ArraySubscript.swift

import Foundation

extension Array {
  subscript(index: Index, default defaultValue: @autoclosure () -> Element) -> Element {
    
    if indices.contains(index) {
      //If the index is valid we have no problem returning the value
      return self[index]
    } else {
      //Otherwise we return the default which can't be nil
      return defaultValue()
    }
  }
  subscript(index: Index, default defaultValue: @autoclosure () -> Element?) -> Element? {
    
    if indices.contains(index) {
      //If the index is valid we have no problem returning the value
      return self[index]
    } else {
      //Otherwise we return the default which may be nil
      return defaultValue()
    }
  }
}