/*

  * Copyright (C) 1998-2001 Semiotek Inc.  All Rights Reserved.

  *

  * Redistribution and use in source and binary forms, with or without

  * modification, are permitted under the terms of either of the following

  * Open Source licenses:

  *

  * The GNU General Public License, version 2, or any later version, as

  * published by the Free Software Foundation

  * (http://www.fsf.org/copyleft/gpl.html);

  *

  *  or

  *

  * The Semiotek Public License (http://webmacro.org/LICENSE.)

  *

  * This software is provided "as is", with NO WARRANTY, not even the

  * implied warranties of fitness to purpose, or merchantability. You

  * assume all risks and liabilities associated with its use.

  *

  * See www.webmacro.org for more information on the WebMacro project.

  */

 package org.webmacro.engine;

 import org.webmacro.Context;

 import org.webmacro.FastWriter;

 import org.webmacro.Macro;

 import org.webmacro.PropertyException;

 import org.webmacro.TemplateVisitor;

 import org.webmacro.Visitable;

 /**

  * Looks like a Macro, but really it's an undefined variable.

  */

 public final class UndefinedMacro implements Macro, Visitable

 {

     final static private UndefinedMacro _singleton = new UndefinedMacro();

     private UndefinedMacro ()

     {

     }

     static public UndefinedMacro getInstance ()

     {

         return _singleton;

     }

     public final String toString ()

     {

         throw new UnsupportedOperationException(

                 "Cannot invoke toString() on an undefined variable.");

     }

     public void accept (TemplateVisitor v)

     {

         v.visitString(toString());

     }

     /**

      * Returns the wrapped object, context is ignored.

      */

     public final Object evaluate (Context context)

     {

         return _singleton;

     }

     public Object get (Object key)

     {

         return _singleton;

     }

     /**

      * Throws an exception -- cannot write an undefined.

      */

     public final void write (FastWriter out, Context context)

             throws PropertyException

     {

         throw new PropertyException.UndefinedVariableException();

     }

 }