bits.swift

enum Direction {
    case up, down, left, right
}

protocol PositionType {
    var x: Int { get }
    var y: Int { get }
}

func euclideanDistance(_ lhs: PositionType, _ rhs: PositionType) -> Double {
    return sqrt(pow(Double(rhs.x - lhs.x), 2) + pow(Double(rhs.y - lhs.y), 2))
}

let moves = [Direction.left, .up, .up, .up, .right, .right]

reference_semantics.swift

class MutatingPosition: CustomDebugStringConvertible {
    var x: Int
    var y: Int
    
    init(x: Int, y: Int) {
        self.x = x
        self.y = y
    }
    
    func move(direction: Direction) {
        switch direction {
        case .down: y += 1
        case .up: y -= 1
        case .left: x -= 1
        case .right: x += 1
        }
    }
    
    var debugDescription: String {
        return "Position(x: \(x), y: \(y))"
    }
}

let startingPosition_mutating = MutatingPosition(x: 5, y: 5)
//let startingPosition2 = startingPosition_mutating // This is a bug. Reference semantics.
let startingPosition2 = MutatingPosition(x: startingPosition_mutating.x, y: startingPosition_mutating.y)

moves.forEach {
    startingPosition_mutating.move(direction: $0)
}

// our destination is, unfortunately, also our "starting position"
lets destination2 = startingPosition_mutating

print("Euclidian Distance of mutating class:")
print(euclideanDistance(startingPosition2, destination2))

value_semantics.swift

struct Position: PositionType {
    let x: Int
    let y: Int
}

func navigate(_ direction: Direction) -> (Position) -> (Position) {
    return { position in
        switch direction {
        case .down: return Position(x: position.x, y: position.y + 1)
        case .up: return Position(x: position.x, y: position.y - 1)
        case .left: return Position(x: position.x - 1, y: position.y)
        case .right: return Position(x: position.x + 1, y: position.y)
        }
    }
}

let startingPosition = Position(x: 5, y: 5)
let destination = moves
    .map { navigate($0) }
    .reduce(startingPosition) { $1($0) }

print("Euclidian Distance of curried structs:")
print(euclideanDistance(startingPosition, destination))