package schemeFactory;
/**
 * Represents an abstract Scheme-like list, an  immutable linear recursive
 * structure of data. Has abstract methods to provide its internal data and
 * structure to the client.
 * @author Dung X. Nguyen
 * @version 3.1
 * @since 02/24/02
 * @Custom Copyright 2002 -All rights reserved
 */
public abstract class AList {
    /**
     * Returns the first element in this <code>AList</code>, if any.
     * The behavior for the empty list is undefined.
     */
    public abstract Object getFirst();

    /**
     * Returns the tail ("rest") of this <code>AList</code>, if any.
     * The behavior for the empty list is undefined.
     */
    public abstract AList getRest();

    /**
     * "Hook" method to execute any IListAlgo visitor.
     * @param algo the algorithm operating on this AList.
     * @param inp the input needed by algo to perform its task.
     * @return the output Object of algo.
     */
    public abstract Object execute(IListAlgo algo, Object inp);

    /**
     * Calls anonymous visitor to compute a String representation for AList.
     */
    public String toString() {
        return (String)execute(new IListAlgo() {
            /**
             * Returns "()"
             * @param host an empty list
             * @param inp not used
             * @return String
             */
            public Object emptyCase(AList host, Object inp) {
                return "()";
            }

            /**
             * Returns "(", followed by String reperesentation of elements in
             * host separated by spaces, followed by ")".
             * @param host a non-empty list.
             * @param inp not used
             * @return String
             */
            public Object nonEmptyCase(AList host, Object inp) {
                return "(" + host.getFirst() + host.getRest().execute(new IListAlgo() {
                    public Object emptyCase(AList h, Object i) {
                        return ")";
                    }

                    public Object nonEmptyCase(AList h, Object i) {
                        return " " + h.getFirst() + h.getRest().execute (this, null);
                    }
                }, null);
            }
        }, null);
    }
}