Type Unions in Swift

Type unions are one of my favorite patterns to enforce stricter typing in a codebase. The basic idea is to allow a single type to represent a value that is one of a set of different, distinct types.

One of my frequent use cases is iteration over a collection of varied objects. A type union allows me to avoid complicated merging and ordering logic and use simple conditional logic within the iteration body. It’s also used commonly in APIs I interact with, where a single resource can produce different but similar things, or I want to reduce ambiguity between conditional fields.

Type unions can be a stronger pattern than inheritance. In class-based inheritance, the possible set of types is unbounded, while type unions have an explicit set known at compile time. In languages like TypeScript (and Swift, as we’ll see), sub-types of a type union can be unrelated and have distinct inheritance chains.

In other languages

Type unions are simple in weakly typed languages like JavaScript or Python1 since a variable can contain any value. Strongly typed languages take different approaches to support them.

TypeScript

TypeScript has first-class support with union types. Type narrowing is done with type predicates and custom functions.

interface Rabbit {
	name: string;
	eat(food: Carrot): void;
}
class Dog {
	name: string;
	pet() {}
}
interface Turtle {
	name: "turtle"
}

type Pet = Rabbit | Dog | Turtle

function whatIsIt(pet: Pet) {
	if (typeof pet == "string") {
		return "turtle"
	} else if (pet instanceof Dog) {
		pet.pet()
		return "dog"
	} else {
		return "rabbit"
	}
}

TypeScript is nice because it automatically casts the inspected variable’s type: within the conditional blocks, pet assumes the narrowed type. TypeScript automatically infers common attributes of the sub-types, allowing access to the common name property (pet.name).

Kotlin

Kotlin supports type unions with sealed classes. Type narrowing is done with when expressions.

sealed class Pet()
data class Rabbit(
	val name: String,
	val carrot: Double
) : Pet() {
	fun eat(food: Carrot) {}
}
class Dog(val name: String) : Pet() {
	fun pet() {}
}
object Turtle : Pet() {
	val name: String = "turtle"
}

fun whatIsIt(pet: Pet): String =
	when (pet) {
		is Rabbit -> "rabbit"
		is Dog -> {
			pet.pet()
			"dog"
		}
		is Turtle -> "turtle"
	}

Like TypeScript, variable types are automatically narrowed. Kotlin does not automatically infer common attributes, but they can be defined on the base sealed class, following standard inheritance patterns.

In Swift

The core construct for a type union in Swift is an enumeration with associated values. Type narrowing is done with case statements.

protocol Rabbit {
	var name: String { get }
	func eat(food: Carrot)
}
struct Dog {
	var name: String
	func pet() {}
}
typealias Turtle = String

enum Pet {
	case rabbit(Rabbit)
	case dog(Dog)
	case turtle(Turtle)
}

func whatIsIt(pet: Pet) -> String {
	switch pet {
	case .rabbit:
		return "rabbit"
	case .dog(let dog):
		dog.pet()
		return "dog"
	case .turtle:
		return "turtle"
	}
}

Notice that to get a variable of the narrowed type, a new variable is directly defined within the case expression (let dog).

A more concise form of type narrowing is also possible within an if statement2.

if case .rabbit(let rabbit) = pet {
	// ...
}

The main downside to Swift’s form of type unions is that the value is nested within another. It’s not possible to directly use attributes on a Pet variable. To work around that, you need to define accessors on the enum itself3.

enum Pet {
	case rabbit(Rabbit)
	case dog(Dog)
	case turtle(Turtle)
	
	var name: String {
		switch self {
		case .rabbit(let rabbit):
			return rabbit.name
		case .dog(let dog):
			return dog.name
		case .turtle:
			return turtle
		}
	}
}

func example(pet: Pet) {
	pet.name
}

If the different sub-types share a protocol, you can define a single accessor to access the protocol.

protocol Mammal {
	var furColor: String { get }
}

enum Pet {
	case rabbit(Rabbit)
	case dog(Dog)
	case turtle(Turtle)
	
	var mammal: Mammal? {
		switch self {
		case .rabbit(let mammal as Mammal), .dog(let mammal as Mammal):
			return mammal
		case .turtle:
			return nil
		}
	}
}

func example(pet: Pet) {
	pet.mammal?.furColor
}

Taking this further, if all sub-types share a protocol, you can extend the enum itself from that protocol and delegate to that accessor.

protocol Animal {
	func sleep()
}

enum Pet: Animal {
	case rabbit(Rabbit)
	case dog(Dog)
	case turtle(Turtle)
	
	private var animal: Animal {
		switch self {
		case .rabbit(let animal as Animal),
			 .dog(let animal as Animal),
			 .turtle(let animal as Animal):
			return animal
		}
	}
	
	func sleep() {
		animal.sleep()
	}
}

func example(pet: Pet) {
	pet.sleep()
}

I find type unions significantly simplify my code and prevent mistakes by strengthening my type semantics. I hope this helps you out as well.


  1. Unless you’re using Python 3.5 type hinting. ↩︎

  2. Here’s another good article about working with Swift enums ↩︎

  3. There’s some discussion on the Swift forums about proposed syntax changes to make it easier to access the associated value within an enum, so keep your eyes out for changes in this space. ↩︎