Write a dispatcher over a sealed Shape hierarchy with Circle, Square, and Triangle.

← Design Patterns and Idioms · Ref: Q1121

Define sealed trait, then dispatcher method with overloads:

  describe() as dispatcher
    -> shape as Shape
    <- rtn as String: "Unknown"

  describe()
    -> shape as Circle
    <- rtn as String: "Circle"

Mark the base method 'as dispatcher'. The compiler generates dispatch logic -- no instanceof, no casting.

See Q1122 for dispatcher with fallback. See Q1123 for deep hierarchy.

Example

defines module qa.designpatterns.dispatchersealed

  defines trait

    Shape allow only Circle, Square, Triangle
      name() as abstract
        <- rtn as String?

  defines class

    Circle with trait of Shape
      override name()
        <- rtn as String: "Circle"

    Square with trait of Shape
      override name()
        <- rtn as String: "Square"

    Triangle with trait of Shape
      override name()
        <- rtn as String: "Triangle"

    ShapeProcessor

      describe() as dispatcher
        -> shape as Shape
        <- rtn as String: "Unknown"

      describe()
        -> shape as Circle
        <- rtn as String: "Handled Circle"

      describe()
        -> shape as Square
        <- rtn as String: "Handled Square"

      describe()
        -> shape as Triangle
        <- rtn as String: "Handled Triangle"

  defines program

    DispatcherSealedDemo()
      stdout <- Stdout()
      processor <- ShapeProcessor()

      stdout.println(processor.describe(Circle()))
      stdout.println(processor.describe(Square()))
      stdout.println(processor.describe(Triangle()))

Common mistakes

E01010 — EK9 has no instanceof. Use 'as dispatcher' to dispatch on runtime types.

Incorrect:

      if shape instanceof Circle

Correct:

      describe() as dispatcher
        -> shape as Shape

Other ways to ask this

I need to handle different shape types without instanceof or casting
In Java I'd use instanceof checks or visitor pattern. Write the EK9 dispatcher
Given a sealed trait with three permitted types, dispatch on each type at runtime
Replace a chain of type checks with EK9's dispatcher mechanism

Coming from another language?

Java: instanceof + cast chain or visitor pattern. Kotlin: sealed class + when. Rust: enum + match. EK9: sealed trait + dispatcher — compiler generates dispatch, no casting.

Keywords: dispatcher, trait, switch, sealed, pattern matching, type