Answer to Question 30-2

  void addToEnd(const int x, List& L)
  {
    if(L == NULL)
    {
      L = cons(x, NULL);
    }
    else
    {
      addToEnd(x, L->tail);
    }
  }

Since the definition of addToEnd is tail-recursive, an optimizing compiler can turn it into a loop.

  void addToEnd(const int x, List& L)
  {
    List& p = L;
    while(!isEmpty(p))
    {
      p = p->tail;
    }
    p->tail = cons(x, emptyList);
  }

When the loop ends, p is a variable that holds NULL. It is the 'tail' variable in the last cell, or is variable L if L is NULL. It is important to use p->tail instead of tail(p) since tail(p) is not a variable.