/* Author: Jeff Dalton <J.Dalton@ed.ac.uk>
 * Updated: Wed Mar 14 13:14:07 2001 by Jeff Dalton
 */

package ix.util.lisp;

import java.util.*;
import ix.util.*;


/** 
 * A class for delaying evaluation. <p>
 *
 * A standard way to construct a Delay is to instantiate an anonymous
 * class.  For instance:
 *
 * <pre>
 *    new Delay() {
 *        public Object eval() {
 *            ... code that computes the value ...
 *        }
 *    }
 * </pre>
 */

// N.B. Need to use the above style of comment to preserve the indentation
// in the pre block.

public abstract class Delay {

    Object value = null;

    boolean forced_p = false;

    public Object force() {
	if (!forced_p) {
	    value = eval();
	    forced_p = true;
	}
	return value;
    }

    public abstract Object eval();

}