Learning Swift™ Programming

Functions as Types

In Swift, a function is a type. This means that it can be passed as arguments, stored in variables, and used in a variety of ways. Every function has an inherent type that is defined by its arguments and its return type. The basic syntax for expressing a function type looks like this:

 (parameterTypes) -> ReturnType

This is a funky little syntax, but you can use it as you would use any other type in Swift, which makes passing around self-contained blocks of functionality easy.

Let’s next look at a basic function and then break down its type. This function is named double and takes an int named num:

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func double(num: Int) -> Int {    return number * 2}

It also returns an int. To express this function ...