Pattern Matching, Part 3: Custom pattern matching & syntactic sugar




In parts 1 and 2 of this article series, we saw some usages of switch on a lot of things, including tuples, Range, String, Character and even type. But what if we can use pattern matching even with our own custom types?

This post is part of an article series. You can read all the parts here: part 1, part 2, part 3, part 4

Switch and the pattern matching operator

When you write case 1900..<2000 for example in a switch, how does Swift know how to compare that Range with the single value you’re switch-ing onto?

Well the answer is simple: Swift uses the ~= operator. You can use a Range<I> in a case when switching over a I (e.g. an Int) simply because the ~= operator is declared between a Range<I> and a I:

func ~=<I : ForwardIndexType where I : Comparable>(pattern: Range<I>, value: I) -> Bool

And in fact when you write switch someI with a case aRangeOfI then Swift will try to match it by calling aRangeOfI ~= someI (which returns a Bool telling if it matched).

This means that you can actually define the same operator ~= on your own custom types to make them usable in a switch/case statement, the same way you can use Range!

Making your types respond to pattern matching

So let’s do that with a custom struct:

struct Affine {
  var a: Int
  var b: Int
}

func ~= (lhs: Affine, rhs: Int) -> Bool {
  return rhs % lhs.a == lhs.b
}

switch 5 {
case Affine(a: 2, b: 0): print("Even number")
case Affine(a: 3, b: 1): print("3x+1")
case Affine(a: 3, b: 2): print("3x+2")
default: print("Other")
}

And this works and prints 3x+2!

Note however that Swift can’t know if the switch is exhaustive with custom pattern matching. For example, even if we add a case Affine(a: 2, b: 1) and a case Affine(a: 2, b: -1) which would cover every positive and negative integer case, Swift wouldn’t know and would still force us to use a default: statement.

Also, don’t mix up the parameters order: the first parameter of the infix ~= operator (commonly named lhs, for left-hand side) is the object you’re gonna use in your case statements. The second parameter (commonly named rhs for right-hand side) is the object you’re switch-ing over.

Other uses of ~=

There can be plenty of other uses of ~=

For example, we can add support for pattern matching on our Book struct from part 2 article:

func ~= (lhs: Range<Int>, rhs: Book) -> Bool {
  return lhs ~= rhs.year
}

Now let’s test it:

let aBook = Book(title: "20,000 leagues under the sea", author: "Jules Vernes", year: 1870)
switch aBook {
case 1800..<1900: print("19th century book")
case 1900..<2000: print("20th century book")
default: print("Other century")
}

Of course, I discourage this kind of usage, as comparing a book directly with a range of integers does not make it obvious that it compares the book’s year. Better switch over the aBook.year directly. But that’s just to show the power of the ~= operator (and because I didn’t have a better example in mind 🙃).

Another usage example of ~= could be to check if a String is “close enough” to another one. For example, if you’re creating a quizz game and expect the player to type the answer from the keyboard but want to be case insensitive, diacritics insensitive, and even tolerate small typos, you could imagine this usage:

struct Answer {
  let text: String
  let compareOptions: NSStringCompareOptions = [.CaseInsensitiveSearch, .DiacriticInsensitiveSearch, .WidthInsensitiveSearch]
}

func ~= (lhs: Answer, rhs: String) -> Bool {
  return lhs.text.compare(rhs, options: lhs.compareOptions, range: nil, locale: nil) == NSComparisonResult.OrderedSame
}

let question = "What's the French word for a face-to-face meeting?"
let userAnswer = "Tete a Tete"

switch userAnswer {
case Answer(text: "tête-à-tête"): print("Good answer!")
case Answer(text: "tête à tête"): print("Almost… don't forget dashes!")
default: print("Sorry, wrong answer!")
}
// prints "Almost… don't forget dashes!"

See how the comparison uses a case-sensitive, diacritics-insensitive and width-insensitive comparison to be lenient about the answer?

Syntactic sugar on Optionals

But the syntactic sugar of switch and pattern matching doesn’t stop at the transparent use of ~= by the switch/case statement.

Another useful syntactic sugar to know when dealing with switch is simply x?. You recognize with the question mark the relation with Optionals of course.

In this specific context, using x? is syntactic sugar for .Some(x). This means that you can write stuff like this:

let anOptional: Int? = 2
switch anOptional {
case 0?: print("Zero")
case 1?: print("One")
case 2?: print("Two")
case nil: print("None")
default: print("Other")
}

In fact, if you don’t use ? but write case 2: instead of case 2?: then the compiler will complain with an error like: expression pattern of type 'Int' cannot match values of type 'Int?' because it would be trying to match an Int? (anOptional) with an Int (2).

But using case 2?: is the exact equivalent of writing case Optional.Some(2), which produces an Int? containing 2, which can be matched against another Int? like anOptional. case 2?: is just a more compact form of .Some(2).

Switch on enums from rawValue

Talking about this, I recently stumbled upon some code which used enum (with an Int rawValue) to organize a UITableView used as a Menu. This is a good idea to manipulate enum MenuItem instead of the indexPath.row, right?

enum MenuItem: Int {
  case home
  case account
  case settings
}

But then to implement each tableView row based in the MenuItem, the code was looking like this:

switch indexPath.row {
case MenuItem.home.rawValue: 
case MenuItem.account.rawValue: 
case MenuItem.settings.rawValue: 
default: ()

First of all, notice how the switch is done on an Int (indexPath.row) and then each case uses a rawValue. This is wrong for multiple reasons.

  • the first being that nothing prevents you to use any other value, like a copy/pasting could make you write case FooBar.baz.rawValue and the compiler won’t even complain. But you’re dealing with MenuItems, so you should leverage the compiler to ensure you only deal with MenuItems, right?
  • the other problem is that this switch is not exhaustive by itself, this is why the default statement was necessary. I strongly recommand you to not use default when possible, and instead make your switch exhaustive, this way if you happen to add a new value to your enum you’ll be forced to think about what to do with it instead of it being ignored or eaten up by the default without you realizing.

So instead of switching on indexPath and case ….rawValue, you should rather build the enum from the rawValue first. This way you can then only switch over cases that use MenuItem enum cases, not anything else like FooBar.baz or whatnot.

And to do that, because MenuItem(rawValue:) is a failable initializer and will in fact return a MenuItem?, you can leverage the syntactic sugar we discovered above!

switch MenuItem(rawValue: indexPath.row) {
case .home?: 
case .account?: 
case .settings?: 
case nil: fatalError("Invalid indexPath!")
}

Well to be honest, I rather prefer using a guard let for that kind of stuff, as I find it way more readable than using ? in every case:

guard let menuItem = MenuItem(rawValue: indexPath.row) else { fatalError("Invalid indexPath!") }
switch menuItem {
case .home: 
case .account: 
case .settings: 
}

But hey, it’s good to know all the possible alternatives!

Conclusion

That’s it for today. Next (and probably last) part of this article series will talk about using pattern matching in contexts other than switch, especially if, guard, but also for loops, and using these features in a whole new level. Can’t wait!

⏩ Read last part of this article series here: part 4

Thanks to Frank Manno for updating the code samples of this article to Swift 3!