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<?php
/*
* This file is part of the webmozart/expression package.
*
* (c) Bernhard Schussek <bschussek@gmail.com>
*
* For the full copyright and license information, please view the LICENSE
* file that was distributed with this source code.
*/
namespace Webmozart\Expression\Traversal;
use Webmozart\Expression\Expression;
/**
* Visits the nodes of an {@link Expression} tree.
*
* The visitor needs to be attached to a {@link ExpressionTraverser}. The
* traverser invokes the visitor for every node of the expression tree.
*
* @since 1.0
*
* @author Bernhard Schussek <bschussek@gmail.com>
*/
interface ExpressionVisitor
{
/**
* Called when the traverser enters an expression.
*
* @param Expression $expr The expression.
*
* @return Expression The modified expression.
*/
public function enterExpression(Expression $expr);
/**
* Called when the traverser leaves an expression.
*
* @param Expression $expr The expression.
*
* @return Expression|null The modified expression or `null` if the
* expression should be removed from the tree.
*/
public function leaveExpression(Expression $expr);
}