iOS Interview - Understanding and Implementing Phantom Types in Swift

April 09, 20244 min read#swift, #ios, #interview

Swift is a strong-typed programing language. The Swift compiler is doing its best effort to ensure the correctness of type usages in our apps. Phantom types are a powerful concept that can help the compiler to work apply more type restrictions to make our code more robust and type-safe. In this blog post, we’ll dive into what phantom types are, how they work, and explore a practical example of using them in your Swift projects.

What are Phantom Types?

Phantom types are a way to extend the type system in Swift to express additional constraints or properties that are not represented by the value itself. They allow you to encode additional type-level information that can be used by the compiler to enforce certain rules and catch potential errors at compile-time.

In Swift, a phantom type is a generic type parameter that is not used in the representation of the type. This means that the phantom type parameter does not affect the size, layout, or behavior of the type. Instead, it is used to carry additional type-level information that can be used by the compiler to enforce certain constraints or properties.

Practical Example

Let’s consider a practical example of using phantom types in Swift. Imagine you’re building an app that deals with various measurements, such as length, weight, or volume. You want to ensure that you don’t accidentally mix up different types of measurements, which could lead to serious bugs in your application.

Here’s how you can use phantom types to create a type-safe Measurement struct in Swift:

struct Measurement<Unit>: Comparable {
    let value: Double

    init(_ value: Double) {
        self.value = value

    static func < (lhs: Measurement<Unit>, rhs: Measurement<Unit>) -> Bool {
        lhs.value < rhs.value

// Length measurements
struct Meters {
    static let zero = Measurement<Meters>(0)

let fiveMeters = Measurement<Meters>(5.0)
let tenMeters = Measurement<Meters>(10.0)

// Weight measurements
struct Kilograms {
    static let zero = Measurement<Kilograms>(0)

let twoKilograms = Measurement<Kilograms>(2.0)
let fiveKilograms = Measurement<Kilograms>(5.0)

// Comparing measurements
print(fiveMeters > tenMeters) // false
print(twoKilograms < fiveKilograms) // true

// Mixing measurements will not compile
let mixedMeasurement = Measurement<Meters>(2.0) + Measurement<Kilograms>(3.0) // <-- compiler errors

In this example, we define a Measurement struct that takes a phantom type parameter Unit. The Unit type represents the type of measurement, such as Meters or Kilograms. The Measurement struct itself only stores a Double value, but the phantom type parameter allows us to create distinct types for different measurement units.


By using phantom types, we can ensure that you can only add, subtract, or compare measurements of the same unit. Trying to mix different measurement types will result in a compile-time error, preventing potential runtime errors and making your code more robust.

Benefits of Phantom Types

Phantom types in Swift offer several benefits:

  1. Type Safety: Phantom types help you create more type-safe code by encoding additional type-level information that the compiler can use to catch potential errors at compile-time.
  2. Clarity: Phantom types make your code more expressive and self-documenting, as the type system clearly communicates the intended use of your types.
  3. Flexibility: Phantom types can be used in a variety of scenarios, from representing measurements (as shown in the example) to modeling domain-specific concepts in your application.


Phantom types are a powerful feature in Swift that can help you write more robust, type-safe, and expressive code. By using phantom types, you can encode additional type-level information and leverage the compiler to catch potential errors early in the development process.

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