Working example of calbacks in swift

Callback.swift

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

import UIKit

func firstFunction(input: String, completion: (result: String) -> Void) {
    print("run first func")
    
    // Need to include parameter labels in function call if 
    // you also use the labels in the call back definition
    completion(result: "we finished!") 
}

firstFunction("commands") { (result: String) in
    print("got back: \(result)")
}