Swift Functional Programming - Second Edition

Returning values from functions

Swift functions can return a value, tuple, closure, or another function. The ability of a function to return a closure or function is an essential concept for FP as it empowers us to compose with functions. We will get into the use of it when we talk about first-class and higher-order functions.

Syntax-wise, we can specify that a function returns by providing ReturnType after ->. For instance, the following example returns String:

func functionName() -> String { } 

Any function that has ReturnType in its definition should have a return keyword with the matching type in its body.

return types can be optionals in Swift, so the function becomes as follows for our previous example:

 func functionName() -> String? ...