takes no parameters and returns no value.
Let’s break it down:

Part Meaning
() No input parameters
-> Indicates a return type
Void No return value (Void means nothing)

Ⅱ.In plain language:

() -> Void means:

“A block of code that takes nothing in, returns nothing out — it just runs.”

Example:

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let sayHello: () -> Void = {
    print("Hello!")
}

sayHello()  // Output: Hello!

This sayHello is a closure of type () -> Void:

  • It takes no arguments
  • Returns nothing
  • Just performs a side effect when called (printing text)

Ⅲ.Bonus: Void is just ()

In Swift:

  • Void is just a typealias for an empty tuple ()
  • So these two are equivalent:
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() -> Void
() -> ()

Both mean “takes nothing, returns nothing.”

Ⅳ.Comparing Different Closure Types

Closure Type Meaning Example
() -> Void No parameters, no return print(“Hi”) }
(Int) -> Void Takes an Int, returns nothing { num in print(num) }
(String) -> Int Takes a String, returns Int str in str.count }
(Int, Int) -> Int Takes two Int, returns Int { a, b in a + b }