/*
 * Name: Mike Cifelli
 * Course: CIS 443 - Programming Languages
 * Assignment: Lisp Parser
 */

package parser;

/**
 * This class represents NIL in the PL-Lisp implementation.
 */
public class Nil extends Cons {

    private static Nil uniqueInstance = new Nil();

    /**
     * Retrieve the single unique instance of NIL.
     *
     * @return
     *  the single unique instance of NIL
     */
    public static Nil getUniqueInstance() {
        return uniqueInstance;
    }

    // We are using the Singleton pattern, so the constructor for 'Nil' is
    // private.
    private Nil() {
        super(null, null);

        // the car and cdr of NIL both refer to NIL
        super.setCar(this);
        super.setCdr(this);
    }

    /**
     * Test if this S-expression is NULL.
     *
     * @return
     *  <code>true</code>
     */
    public boolean nullp() {
        return true;
    }

    /**
     * Test if this S-expression is an ATOM.
     *
     * @return
     *  <code>true</code>
     */
    public boolean atomp() {
        return true;
    }

    /**
     * Test if this S-expression is a CONS cell.
     *
     * @return
     *  <code>false</code>
     */
    public boolean consp() {
        return false;
    }

    /**
     * Test if this S-expression is a SYMBOL.
     *
     * @return
     *  <code>true</code>
     */
    public boolean symbolp() {
        return true;
    }

    /**
     * Set the car of this CONS cell to the specified value. This method does
     * nothing (the car of NIL can not be changed).
     *
     * @param newCar
     *  the value to assign to the car of this CONS cell
     */
    public void setCar(SExpression newCar) {}

    /**
     * Set the cdr of this CONS cell to the specified value. This method does
     * nothing (the cdr of NIL can not be changed).
     *
     * @param newCdr
     *  the value to assign to the cdr of this CONS cell
     */
    public void setCdr(SExpression newCdr) {}

    /**
     * Returns a string representation of NIL.
     *
     * @return
     *  a string representation of NIL
     */
    public String toString() {
        return "NIL";
    }

}