Dynamic dispatch selects a multimethod implementation from runtime argument information, not only static types. The dispatch function can emit values such as numbers or Booleans, enabling polymorphism without objects. section 13.0 section 13.4
A dynamic multimethod separates selection from implementation:
- The dispatch function receives and validates the arguments. section 13.6
- It emits a composite value based on runtime data. section 13.4
- Methods implement each emitted value and remain extensible independently of initialization. section 13.6
Greeting animals by type and name length
The dysGreet dispatch function returns [animal.type, animal.name.length > 5]. Separate dog, cat, and cow methods then produce different greetings for long and short names; a cow named Clarabelle is greeted with “Moo! Call me Clarabelle.” section 13.4 section 13.6
Listing 13.18 A multimethod using a dispatch function with dynamic dispatch
function dysGreetDispatch(animal) {
if(dev()) {
if(!ajv.validate(animalSchema, animal)) {
var errors = ajv.errorsText(ajv.errors);
throw ("dysGreet called with invalid arguments: " + errors);
}
}
var hasLongName = animal.name.length > 5;
return [animal.type, hasLongName];
};
var dysGreet = multi(dysGreetDispatch);
Inspect the JavaScript dispatch function that validates an animal and emits the type-and-name-length dispatch value. section 13.4
