Multiple Dispatch

“Multiple dispatch” is one of the most powerful and elegant features of Dylan. As explained in the section on generic functions and objects, Dylan methods are declared separately from the classes upon which they act. Polymorphism, the specialization of methods for use with particular classes, can be implemented by declaring several methods with different parameters and attaching them to one generic function:

define generic inspect-vehicle (v :: <vehicle>, i :: <inspector>) => ();

define method inspect-vehicle (v :: <vehicle>, i :: <inspector>) => ()

define method inspect-vehicle (car :: <car>, i :: <inspector>) => ()
  next-method();  // perform vehicle inspection

define method inspect-vehicle (truck :: <truck>, i :: <inspector>) => ()
  next-method();  // perform vehicle inspection

However, different types of vehicle inspectors may have different policies. A state inspector, in addition to the usual procedures, will also typically check a car’s insurance policy. To implement this, add another method to the generic function inspect-vehicle:

define method inspect-vehicle (car :: <car>, i :: <state-inspector>) => ()
  next-method();  // perform car inspection

let inspector = make(<state-inspector>);
let car = make(<car>);
inspect-vehicle(car, inspector);

Calling the generic function inspect-vehicle with these arguments performs three separate tasks: look-for-rust, check-seat-belts and check-insurance. The most specific method on inspect-vehicle – the one for the classes <car> and <state-inspector> – is invoked first and calls next-method to invoke the less-specific methods in turn.

For an exact definition of “specific”, see method dispatch in the DRM.

Dispatching on Specific Objects

Dylan also allows functions to dispatch on specific objects. For example, state inspectors might pass the governor’s car without actually looking at it. Dylan expresses this situation using singletons, objects which are treated as though they were in a class of their own. For example:

define constant $governors-car = make(<car>);

define method inspect-vehicle
    (car == $governors-car, i :: <state-inspector>) => ()

The car parameter is specialized via == $governors-car, meaning “the object identical to $governors-car”. (In this example, none of the usual inspection methods are invoked since the above code doesn’t call next-method.)