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sealed

The sealed keyword marks a type as closed to extension: only a known set of subtypes (declared in the same scope/module) may inherit from it. This enables exhaustive pattern matching over a fixed family of types.

Syntax & Example​

sealed interface Shape {
    fn area(): f64;
}

struct Circle(radius: f64): Shape { /* ... */ }
struct Square(side: f64): Shape { /* ... */ }
// No further subtypes of Shape are permitted outside this set.

Use Cases​

  • Modeling a closed algebraic data type where all variants are known up front.
  • Enabling the compiler to verify exhaustive match arms over a sealed hierarchy.
  • Preventing third-party code from adding new subtypes to a domain type.