[SPARK-16318][SQL] Implement all remaining xpath functions

## What changes were proposed in this pull request?
This patch implements all remaining xpath functions that Hive supports and not natively supported in Spark: xpath_int, xpath_short, xpath_long, xpath_float, xpath_double, xpath_string, and xpath.

## How was this patch tested?
Added unit tests and end-to-end tests.

Author: petermaxlee <petermaxlee@gmail.com>

Closes #13991 from petermaxlee/SPARK-16318.

6 files changed, 364 insertions, 93 deletions

diff --git a/sql/catalyst/src/main/java/org/apache/spark/sql/catalyst/expressions/xml/UDFXPathUtil.java b/sql/catalyst/src/main/java/org/apache/spark/sql/catalyst/expressions/xml/UDFXPathUtil.java
index 410e9e51ba..d224332d8a 100644
--- a/sql/catalyst/src/main/java/org/apache/spark/sql/catalyst/expressions/xml/UDFXPathUtil.java
+++ b/sql/catalyst/src/main/java/org/apache/spark/sql/catalyst/expressions/xml/UDFXPathUtil.java
@@ -43,7 +43,7 @@ public class UDFXPathUtil {
   private XPathExpression expression = null;
   private String oldPath = null;
 
-  public Object eval(String xml, String path, QName qname) {
+  public Object eval(String xml, String path, QName qname) throws XPathExpressionException {
     if (xml == null || path == null || qname == null) {
       return null;
     }
@@ -56,7 +56,7 @@ public class UDFXPathUtil {
       try {
         expression = xpath.compile(path);
       } catch (XPathExpressionException e) {
-        expression = null;
+        throw new RuntimeException("Invalid XPath '" + path + "'" + e.getMessage(), e);
       }
       oldPath = path;
     }
@@ -66,31 +66,30 @@ public class UDFXPathUtil {
     }
 
     reader.set(xml);
-
     try {
       return expression.evaluate(inputSource, qname);
     } catch (XPathExpressionException e) {
-      throw new RuntimeException("Invalid expression '" + oldPath + "'", e);
+      throw new RuntimeException("Invalid XML document: " + e.getMessage() + "\n" + xml, e);
     }
   }
 
-  public Boolean evalBoolean(String xml, String path) {
+  public Boolean evalBoolean(String xml, String path) throws XPathExpressionException {
     return (Boolean) eval(xml, path, XPathConstants.BOOLEAN);
   }
 
-  public String evalString(String xml, String path) {
+  public String evalString(String xml, String path) throws XPathExpressionException {
     return (String) eval(xml, path, XPathConstants.STRING);
   }
 
-  public Double evalNumber(String xml, String path) {
+  public Double evalNumber(String xml, String path) throws XPathExpressionException {
     return (Double) eval(xml, path, XPathConstants.NUMBER);
   }
 
-  public Node evalNode(String xml, String path) {
+  public Node evalNode(String xml, String path) throws XPathExpressionException {
     return (Node) eval(xml, path, XPathConstants.NODE);
   }
 
-  public NodeList evalNodeList(String xml, String path) {
+  public NodeList evalNodeList(String xml, String path) throws XPathExpressionException {
     return (NodeList) eval(xml, path, XPathConstants.NODESET);
   }
 
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala
index c8bbbf8853..54568b7445 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/FunctionRegistry.scala
@@ -310,7 +310,15 @@ object FunctionRegistry {
     expression[UnBase64]("unbase64"),
     expression[Unhex]("unhex"),
     expression[Upper]("upper"),
+    expression[XPathList]("xpath"),
     expression[XPathBoolean]("xpath_boolean"),
+    expression[XPathDouble]("xpath_double"),
+    expression[XPathDouble]("xpath_number"),
+    expression[XPathFloat]("xpath_float"),
+    expression[XPathInt]("xpath_int"),
+    expression[XPathLong]("xpath_long"),
+    expression[XPathShort]("xpath_short"),
+    expression[XPathString]("xpath_string"),
 
