Swift 2 Error Handling Demo

README.md

Swift 2 Error Handling Demo

This small example is a little CLI tool to demonstrate how you can make use of the new error handling of Swift 2. It was part of my talk "Swift 2 - Error Handling".

Build & Run

1. Clone the gist.
2. Compile by xcrun -sdk macosx swiftc main.swift.
3. Run the executable and pass the amount as first argument. (./main AMOUNT)

main.swift

//
//  main.swift
//  WithdrawTool
//
// The MIT License (MIT)
// 
// Copyright (c) 2015 Marius Rackwitz
// 
// Permission is hereby granted, free of charge, to any person obtaining a copy of
// this software and associated documentation files (the "Software"), to deal in
// the Software without restriction, including without limitation the rights to
// use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
// the Software, and to permit persons to whom the Software is furnished to do so,
// subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
// FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
// COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
// IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
// CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
//

import Foundation

public struct Account {
    public enum Error : ErrorType {
        case TransactionExceedsFunds
        case NonPositiveTransactionNotAllowed(amount: Int)
    }
    
    var fund: Int
    
    public mutating func withdraw(amount: Int) throws {
        guard amount < fund else {
            throw Error.TransactionExceedsFunds
        }
        guard amount > 0 else {
            throw Error.NonPositiveTransactionNotAllowed(amount: amount)
        }
        fund -= amount
    }
}

enum Error : ErrorType {
    case NoArgumentGiven
    case ArgumentIsNotAnInteger
}

var arguments = Process.arguments

// Remove the executable name.
assert(arguments.count >= 1)
arguments.removeAtIndex(0)

guard let firstArg = arguments.first else {
    throw Error.NoArgumentGiven
}
guard let amount = Int(firstArg) else {
    throw Error.ArgumentIsNotAnInteger
}

let dict = NSMutableDictionary(contentsOfFile: "account.plist") ?? NSMutableDictionary()
let fund = dict["fund"] as? Int ?? 100

var account = Account(fund: fund)

do {
    try account.withdraw(amount)
    dict["fund"] = account.fund
    dict.writeToFile("account.plist", atomically: true)
} catch Account.Error.NonPositiveTransactionNotAllowed(let amount) {
    print("Amount (\(amount)) must be positive.")
} catch Account.Error.TransactionExceedsFunds {
    print("Amount (\(amount)) exceeds account balance (\(account.fund)).")
} catch {
    print("An unexpected error <\(error)> occured.")
}