1 May 2024

Extension Types in Dart 3.3

Dart 3.3 introduced extension types — a way to wrap an existing type and give it a different interface, with zero runtime overhead.

Why they matter

Before extension types, a common pattern was:

typedef UserId = String;
typedef ProductId = String;

void fetchUser(UserId id) { ... }

// Nothing stops this mistake:
fetchUser(productId); // compiles fine

Both UserId and ProductId are just String at runtime. The compiler can’t help.

Extension types to the rescue

extension type UserId(String value) {}
extension type ProductId(String value) {}

void fetchUser(UserId id) { ... }

fetchUser(ProductId('123')); // compile error!

Now the compiler enforces the distinction. At runtime, it’s still just a String — no wrapper object, no allocation, no overhead.

Implementing interfaces

extension type Celsius(double value) implements double {
  Celsius operator +(Celsius other) => Celsius(value + other.value);
  
  Fahrenheit toFahrenheit() => Fahrenheit(value * 9 / 5 + 32);
}

extension type Fahrenheit(double value) implements double {}

When to use them

  • Domain identifiers (UserId, OrderId, Email)
  • Unit types (Meters, Kilograms, Seconds)
  • Restricted views of existing types (expose only a subset of List methods)

Extension types are a lightweight alternative to full wrapper classes when you only need type safety, not behavioural extension.

© Ildeberto Silva.