     // datetime functions
     expression[AddMonths]("add_months"),
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/xml/XPathBoolean.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/xml/XPathBoolean.scala
deleted file mode 100644
index 2a5256c7f5..0000000000
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/xml/XPathBoolean.scala
+++ /dev/null
@@ -1,58 +0,0 @@
-/*
- * Licensed to the Apache Software Foundation (ASF) under one or more
- * contributor license agreements.  See the NOTICE file distributed with
- * this work for additional information regarding copyright ownership.
- * The ASF licenses this file to You under the Apache License, Version 2.0
- * (the "License"); you may not use this file except in compliance with
- * the License.  You may obtain a copy of the License at
- *
- *    http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package org.apache.spark.sql.catalyst.expressions.xml
-
-import org.apache.spark.sql.catalyst.expressions._
-import org.apache.spark.sql.catalyst.expressions.codegen.CodegenFallback
-import org.apache.spark.sql.types.{AbstractDataType, BooleanType, DataType, StringType}
-import org.apache.spark.unsafe.types.UTF8String
-
-
-@ExpressionDescription(
-  usage = "_FUNC_(xml, xpath) - Evaluates a boolean xpath expression.",
-  extended = "> SELECT _FUNC_('<a><b>1</b></a>','a/b');\ntrue")
-case class XPathBoolean(xml: Expression, path: Expression)
-  extends BinaryExpression with ExpectsInputTypes with CodegenFallback {
-
-  @transient private lazy val xpathUtil = new UDFXPathUtil
-
-  // If the path is a constant, cache the path string so that we don't need to convert path
-  // from UTF8String to String for every row.
-  @transient lazy val pathLiteral: String = path match {
-    case Literal(str: UTF8String, _) => str.toString
-    case _ => null
-  }
-
-  override def prettyName: String = "xpath_boolean"
-
-  override def dataType: DataType = BooleanType
-
-  override def inputTypes: Seq[AbstractDataType] = Seq(StringType, StringType)
-
-  override def left: Expression = xml
-  override def right: Expression = path
-
-  override protected def nullSafeEval(xml: Any, path: Any): Any = {
-    val xmlString = xml.asInstanceOf[UTF8String].toString
-    if (pathLiteral ne null) {
-      xpathUtil.evalBoolean(xmlString, pathLiteral)
-    } else {
-      xpathUtil.evalBoolean(xmlString, path.asInstanceOf[UTF8String].toString)
-    }
-  }
-}
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/xml/xpath.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/xml/xpath.scala
new file mode 100644
index 0000000000..47f039e6a4
--- /dev/null
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/xml/xpath.scala
@@ -0,0 +1,174 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.catalyst.expressions.xml
+
+import org.apache.spark.sql.catalyst.analysis.TypeCheckResult
+import org.apache.spark.sql.catalyst.analysis.TypeCheckResult.TypeCheckFailure
+import org.apache.spark.sql.catalyst.expressions._
+import org.apache.spark.sql.catalyst.expressions.codegen.CodegenFallback
+import org.apache.spark.sql.catalyst.util.GenericArrayData
+import org.apache.spark.sql.types._
+import org.apache.spark.unsafe.types.UTF8String
+
+/**
+ * Base class for xpath_boolean, xpath_double, xpath_int, etc.
