Contents.swift

/*:
 # Hacking with Swift
 
 [Day 12 - Optionals](https://www.hackingwithswift.com/100/12)
 
 ## Handling missing data
 
 We’ve used types such as `Int` to hold values like `5`.
 But if you wanted to store an age property for users, what would you do if you didn’t know someone’s age?
 
 You might say “well, I’ll store 0”, but then you would get confused between new-born babies and people whose age you don’t know.
 You could use a special number such as `1000` or `-1` to represent “unknown”, both of which are impossible ages,
 but then would you really remember that number in all the places it’s used?
 
 Swift’s solution is called *optionals*, and you can make optionals out of any type.
 An optional integer might have a number like `0` or `40`,
 but it might have no value at all – it might literally be missing, which is nil in Swift.
 
 To make a type optional, add a question mark after it.
 For example, we can make an optional integer like this:
 */
var age: Int? = nil
//: That doesn’t hold any number – it holds nothing. But if we later learn that age, we can use it:
age = 38
/*:
 ## Unwrapping optionals
 
 Optional strings might contain a string like “Hello” or they might be nil – nothing at all.
 
 Consider this optional string:
 */
var name: String? = nil
/*:
 What happens if we use `name.count`? A real string has a count property that stores how many letters it has,
 but this is `nil` – it’s empty memory, not a string, so it doesn’t have a `count`.
 
 Because of this, trying to read `name.count` is unsafe and Swift won’t allow it.
 Instead, we must look inside the optional and see what’s there – a process known as *unwrapping*.
 
 A common way of unwrapping optionals is with `if let` syntax, which unwraps with a condition.
 If there was a value inside the optional then you can use it, but if there wasn’t the condition fails.
 
 For example:
 */
if let unwrapped = name {
    print("\(unwrapped.count) letters")
} else {
    print("Missing name.")
}
/*:
 If `name` holds a string, it will be put inside `unwrapped` as a regular `String` and we can read its `count` property inside the condition.
 Alternatively, if `name` is empty, the `else` code will be run.
 
 ## Unwrapping with guard
 
 An alternative to `if let` is `guard let`, which also unwraps optionals.
 `guard` let will unwrap an optional for you, but if it finds `nil` inside it expects you to exit the function, loop, or condition you used it in.
 
 However, the major difference between `if let` and `guard let` is that your unwrapped optional remains usable after the `guard` code.
 
 Let’s try it out with a `greet()` function. This will accept an optional string as its only parameter and try to unwrap it,
 but if there’s nothing inside it will print a message and exit.
 Because optionals unwrapped using `guard let` stay around after the `guard` finishes, we can print the unwrapped string at the end of the function:
 */
func greet(_ name: String?) {
    guard let unwrapped = name else {
        print("You didn't provide a name!")
        return
    }
    
    print("Hello, \(unwrapped)!")
}
/*:
 Using `guard let` lets you deal with problems at the start of your functions, then exit immediately.
 This means the rest of your function is the happy path – the path your code takes if everything is correct.
 
 ## Force unwrapping
 
 Optionals represent data that may or may not be there, but sometimes you know for *sure* that a value isn’t `nil`.
 In these cases, Swift lets you force unwrap the optional: convert it from an optional type to a non-optional type.
 
 For example, if you have a string that contains a number, you can convert it to an `Int` like this:
 */
let str = "5"
let num = Int(str)
/*:
 That makes `num` an optional `Int` because you might have tried to convert a string like “Fish” rather than “5”.
 
 Even though Swift isn’t sure the conversion will work,
 you can see the code is safe so you can force unwrap the result by writing `!` after `Int(str)`, like this:
 */
let numUnwrapped = Int(str)!
/*:
 Swift will immediately unwrap the optional and make `numUnwrapped` a regular `Int` rather than an `Int?`.
 But if you’re *wrong* – if `str` was something that couldn’t be converted to an integer – your code will crash.
 
 As a result, you should force unwrap only when you’re sure it’s safe – there’s a reason it’s often called the crash operator.
 
 ## Implicitly unwrapped optionals
 
 Like regular optionals, implicitly unwrapped optionals might contain a value or they might be `nil`.
 However, unlike regular optionals you don’t need to unwrap them in order to use them: you can use them as if they weren’t optional at all.
 
 Implicitly unwrapped optionals are created by adding an exclamation mark after your type name, like this:
 */
let ageUnwrapped: Int! = nil
/*:
 Because they behave as if they were already unwrapped, you don’t need `if let` or `guard let` to use implicitly unwrapped optionals.
 However, if you try to use them and they have no value – if they are `nil` – your code crashes.
 
 Implicitly unwrapped optionals exist because sometimes a variable will start life as `nil`,
 but will always have a value before you need to use it.
 Because you know they will have a value by the time you need them, it’s helpful not having to write if let all the time.
 
 That being said, if you’re able to use regular optionals instead it’s generally a good idea.
 
 ## Nil coalescing
 
 The nil coalescing operator unwraps an optional and returns the value inside if there is one.
 If there isn’t a value – if the optional was `nil` – then a default value is used instead.
 Either way, the result won’t be optional: it will either by the value from inside the optional or the default value used as a back up.
 
