--- /dev/null
+<?php
+/*
+ * 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.
+ *
+ * This software consists of voluntary contributions made by many individuals
+ * and is licensed under the MIT license. For more information, see
+ * <http://www.doctrine-project.org>.
+ */
+
+namespace Doctrine\Common\Collections\Expr;
+
+/**
+ * An Expression visitor walks a graph of expressions and turns them into a
+ * query for the underlying implementation.
+ *
+ * @author Benjamin Eberlei <kontakt@beberlei.de>
+ */
+abstract class ExpressionVisitor
+{
+ /**
+ * Converts a comparison expression into the target query language output.
+ *
+ * @param Comparison $comparison
+ *
+ * @return mixed
+ */
+ abstract public function walkComparison(Comparison $comparison);
+
+ /**
+ * Converts a value expression into the target query language part.
+ *
+ * @param Value $value
+ *
+ * @return mixed
+ */
+ abstract public function walkValue(Value $value);
+
+ /**
+ * Converts a composite expression into the target query language output.
+ *
+ * @param CompositeExpression $expr
+ *
+ * @return mixed
+ */
+ abstract public function walkCompositeExpression(CompositeExpression $expr);
+
+ /**
+ * Dispatches walking an expression to the appropriate handler.
+ *
+ * @param Expression $expr
+ *
+ * @return mixed
+ *
+ * @throws \RuntimeException
+ */
+ public function dispatch(Expression $expr)
+ {
+ switch (true) {
+ case ($expr instanceof Comparison):
+ return $this->walkComparison($expr);
+
+ case ($expr instanceof Value):
+ return $this->walkValue($expr);
+
+ case ($expr instanceof CompositeExpression):
+ return $this->walkCompositeExpression($expr);
+
+ default:
+ throw new \RuntimeException("Unknown Expression " . get_class($expr));
+ }
+ }
+}