Update Expression's documentation and license.

/*

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 * Copyright (C) 2014 Matthew Dawson <matthew@mjdsystems.ca>

 *

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

 * modification, are permitted provided that the following conditions are

 * met:

 *

 *     * Redistributions of source code must retain the above copyright

 *       notice, this list of conditions and the following disclaimer.

 *     * Redistributions in binary form must reproduce the above copyright

 *       notice, this list of conditions and the following disclaimer in

 *       the documentation and/or other materials provided with the distribution

 *     * Neither the name of the McMaster Centre for Software Certification nor the names

 *       of its contributors may be used to endorse or promote products derived

 *       from this software without specific prior written permission.

 *

 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"

 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE

 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE

 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE

 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR

 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF

 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS

 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN

 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)

 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE

 * POSSIBILITY OF SUCH DAMAGE.

 */

package ca.mcscert.jtet.expression;

/**

 * Base interface to build an abstract syntax tree in jTET.

 * <p>

 * Expression provides the base class from which any abstract syntax tree is built in jTET.  How exactly any given

 * part of the AST generates is left up to an implementation of this interface.  Expression simply provides a way to

 * generate a string representing the expression while being appropriately typed.

 *

 * @author matthew

 * @author Matthew Dawson

 */

public interface Expression {

    /* This methods returns an actual variable type for the requested input type.  If due to the setup of the system,

    *  this is not possible, then this function should return null.

    /**

     * Attempts to resolve the type the expression will output, limited to the class of types given by the type marker.

     *

     * This function finds if the requested class of types can represent the output from this expression, and what that

     * output type would be.  This is mostly used internally to make sure call to getCheckerOutput can succeed.

     *

     * @param requestedType The class of type requested.

     * @return The concrete type that will be used, or null if none can be found.

     */

    VariableType actualOutputTypeForRequestedOutput(VariableTypeMarker requestedType);

    /**

     * Returns a string representing this expression generated from the given generator.

     * <p>

     * This function should make appropriate calls into the generator to generate an expression in its language that

     * outputs requested type.  If it is impossible to satisfy this request, it should throw an appropriate exception.

     * <p>

     * Note that the AST should not modify the string returned from the generator, as its format is left unspecified.

     *

     * @param generator The expression generator that is used to generate the string output.

     * @param requestedType The type the expression output.

     * @return A string from the generator representing the expression.

     */

    String getCheckerOutput(ExpressionGenerator generator, VariableType requestedType);

}