+ *
+ * This is not the world's most efficient implementation due to type conversion, but works.
+ */
+abstract class XPathExtract extends BinaryExpression with ExpectsInputTypes with CodegenFallback {
+  override def left: Expression = xml
+  override def right: Expression = path
+
+  /** XPath expressions are always nullable, e.g. if the xml string is empty. */
+  override def nullable: Boolean = true
+
+  override def inputTypes: Seq[AbstractDataType] = Seq(StringType, StringType)
+
+  override def checkInputDataTypes(): TypeCheckResult = {
+    if (!path.foldable) {
+      TypeCheckFailure("path should be a string literal")
+    } else {
+      super.checkInputDataTypes()
+    }
+  }
+
+  @transient protected lazy val xpathUtil = new UDFXPathUtil
+  @transient protected lazy val pathString: String = path.eval().asInstanceOf[UTF8String].toString
+
+  /** Concrete implementations need to override the following three methods. */
+  def xml: Expression
+  def path: Expression
+}
+
+@ExpressionDescription(
+  usage = "_FUNC_(xml, xpath) - Evaluates a boolean xpath expression.",
+  extended = "> SELECT _FUNC_('<a><b>1</b></a>','a/b');\ntrue")
+case class XPathBoolean(xml: Expression, path: Expression) extends XPathExtract {
+
+  override def prettyName: String = "xpath_boolean"
+  override def dataType: DataType = BooleanType
+
+  override def nullSafeEval(xml: Any, path: Any): Any = {
+    xpathUtil.evalBoolean(xml.asInstanceOf[UTF8String].toString, pathString)
+  }
+}
+
+@ExpressionDescription(
+  usage = "_FUNC_(xml, xpath) - Returns a short value that matches the xpath expression",
+  extended = "> SELECT _FUNC_('<a><b>1</b><b>2</b></a>','sum(a/b)');\n3")
+case class XPathShort(xml: Expression, path: Expression) extends XPathExtract {
+  override def prettyName: String = "xpath_int"
+  override def dataType: DataType = ShortType
+
+  override def nullSafeEval(xml: Any, path: Any): Any = {
+    val ret = xpathUtil.evalNumber(xml.asInstanceOf[UTF8String].toString, pathString)
+    if (ret eq null) null else ret.shortValue()
+  }
+}
+
+@ExpressionDescription(
+  usage = "_FUNC_(xml, xpath) - Returns an integer value that matches the xpath expression",
+  extended = "> SELECT _FUNC_('<a><b>1</b><b>2</b></a>','sum(a/b)');\n3")
+case class XPathInt(xml: Expression, path: Expression) extends XPathExtract {
+  override def prettyName: String = "xpath_int"
+  override def dataType: DataType = IntegerType
+
+  override def nullSafeEval(xml: Any, path: Any): Any = {
+    val ret = xpathUtil.evalNumber(xml.asInstanceOf[UTF8String].toString, pathString)
+    if (ret eq null) null else ret.intValue()
+  }
+}
+
+@ExpressionDescription(
+  usage = "_FUNC_(xml, xpath) - Returns a long value that matches the xpath expression",
+  extended = "> SELECT _FUNC_('<a><b>1</b><b>2</b></a>','sum(a/b)');\n3")
+case class XPathLong(xml: Expression, path: Expression) extends XPathExtract {
+  override def prettyName: String = "xpath_long"
+  override def dataType: DataType = LongType
+
+  override def nullSafeEval(xml: Any, path: Any): Any = {
+    val ret = xpathUtil.evalNumber(xml.asInstanceOf[UTF8String].toString, pathString)
+    if (ret eq null) null else ret.longValue()
+  }
+}
+
+@ExpressionDescription(
