Dispatch on a deep class hierarchy where exact match wins over parent handler.

← Design Patterns and Idioms · Ref: Q1123

Most specific handler wins via cost-based selection:

  render() as dispatcher
    -> shape as Shape
    <- rtn as String: "Shape handler"

  render()
    -> shape as Polygon
    <- rtn as String: "Polygon handler"

  render()
    -> shape as Square
    <- rtn as String: "Square handler"

Passing Quadrilateral selects Polygon handler (closest match). Passing Square selects Square handler (exact), not Polygon.

See Q1121 for sealed dispatch. See Q1122 for fallback handling.

Example

#!ek9
defines module qa.designpatterns.dispatcherdeep

  defines class

    Shape as abstract
      default operator ?

    Polygon is Shape as open
      default operator ?

    Quadrilateral is Polygon as open
      default operator ?

    Square is Quadrilateral
      default operator ?

    GeometryRenderer

      render() as dispatcher
        -> shape as Shape
        <- rtn as String: "Shape handler"

      render()
        -> shape as Polygon
        <- rtn as String: "Polygon handler"

      render()
        -> shape as Square
        <- rtn as String: "Square handler"

  defines program

    DispatcherDeepDemo()
      stdout <- Stdout()
      renderer <- GeometryRenderer()

      //Polygon -> Polygon handler
      stdout.println(renderer.render(Polygon()))

      //Quadrilateral -> Polygon handler (closest match)
      stdout.println(renderer.render(Quadrilateral()))

      //Square -> Square handler (exact match beats Polygon)
      stdout.println(renderer.render(Square()))

Common mistakes

E05030 — Every level of the hierarchy that is extended must be 'as open'. Square extends Quadrilateral, so a Quadrilateral that is not open is rejected with E05030. See ek9 -h E05030 for details.

Incorrect:

Quadrilateral is Polygon

Correct:

Quadrilateral is Polygon as open

Other ways to ask this

I have Shape -> Polygon -> Quadrilateral -> Square and need the most specific handler
In Java I'd check instanceof in order. Show how EK9 dispatcher picks the best match
Given a deep hierarchy, verify that a Square handler wins over a Polygon handler
Set up dispatcher resolution across multiple inheritance levels

Coming from another language?

Java: ordered instanceof checks (fragile). C#: pattern matching with type guards. EK9: dispatcher with automatic cost-based resolution — most specific handler wins.

Keywords: cost-based, deep, dispatcher, hierarchy, resolution, specific