/* Polymorphic linked lists - first version */

using System;
using numbers;

public class cell
{
 private object head;  // general head
 private cell tail;
 public object CAR { get { return head; } }
 public cell CDR   { get { return tail; } }
 public cell(object h, cell t) {head=h; tail=t;}

 public int length()   // internal visitor
  { int ax = 0;  // accumulator
    for (cell i=this;i!=null;i=i.tail) ax++;
    return ax;
  }
 public void print()
  {
    Console.Write(head + " ");
    if (tail==null) Console.Write("\n");
      else tail.print();
  }

  // but the above form of polymorphism is trivial.
  // now I want to be able to add one to the head, but I don't know
  // if the head is a numerical type, or an int or rational!  
  // Another way to do it would be to use a superclass for all numerical
  // types, but that would disallow other lists.

  public void add1()
   {
     if (head is int) head = (int)head + 1;
       else if (head is rational) ((rational)head).add(1);
     if (tail !=null) tail.add1();  

     // Might as well do it in C - there's no auto-dispatch here.
     // you might think the if-else can be avoided if we could just
     // overload the ++ operator, but that won't work either, because
     // the static type of 'head' is object, which doesn't have a ++
     // operator.  You must type cast it to a type that allows for ++.
     // that is, you'll be able to do ((number)head)++; assuming the
     // type number (be it a superclass or interface), allows for ++.
   }
}  // cell class

public class polylist1
{
  public static int length(cell l) // external visitor
  { 
    if (l==null) return 0; 
      else return 1+length(l.CDR);
  }

  public static void Main()
  { 
    cell l;
    l = new cell(3,new cell(new rational(1,2),new cell(5,null)));
    Console.WriteLine(length(l));
    l.print();
    l.add1();
    l.print();
  }
}

//complie with csc /r:number.dll polylist.cs