Interfaces

An interface declares a structural contract — a set of fields a type must have to satisfy it. Any type with the right fields satisfies it automatically, with no explicit implements.

Declaring an interface

interface Drawable {
  x: f64
  y: f64
}

interface Sized extends Drawable {
  width:  f64
  height: f64
}

Structural, not nominal

A type need not be declared as a parent or implement anything explicitly. If it happens to have all the interface's fields with compatible types, it satisfies the interface.

Interfaces have one common consumer: reference fields (&T), which accept any value that structurally satisfies the interface. See References.