How do I avoid dispatcher handler ambiguity with multiple traits?

← Dispatcher Validation · Ref: Q858

When a type implements two traits and handlers exist for both traits at equal cost, the dispatcher cannot choose. Add a specific handler for the ambiguous type.

THIS EXAMPLE

Duck implements Flyable. The processor has a handler for Dog (concrete). Replacing the Dog handler with a Flyable handler would create ambiguity for Duck if Swimmable were also handled.

See Q616 for dispatcher ambiguity. See Q621 for dispatch rules.

Example

defines module qa.dispatcher.no.ambiguity

  defines trait

    Flyable
      fly()
        Stdout().println("Flying")

    Swimmable
      swim()
        Stdout().println("Swimming")

  defines class

    Animal as abstract
      speak() as abstract
      default operator ?

    Dog is Animal
      override speak()
        Stdout().println("Woof")

    Duck is Animal with trait of Flyable, Swimmable
      override speak()
        Stdout().println("Quack")
      override operator ? as pure
        <- rtn as Boolean: true

    AnimalProcessor

      process() as dispatcher
        -> animal as Animal
        animal.speak()

      process()
        -> animal as Dog
        animal.speak()

      default operator ?

  defines program

    DispatcherDemo()
      processor <- AnimalProcessor()
      dog as Animal: Dog()
      if processor? and dog?
        processor.process(dog)

Other ways to ask this

What triggers E05230 DISPATCHER_HANDLER_AMBIGUITY?
Why is Duck ambiguous when it implements two traits?
How do I resolve dispatcher ambiguity in EK9?

Coming from another language?

Java: method overloading is compile-time. C#: dynamic dispatch at runtime. Kotlin: sealed when is exhaustive. EK9: compile-time ambiguity detection for all reachable runtime types.

Keywords: E05230, dispatcher, trait, ambiguity, multiple, handler