COW Example

main.swift

//
//  main.swift
//  COW Example
//
//  Created by The Northstar✨ System on 2023-10-16.
//

import Foundation

struct Person {
    var name: String
    var birthdate: Date
    var favoriteNumber: Int

    mutating func newFavoriteNumber() {
        favoriteNumber = .random(in: 1 ... 1000)
    }
}


func main() {
    
    // `bate` allocated & initialized at A1 (memory address A in stack frame 1)
    var bate = Person(
        name: "Bate",
        birthdate: Date(timeIntervalSince1970: 665046000),
        favoriteNumber: 42)
    
    // `bate` is read, but not modified
    dump(bate, name: "bate")
    
    
    // `dax` is allocated at A1
    var dax = bate
    
    // `dax` is printed by reading the same value in-memory as `bate` at A1
    dump(dax, name: "dax")
    
    // B1 is allocated,
    // the value of `bate` is copied to it,
    // `dax` recontextualized to be at B1,
    // field `name` at B1 set to `"Dax"`
    dax.name = "Dax"
    
    // field `favoriteNumber` at B1 is set to `7`
    dax.favoriteNumber = 7
    
    
    // field `birthdate` at memory address A1 is set one second later
    bate.birthdate += 1
    
    
    // `bate` at A1 is printed
    dump(bate, name: "bate")
    
    // `dax` at B1 is printed
    dump(dax, name: "dax")
    
    
    // Value at A1 passed to the new stack frame
    anotherStackFrame(bate)
    
    
    // `bate` at A1 is printed
    dump(bate, name: "bate")
    
    // `dax` at B1 is printed
    dump(dax, name: "dax")
}


// `person` is immutable, so its value is that same value which was passed in (for this example, let's say A1)
func anotherStackFrame(_ person: Person) {
    
    // value at A1 is printed
    dump(person, name: "person", indent: 1)
    
    // `rando` is allocated at A1
    var rando = person
    
    // A2 is allocated,
    // the value of the `person` is copied to it,
    // `person` recontextualized to be at A2,
    // field `name` is set to `"Rando"`
    rando.name = "Rando"
    
    // The field `favoriteNumber` at A2 is changed to be a random number
    rando.newFavoriteNumber()
    
    // `rando` at A2 is printed
    dump(rando, name: "rando")
    
    // value at A1 is printed
    dump(person, name: "person")
    
    // Stack frame is popped,
    // `rando` is no longer a symbol,
    // A2 released for future use
    return
}



main()

/*
 ▿ bate:
   - name: "Bate"
   ▿ birthdate: 1991-01-28 07:00:00 +0000
   - favoriteNumber: 42
 ▿ dax:
   - name: "Bate"
   ▿ birthdate: 1991-01-28 07:00:00 +0000
   - favoriteNumber: 42
 ▿ bate:
   - name: "Bate"
   ▿ birthdate: 1991-01-28 07:00:01 +0000
   - favoriteNumber: 42
 ▿ dax:
   - name: "Dax"
   ▿ birthdate: 1991-01-28 07:00:00 +0000
   - favoriteNumber: 7
  ▿ person:
    - name: "Bate"
    ▿ birthdate: 1991-01-28 07:00:01 +0000
    - favoriteNumber: 42
 ▿ rando:
   - name: "Rando"
   ▿ birthdate: 1991-01-28 07:00:01 +0000
   - favoriteNumber: 171
 ▿ person:
   - name: "Bate"
   ▿ birthdate: 1991-01-28 07:00:01 +0000
   - favoriteNumber: 42
 ▿ bate:
   - name: "Bate"
   ▿ birthdate: 1991-01-28 07:00:01 +0000
   - favoriteNumber: 42
 ▿ dax:
   - name: "Dax"
   ▿ birthdate: 1991-01-28 07:00:00 +0000
   - favoriteNumber: 7
 */