+  usage = "_FUNC_(xml, xpath) - Returns a float value that matches the xpath expression",
+  extended = "> SELECT _FUNC_('<a><b>1</b><b>2</b></a>','sum(a/b)');\n3.0")
+case class XPathFloat(xml: Expression, path: Expression) extends XPathExtract {
+  override def prettyName: String = "xpath_float"
+  override def dataType: DataType = FloatType
+
+  override def nullSafeEval(xml: Any, path: Any): Any = {
+    val ret = xpathUtil.evalNumber(xml.asInstanceOf[UTF8String].toString, pathString)
+    if (ret eq null) null else ret.floatValue()
+  }
+}
+
+@ExpressionDescription(
+  usage = "_FUNC_(xml, xpath) - Returns a double value that matches the xpath expression",
+  extended = "> SELECT _FUNC_('<a><b>1</b><b>2</b></a>','sum(a/b)');\n3.0")
+case class XPathDouble(xml: Expression, path: Expression) extends XPathExtract {
+  override def prettyName: String = "xpath_float"
+  override def dataType: DataType = DoubleType
+
+  override def nullSafeEval(xml: Any, path: Any): Any = {
+    val ret = xpathUtil.evalNumber(xml.asInstanceOf[UTF8String].toString, pathString)
+    if (ret eq null) null else ret.doubleValue()
+  }
+}
+
+// scalastyle:off line.size.limit
+@ExpressionDescription(
+  usage = "_FUNC_(xml, xpath) - Returns the text contents of the first xml node that matches the xpath expression",
+  extended = "> SELECT _FUNC_('<a><b>b</b><c>cc</c></a>','a/c');\ncc")
+// scalastyle:on line.size.limit
+case class XPathString(xml: Expression, path: Expression) extends XPathExtract {
+  override def prettyName: String = "xpath_string"
+  override def dataType: DataType = StringType
+
+  override def nullSafeEval(xml: Any, path: Any): Any = {
+    val ret = xpathUtil.evalString(xml.asInstanceOf[UTF8String].toString, pathString)
+    UTF8String.fromString(ret)
+  }
+}
+
+// scalastyle:off line.size.limit
+@ExpressionDescription(
+  usage = "_FUNC_(xml, xpath) - Returns a string array of values within xml nodes that match the xpath expression",
+  extended = "> SELECT _FUNC_('<a><b>b1</b><b>b2</b><b>b3</b><c>c1</c><c>c2</c></a>','a/b/text()');\n['b1','b2','b3']")
+// scalastyle:on line.size.limit
+case class XPathList(xml: Expression, path: Expression) extends XPathExtract {
+  override def prettyName: String = "xpath"
+  override def dataType: DataType = ArrayType(StringType, containsNull = false)
+
+  override def nullSafeEval(xml: Any, path: Any): Any = {
+    val nodeList = xpathUtil.evalNodeList(xml.asInstanceOf[UTF8String].toString, pathString)
+    if (nodeList ne null) {
+      val ret = new Array[UTF8String](nodeList.getLength)
+      var i = 0
+      while (i < nodeList.getLength) {
+        ret(i) = UTF8String.fromString(nodeList.item(i).getNodeValue)
+        i += 1
+      }
+      new GenericArrayData(ret)
+    } else {
+      null
+    }
+  }
+}
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/xml/UDFXPathUtilSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/xml/UDFXPathUtilSuite.scala
index a5614f8384..c4cde70911 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/xml/UDFXPathUtilSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/xml/UDFXPathUtilSuite.scala
@@ -43,8 +43,9 @@ class UDFXPathUtilSuite extends SparkFunSuite {
     assert(util.eval("<a><b>b1</b><b>b2</b><b>b3</b><c>c1</c><c>c2</c></a>", "", STRING) == null)
 
