package com.googlecode.prolog_cafe.lang;

import com.googlecode.prolog_cafe.exceptions.PrologException;

/**
 * Initial backtrak point.<br>
 *
 * @author Mutsunori Banbara (banbara@kobe-u.ac.jp)
 * @author Naoyuki Tamura (tamura@kobe-u.ac.jp)
 * @version 1.2
 */
public class Failure extends Operation {
    public static final Failure FAILURE = new Failure();
    private Failure() {}

    public Operation exec(Prolog engine) {
	engine.control.fail();
	engine.halt = 1; // halt(0)
	return null;
    }

    /** Returns a string representation of this <code>Failure</code>. */
    public String toString(){ return "Failure"; }

    public static final Operation FAIL_0 = new PRED_fail_0();
    private static final class PRED_fail_0 extends Operation {
      @Override
      public Operation exec(Prolog engine) throws PrologException {
        engine.setB0();
        return engine.fail();
      }
    }
}