code: //==============================================================================
  //	
  //	Copyright (c) 2002-
  //	Authors:
  //	* Dave Parker <david.parker@comlab.ox.ac.uk> (University of Oxford)
  //	
  //------------------------------------------------------------------------------
  //	
  //	This file is part of PRISM.
  //	
  //	PRISM is free software; you can redistribute it and/or modify
  //	it under the terms of the GNU General Public License as published by
  //	the Free Software Foundation; either version 2 of the License, or
  //	(at your option) any later version.
  //	
  //	PRISM is distributed in the hope that it will be useful,
  //	but WITHOUT ANY WARRANTY; without even the implied warranty of
  //	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  //	GNU General Public License for more details.
  //	
  //	You should have received a copy of the GNU General Public License
  //	along with PRISM; if not, write to the Free Software Foundation,
  //	Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  //	
  //==============================================================================
  
  package parser.type;
  
  import param.BigRational;
  import prism.PrismLangException;
  
  public abstract class Type 
  {
  	/**
  	 * Returns the string denoting this type, e.g. "int", "bool".
  	 */
  	public abstract String getTypeString();
  	
  	/**
  	 * Returns the default value for this type, assuming no initialisation specified.
  	 */
  	public Object defaultValue()
  	{
  		// Play safe: assume null
  		return null;
  	}
  	
  	/**
  	 * Returns true iff a variable of this type can be assigned a value that is of type {@code type}. 
  	 */
  	public boolean canAssign(Type type)
  	{
  		// Play safe: assume not possible, unless explicitly overridden.
  		return false;
  	}
  	
  	/**
  	 * Make sure that a value, stored as an Object (Integer, Boolean, etc.) is of this type.
  	 * Basically, implement some implicit casts, e.g. Integer -> Double.
  	 * This should only only work for combinations of types that satisfy {@code #canAssign(Type)}.
  	 * If not, an exception is thrown (but such problems should have been caught earlier by type checking)
  	 */
  	public Object castValueTo(Object value) throws PrismLangException
  	{
  		// Play safe: assume error unless explicitly overridden.
  		throw new PrismLangException("Cannot cast a value to type " + getTypeString());
  	}
  
  	/**
  	 * Cast a BigRational value to the Java data type (Boolean, Integer, Double, ...)
  	 * corresponding to this type.
  	 * <br>
  	 * For boolean and integer, this throws an exception if the value can not be
  	 * precisely represented by the Java data type; for double, loss of precision
  	 * is expected and does not raise an exception.
  	 */
  	public Object castFromBigRational(BigRational value) throws PrismLangException
  	{
  		// Play safe: assume error unless explicitly overridden.
  		throw new PrismLangException("Cannot cast rational number to type " + getTypeString());
  	}
  
  	@Override
  	public String toString()
  	{
  		return getTypeString();
  	}
  }
  