     // wrong expression:
-    assert(
-      util.eval("<a><b>b1</b><b>b2</b><b>b3</b><c>c1</c><c>c2</c></a>", "a/text(", STRING) == null)
+    intercept[RuntimeException] {
+      util.eval("<a><b>b1</b><b>b2</b><b>b3</b><c>c1</c><c>c2</c></a>", "a/text(", STRING)
+    }
   }
 
   test("generic eval") {
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/xml/XPathExpressionSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/xml/XPathExpressionSuite.scala
index f7c65c667e..bfa18a0919 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/xml/XPathExpressionSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/xml/XPathExpressionSuite.scala
@@ -18,8 +18,7 @@
 package org.apache.spark.sql.catalyst.expressions.xml
 
 import org.apache.spark.SparkFunSuite
-import org.apache.spark.sql.catalyst.dsl.expressions._
-import org.apache.spark.sql.catalyst.expressions.{ExpressionEvalHelper, Literal}
+import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.types.StringType
 
 /**
@@ -27,35 +26,183 @@ import org.apache.spark.sql.types.StringType
  */
 class XPathExpressionSuite extends SparkFunSuite with ExpressionEvalHelper {
 
-  private def testBoolean[T](xml: String, path: String, expected: T): Unit = {
-    checkEvaluation(
-      XPathBoolean(Literal.create(xml, StringType), Literal.create(path, StringType)),
-      expected)
+  /** A helper function that tests null and error behaviors for xpath expressions. */
+  private def testNullAndErrorBehavior[T <: AnyRef](testExpr: (String, String, T) => Unit): Unit = {
+    // null input should lead to null output
+    testExpr("<a><b>b1</b><b id='b_2'>b2</b></a>", null, null.asInstanceOf[T])
+    testExpr(null, "a", null.asInstanceOf[T])
+    testExpr(null, null, null.asInstanceOf[T])
+
+    // Empty input should also lead to null output
+    testExpr("", "a", null.asInstanceOf[T])
+    testExpr("<a></a>", "", null.asInstanceOf[T])
+    testExpr("", "", null.asInstanceOf[T])
+
+    // Test error message for invalid XML document
+    val e1 = intercept[RuntimeException] { testExpr("<a>/a>", "a", null.asInstanceOf[T]) }
+    assert(e1.getCause.getMessage.contains("Invalid XML document") &&
+      e1.getCause.getMessage.contains("<a>/a>"))
+
+    // Test error message for invalid xpath
+    val e2 = intercept[RuntimeException] { testExpr("<a></a>", "!#$", null.asInstanceOf[T]) }
+    assert(e2.getCause.getMessage.contains("Invalid XPath") &&
+      e2.getCause.getMessage.contains("!#$"))
   }
 
   test("xpath_boolean") {
-    testBoolean("<a><b>b</b></a>", "a/b", true)
-    testBoolean("<a><b>b</b></a>", "a/c", false)
-    testBoolean("<a><b>b</b></a>", "a/b = \"b\"", true)
-    testBoolean("<a><b>b</b></a>", "a/b = \"c\"", false)
-    testBoolean("<a><b>10</b></a>", "a/b < 10", false)
-    testBoolean("<a><b>10</b></a>", "a/b = 10", true)
+    def testExpr[T](xml: String, path: String, expected: java.lang.Boolean): Unit = {
+      checkEvaluation(
+        XPathBoolean(Literal.create(xml, StringType), Literal.create(path, StringType)),
+        expected)
+    }
+
+    testExpr("<a><b>b</b></a>", "a/b", true)
+    testExpr("<a><b>b</b></a>", "a/c", false)
+    testExpr("<a><b>b</b></a>", "a/b = \"b\"", true)
+    testExpr("<a><b>b</b></a>", "a/b = \"c\"", false)
+    testExpr("<a><b>10</b></a>", "a/b < 10", false)
+    testExpr("<a><b>10</b></a>", "a/b = 10", true)
 
-    // null input
-    testBoolean(null, null, null)
-    testBoolean(null, "a", null)
-    testBoolean("<a><b>10</b></a>", null, null)
+    testNullAndErrorBehavior(testExpr)
+  }
 
-    // exception handling for invalid input
-    intercept[Exception] {
-      testBoolean("<a>/a>", "a", null)
+  test("xpath_short") {
+    def testExpr[T](xml: String, path: String, expected: java.lang.Short): Unit = {
+      checkEvaluation(
+        XPathShort(Literal.create(xml, StringType), Literal.create(path, StringType)),
+        expected)
     }
+
+    testExpr("<a>this is not a number</a>", "a", 0.toShort)
+    testExpr("<a>try a boolean</a>", "a = 10", 0.toShort)
+    testExpr(
+      "<a><b class=\"odd\">10000</b><b class=\"even\">2</b><b class=\"odd\">4</b><c>8</c></a>",
+      "sum(a/b[@class=\"odd\"])",
+      10004.toShort)
+
+    testNullAndErrorBehavior(testExpr)
   }
 