 Here’s a function that accepts an integer as its only parameter and returns an optional string:
 */
func username(for id: Int) -> String? {
    if id == 1 {
        return "Taylor Swift"
    } else {
        return nil
    }
}
/*:
 If we call that with ID 15 we’ll get back nil because the user isn’t recognized,
 but with nil coalescing we can provide a default value of “Anonymous” like this:
 */
let user = username(for: 15) ?? "Anonymous"
/*:
 That will check the result that comes back from the `username()` function:
 if it’s a string then it will be unwrapped and placed into user, but if it has nil inside then “Anonymous” will be used instead.
 
 ## Optional chaining
 
 Swift provides us with a shortcut when using optionals: if you want to access something like `a.b.c` and `b` is optional,
 you can write a question mark after it to enable optional chaining: `a.b?.c`.
 
 When that code is run,
 Swift will check whether `b` has a value, and if it’s `nil` the rest of the line will be ignored – Swift will return `nil` immediately.
 But if it has a value, it will be unwrapped and execution will continue.
 
 To try this out, here’s an array of names:
 */
let names = ["John", "Paul", "George", "Ringo"]
/*:
 We’re going to use the `first` property of that array, which will return the first name if there is one or `nil` if the array is empty.
 We can then call `uppercased()` on the result to make it an uppercase string:
 */
let beatle = names.first?.uppercased()
/*:
 That question mark is optional chaining – if `first` returns `nil` then Swift won’t try to uppercase it, and will set beatle to `nil` immediately.
 
 ## Optional try
 
 Back when we were talking about throwing functions, we looked at this code:
 */
enum PasswordError: Error {
    case obvious
}

func checkPassword(_ password: String) throws -> Bool {
    if password == "password" {
        throw PasswordError.obvious
    }
    
    return true
}

do {
    try checkPassword("password")
    print("That password is good!")
} catch {
    print("You can't use that password.")
}
/*:
 That runs a throwing function, using `do`, `try`, and `catch` to handle errors gracefully.
 
 There are two alternatives to `try`, both of which will make more sense now that you understand optionals and force unwrapping.
 
 The first is `try?`, and changes throwing functions into functions that return an optional.
 If the function throws an error you’ll be sent `nil` as the result, otherwise you’ll get the return value wrapped as an optional.
 
 Using `try?` we can run `checkPassword()` like this:
 */
if let result = try? checkPassword("password") {
    print("Result was \(result)")
} else {
    print("D'oh.")
}
/*:
 The other alternative is `try!`, which you can use when you know for sure that the function will not fail.
 If the function *does* throw an error, your code will crash.
 
 Using `try!` we can rewrite the code to this:
 */
try! checkPassword("sekrit")
print("OK!")
/*:
 ## Failable initializers
 
 When talking about force unwrapping, I used this code:
 */
let str2 = "5"
let num2 = Int(str)
/*:
 That converts a string to an integer, but because you might try to pass any string there what you actually get back is an optional integer.
 
 This is a failable initializer: an initializer that might work or might not.
 You can write these in your own structs and classes by using `init?()` rather than `init()`, and return `nil` if something goes wrong.
 The return value will then be an optional of your type, for you to unwrap however you want.
 
 As an example, we could code a `Person` struct that must be created using a nine-letter ID string.
 If anything other than a nine-letter string is used we’ll return `nil`, otherwise we’ll continue as normal.
 
 Here’s that in Swift:
 */
struct Person {
    var id: String
    
    init?(id: String) {
        if id.count == 9 {
            self.id = id
        } else {
            return nil
        }
    }
}
/*:
 ## Typecasting
 
 Swift must always know the type of each of your variables, but sometimes you know more information than Swift does.
 For example, here are three classes:
 */
class Animal { }
class Fish: Animal { }

class Dog: Animal {
    func makeNoise() {
        print("Woof!")
    }
}
//: We can create a couple of fish and a couple of dogs, and put them into an array, like this:
let pets = [Fish(), Dog(), Fish(), Dog()]
/*:
 Swift can see both `Fish` and `Dog` inherit from the `Animal` class, so it uses type inference to make pets an array of `Animal`.
 
 If we want to loop over the pets array and ask all the dogs to bark, we need to perform a typecast:
 Swift will check to see whether each pet is a `Dog` object, and if it is we can then call `makeNoise()`.
 
 This uses a new keyword called `as?`, which returns an optional:
 it will be `nil` if the typecast failed, or a converted type otherwise.
 
 Here’s how we write the loop in Swift:
 */
for pet in pets {
    if let dog = pet as? Dog {
        dog.makeNoise()
    }
}
/*:
 ## Summary
 
 You’ve made it to the end of the tenth part of this series, so let’s summarize:
 
 1. Optionals let us represent the absence of a value in a clear and unambiguous way.
 1. Swift won’t let us use optionals without unwrapping them, either using `if let` or using `guard let`.
 1. You can force unwrap optionals with an exclamation mark, but if you try to force unwrap `nil` your code will crash.
 1. Implicitly unwrapped optionals don’t have the safety checks of regular optionals.
 1. You can use nil coalescing, `??`, to unwrap an optional and provide a default value if there was nothing inside.
 1. Optional chaining lets us write code to manipulate an optional, but if the optional turns out to be empty the code is ignored.
 1. You can use `try?` to convert a throwing function into an optional return value, or `try!` to crash if an error is thrown.
 1. If you need your initializer to fail when it’s given bad input, use `init?()` to make a failable initializer.
 1. You can use typecasting, `as?`, to convert one type of object to another.
 */