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2008-11-03T05:27:41Z

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== 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)))

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