Morphism.java

class Morphism {
  /*
   List = Cons(int value, List next) | Nil
  */
  interface ListVisitor<R, P> {
    R Cons(int value, ListAcceptor next, P param);
    R Nil(P param);
  }

  interface ListAcceptor {
    <R, P> R accept(P param, ListVisitor<R, P> visitor);
  }

  private static void test2() {
    //List list = new Cons(3, new Cons(2, new Nil()));
    ListAcceptor list = new ListAcceptor() {
      public <R, P> R accept(P param, ListVisitor<R,P> visitor) {
        return visitor.Cons(3, new ListAcceptor() {
            public <S, Q> S accept(Q param, Morphisms.ListVisitor<S,Q> visitor) {
              return visitor.Cons(2, new ListAcceptor() {
                public <T, U> T accept(U param, ListVisitor<T, U> visitor) {
                  return visitor.Nil(param);
                }
              }, param);
            }
          }, param);
        }
    };

    //int v = Cons(value, next)-> value + rec(next)
    //      | Nil -> 0

    int v = list.accept(null, new ListVisitor<Integer, Void>() {
      public Integer Cons(int value, ListAcceptor next, Void param) { return value + next.accept(param, this); }
      public Integer Nil(Void __) { return 0; }
    });
    System.out.println(v);
  }

  public static void main(String[] args) {
    test2();
  }
}