Modified version of code from blog post https://medium.com/swift-programming/why-swift-guard-should-be-avoided-484cfc2603c5#.y8wcikeez

gistfile1.txt

//: Playground - noun: a place where people can play

import UIKit

//: Playground - noun: a place where people can play

import UIKit

struct Item {
    var price: Int
    var count: Int
}

enum VendingMachineError: ErrorType {
    case InvalidSelection
    case InsufficientFunds(coinsNeeded: Int)
    case OutOfStock
}

class VendingMachine {
    var inventory = [
        "Candy Bar": Item(price: 12, count: 7),
        "Chips": Item(price: 10, count: 4),
        "Pretzels": Item(price: 7, count: 11)
    ]
    
    var coinsDeposited = 0
    
    func dispense(snack: String) {
        print("Dispensing \(snack)")
    }
    
    func vend(itemNamed name: String) throws {
        let item = try validatedItemNamed(name)
        reduceDepositedCoinsBy(item.price)
        removeFromInventory(item, name: name)
        dispense(name)
    }
    
    private func validatedItemNamed(name: String) throws -> Item {
        let item = try itemNamed(name)
        try validate(item)
        return item
    }
    
    private func reduceDepositedCoinsBy(price: Int) {
        coinsDeposited -= price
    }
    
    private func removeFromInventory(var item: Item, name: String) {
        --item.count
        inventory[name] = item
    }
    
    private func itemNamed(name: String) throws -> Item {
        guard let item = inventory[name] else {
            throw VendingMachineError.InvalidSelection
        }
        return item
    }
    
    private func validate(item: Item) throws {
        try validateCount(item.count)
        try validatePrice(item.price)
    }
    
    private func validateCount(count: Int) throws {
        guard count > 0 else {
            throw VendingMachineError.OutOfStock
        }
    }
    
    private func validatePrice(price: Int) throws {
        guard coinsDeposited >= price else {
            throw VendingMachineError.InsufficientFunds(coinsNeeded: price - coinsDeposited)
        }
    }
}