Swift arrays are bullshit.

arrays.swift

import Cocoa

let anArray = [1, 2, 3]

// Given `func foo(arr: Int[])`,
// how can it ensure the immutability of _arr_?

// 1. Won't work because `unshare` can only be called on vars.
//func foo(arr: Int[]) {
//    arr.unshare()
//}

// 2. Won't work because `arr` is a constant.
//func foo(arr: Int[]) {
//    arr = arr.copy()
//}

// 3. This isn't a solution because we don't want _arr_ to be mutable even internally.
//func foo(var arr: Int[]) {
//    arr.unshare()
//}

// 4. Solution: Ignore the argument.
func foo (arr: Int[]) {
    let constArr = arr.copy()
}

// The caller should also pass a copy of _anArray_
// if it's going to use it after the call to _foo_,
// since it can't tell what _foo_ will or won't do.
foo(anArray.copy())

// Two copies at every interface? So much for efficiency.