named_arguments.swift

// mandatory argument names for caller
func add(augend: Int, addend: Int) -> Int {
  return augend + addend
}
let sum = add(augend: 5, addend: 2)

// no argument names for caller (underscore for no name)
func add(_ augend: Int, _ addend: Int) -> Int {
  return augend + addend
}
let sum = add(5, 2)

// Two names for an argument. 1. For caller 2. For function definition
// Note: and, addend for 2nd parameter in below function
// `and` will be used by caller, `addend` will used by function definition
func add(_ augend: Int, and addend: Int) -> Int {
  return augend + addend
}
let sum = add(5, and: 2) // for readability, we can have one name at calling place 
                         // and one name at function definition