// file:    list1.java
// author:  Robert Keller
// purpose: expansion of the List class, open list of integers

// In the open list concept, a List is a REFERENCE to the first cell 
// of the list.  
// Thus the empty List is identified with null, the null reference,
// which is part of the Java language.
//
// cons(F, R)  creates a List from an element F and a list R.
// first(L)    returns the first of a non-empty list
// rest(L)     returns all but the first of a non-empty list
// isEmpty(L)  tells whether L is empty
// print(L, S) prints L in PrintStream S

import java.io.PrintStream;
class List
{
int First;                              // every non-empty list has
List Rest;                              // these two things

static List cons(int First, List Rest)  // creat a new list
  {
  return new List(First, Rest);         // defined in constructor below
  }


static int first(List L)                // get First
  {
  return L.First;
  }


static List rest(List L)                // get Rest
  {
  return L.Rest;
  }


static boolean isEmpty(List L)          // tell whether empty
  {
  return L == null;
  }


static final List nil = null;		// empty list


static void print(List L, PrintStream S)  // print list as (1 2 3 ....)
  {
  S.print("(");                 // print opening "("
  
  if( !isEmpty(L) )
    {
    S.print(first(L));          // print first element if any

    L = rest(L);
  
    while( !isEmpty(L) )          // print rest of elements
      {
      S.print(" ");
      S.print(first(L));
      L = rest(L);
      }
    }

  S.print(")");                 // print closing ")"
  }

public static void main(String arg[])   // main program or test
  {
  List L = cons(1, cons(2, cons(3, cons(4, null))));

  System.out.print("L = "); 
  print(L, System.out); 
  System.out.println();
  }

List(int First, List Rest)      // List constructor
  {
  this.First = First;
  this.Rest = Rest;
  }
}