/*
 * RiskScape™ Copyright New Zealand Institute for Earth Science Limited
 * (Earth Sciences New Zealand) is distributed for research purposes only
 * under the terms of AGPLv3.
 *
 * RiskScape™ Copyright 2026 New Zealand Institute for Earth Science
 * Limited (Earth Sciences New Zealand). All rights reserved. Source code
 * available under the AGPLv3.
 * 
 * This program is free software: you can redistribute it and/or modify it under
 *  the terms of the GNU Affero General Public License as published by the Free
 *  Software Foundation, either version 3 of the License, or (at your option) any
 *  later version.
 * 
 * This program is distributed for RESEARCH PURPOSES ONLY, in the hope that it will
 * be useful for research and education initiatives.
 * 
 * If you are not a researcher, or you are a researcher who wishes to use this
 * program on terms other than AGPLv3 (including those who wish to restrict the
 * distribution of any source code created using this program), please contact:
 * https://riskscape.org.nz
 * 
 * This program is distributed WITHOUT ANY WARRANTY; without even the implied
 * warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Affero General Public License for more details.  You should have received a copy
 * of the GNU Affero General Public License along with this program.  If not, see
 * <http://www.gnu.org/licenses/>.
 * 
 * By way of summary only, under the AGPLv3:
 *     • Permissions of this strongest copyleft license are conditioned
 *       on making available complete source code of licensed works and
 *       modifications, which include larger works using a licensed work,
 *       under the same license.
 *     • Copyright and license notices must be preserved.
 *     • Contributors provide an express grant of patent rights.
 *     • When a modified version is used to provide a service over a
 *       network, the complete source code of the modified version must be made
 *       available.
 */
package nz.org.riskscape.rl.ast;

import nz.org.riskscape.engine.rl.RealizedExpression;

/**
 * An as yet Unrealized expression AST.  Note that an Expression is a bit of executable riskscape code that operates on
 * an individual tuple (as scope), whereas a {@link PipelineDeclaration} is AST that organises the flow of execution.
 *
 * Implementing bits of AST should not consider the underlying source code to be part of the objects identity, only the
 * meaningful bits of the AST.  For the most part, this boils down to ignoring whitespace and location the source.
 */
public interface Expression extends AST {

  <T, R> R accept(ExpressionVisitor<T, R> visitor, T data);

  /**
   * Tests whether this expression is a literal expression - a literal expression is a constant expression that is
   * unaffected by the scope in which it was realized/evaluated and contains no function-defined behaviour.  Therefore
   * a literal expression is one that contains no property access (unless the receiver is a literal) and no function
   * calls.
   *
   * A literal expression will always realize to being a constant (e.g. {@link RealizedExpression#isConstant()} will
   * return true), however there are constant expressions that are not literal.
   *
   * One edge case to be aware of is a limitation of the AST in determining literal expressions in contrived cases like
   * this one - `{foo: 'bar', baz: qux}.foo`.  The struct is not literal, but the property being accessed is.  But
   * because the AST is relatively 'dumb', the expression itself is not literal.  It can, however, be realized as a
   * constant - realization is 'smarter' and can pick out the that the property being accessed is constant.
   */
  boolean isLiteral();

}
