transcendental-lisp/parser/Cons.java

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

package parser;

/**
 * This class represents a Lisp CONS cell in the PL-Lisp implementation.
 */
public class Cons extends SExpression {

    private SExpression car;
    private SExpression cdr;

    /**
     * Create a new CONS cell with the specified car and cdr.
     *
     * @param car
     *  the car of this CONS cell
     * @param cdr
     *  the cdr of this CONS cell
     */
    public Cons(SExpression car, SExpression cdr) {
        this.car = car;
        this.cdr = cdr;
    }

    /**
     * Retrieve the car of this CONS cell.
     *
     * @return
     *  the car of this CONS cell
     */
    public SExpression getCar() {
        return car;
    }

    /**
     * Retrieve the cdr of this CONS cell.
     *
     * @return
     *  the cdr of this CONS cell
     */
    public SExpression getCdr() {
        return cdr;
    }

    /**
     * Set the car of this CONS cell to the specified value.
     *
     * @param newCar
     *  the value to assign to the car of this CONS cell
     */
    public void setCar(SExpression newCar) {
        car = newCar;
    }

    /**
     * Set the cdr of this CONS cell to the specified value.
     *
     * @param newCdr
     *  the value to assign to the cdr of this CONS cell
     */
    public void setCdr(SExpression newCdr) {
        cdr = newCdr;
    }

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

    /**
     * Returns a string representation of this CONS cell.
     *
     * @return
     *  a string representation of this CONS cell
     */
    public String toString() {
        return ("(" + toStringAux());
    }

    // Returns a string representation of the car of a CONS cell followed by
    // its cdr. If the cdr of this CONS cell is not a CONS cell itself, this
    // method places a ')' at the end of its return value. Also, if the cdr of
    // this CONS cell is NIL, it will not be included in the return value of
    // this method. When used in conjunction with the 'toString' method of this
    // class, this method provides a means for creating the correct string
    // representation of a list.
    //
    // Returns: a string representation of the car of a CONS cell followed by
    //          its cdr
    private String toStringAux() {
        if (cdr.nullp()) {
            return (car.toString() + ")");
        } else if (cdr.consp()) {
            return (car.toString() + " " + ((Cons) cdr).toStringAux());
        }

        // the cdr of this CONS cell is not a list
        return (car.toString() + " . " + cdr.toString() + ")");
    }

}