In Swift, functions are considered first-class citizens, which means that they can be treated the same as any other type. They can be assigned to variables and be passed in and out of other functions. When treated this way, we call them closures. This is an extremely critical piece to write more declarative code because it allows us to treat functionalities like objects. Instead of thinking of functions as a collection of code to be executed, we can start to think about them more like a recipe to get something done. Just like you can give just about any recipe to a chef to cook, you can create types and methods that take a closure to perform some customizable behavior.

Closures as variables

Let's take a look at how closures work in Swift. ...

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