SE701:March 14

Generic functions and multiple dispatch

Java has "single dispatch"

class DispatchDemo {

  static class A {
    void foo( B b ) {
      System.out.println( "foo called on A B" );
    }

    void foo( SubB b ) {
      System.out.println( "foo called on A SUB-B" );
    }
    
  }

  static class SubA extends A {
    void foo( B b ) {
      System.out.println( "foo called on SUB-A B" );
    }
    
    void foo( SubB b ) {
      System.out.println( "foo called on SUB-A SUB-B" );
    }
  }
  
  static class B { }
  static class SubB extends B { }

  static public void main( String[] args ) {
    A just_a = new A( );
    B just_b = new B( );
    A sub_a = new SubA( );
    B sub_b = new SubB( );

    just_a.foo( just_b );
    just_a.foo( sub_b );

    sub_a.foo( just_b );
    sub_a.foo( sub_b );
  }

}

CommonLisp has multiple dispatch

(defclass a () ())
(defclass sub-a (a) ())

(defclass b () ())
(defclass sub-b (b) ())

(defgeneric foo (a b))

(defmethod foo ((x a) (y b))
  (format nil "foo called on A and B~%"))

(defmethod foo ((x sub-a) (y b))
  (format nil "foo called on SUB-A and B~%"))

(defmethod foo ((x a) (y sub-b))
  (format nil "foo called on A and SUB-B~%"))

(defmethod foo ((x sub-a) (y sub-b))
  (format nil "foo called on SUB-A and SUB-B~%"))

(defparameter *just-a* (make-instance 'a))
(defparameter *just-b* (make-instance 'b))
(defparameter *sub-a* (make-instance 'sub-a))
(defparameter *sub-b* (make-instance 'sub-b)))