Use Completion Handlers in Xcode playgrounds

playground.swift

//: Playground - noun: a place where people can play

import UIKit
import XCPlayground // we import this so we can get the main XCPlayground "page"

//create instance of current playground page
let page = XCPlayground.XCPlaygroundPage

func myCoolFunctionWithCompletion(input: String, completion: (result: String) -> Void) {
    print("\(input) ... ")
    completion(result: "we finished!")
}

myCoolFunctionWithCompletion("start") {
    (result: String) in
    print("got back: \(result)")
    page.currentPage.finishExecution() // finish execution on main page
}