-  test("xpath_boolean path cache invalidation") {
-    // This is a test to ensure the expression is not reusing the path for different strings
-    val expr = XPathBoolean(Literal("<a><b>b</b></a>"), 'path.string.at(0))
-    checkEvaluation(expr, true, create_row("a/b"))
-    checkEvaluation(expr, false, create_row("a/c"))
+  test("xpath_int") {
+    def testExpr[T](xml: String, path: String, expected: java.lang.Integer): Unit = {
+      checkEvaluation(
+        XPathInt(Literal.create(xml, StringType), Literal.create(path, StringType)),
+        expected)
+    }
+
+    testExpr("<a>this is not a number</a>", "a", 0)
+    testExpr("<a>try a boolean</a>", "a = 10", 0)
+    testExpr(
+      "<a><b class=\"odd\">100000</b><b class=\"even\">2</b><b class=\"odd\">4</b><c>8</c></a>",
+      "sum(a/b[@class=\"odd\"])",
+      100004)
+
+    testNullAndErrorBehavior(testExpr)
+  }
+
+  test("xpath_long") {
+    def testExpr[T](xml: String, path: String, expected: java.lang.Long): Unit = {
+      checkEvaluation(
+        XPathLong(Literal.create(xml, StringType), Literal.create(path, StringType)),
+        expected)
+    }
+
+    testExpr("<a>this is not a number</a>", "a", 0L)
+    testExpr("<a>try a boolean</a>", "a = 10", 0L)
+    testExpr(
+      "<a><b class=\"odd\">9000000000</b><b class=\"even\">2</b><b class=\"odd\">4</b><c>8</c></a>",
+      "sum(a/b[@class=\"odd\"])",
+      9000000004L)
+
+    testNullAndErrorBehavior(testExpr)
+  }
+
+  test("xpath_float") {
+    def testExpr[T](xml: String, path: String, expected: java.lang.Float): Unit = {
+      checkEvaluation(
+        XPathFloat(Literal.create(xml, StringType), Literal.create(path, StringType)),
+        expected)
+    }
+
+    testExpr("<a>this is not a number</a>", "a", Float.NaN)
+    testExpr("<a>try a boolean</a>", "a = 10", 0.0F)
+    testExpr("<a><b class=\"odd\">1</b><b class=\"even\">2</b><b class=\"odd\">4</b><c>8</c></a>",
+      "sum(a/b[@class=\"odd\"])",
+      5.0F)
+
+    testNullAndErrorBehavior(testExpr)
+  }
+
+  test("xpath_double") {
+    def testExpr[T](xml: String, path: String, expected: java.lang.Double): Unit = {
+      checkEvaluation(
+        XPathDouble(Literal.create(xml, StringType), Literal.create(path, StringType)),
+        expected)
+    }
+
+    testExpr("<a>this is not a number</a>", "a", Double.NaN)
+    testExpr("<a>try a boolean</a>", "a = 10", 0.0)
+    testExpr("<a><b class=\"odd\">1</b><b class=\"even\">2</b><b class=\"odd\">4</b><c>8</c></a>",
+      "sum(a/b[@class=\"odd\"])",
+      5.0)
+
+    testNullAndErrorBehavior(testExpr)
+  }
+
+  test("xpath_string") {
+    def testExpr[T](xml: String, path: String, expected: String): Unit = {
+      checkEvaluation(
+        XPathString(Literal.create(xml, StringType), Literal.create(path, StringType)),
+        expected)
+    }
+
+    testExpr("<a><b>bb</b><c>cc</c></a>", "a", "bbcc")
+    testExpr("<a><b>bb</b><c>cc</c></a>", "a/b", "bb")
+    testExpr("<a><b>bb</b><c>cc</c></a>", "a/c", "cc")
+    testExpr("<a><b>bb</b><c>cc</c></a>", "a/d", "")
+    testExpr("<a><b>b1</b><b>b2</b></a>", "//b", "b1")
+    testExpr("<a><b>b1</b><b>b2</b></a>", "a/b[1]", "b1")
+    testExpr("<a><b>b1</b><b id='b_2'>b2</b></a>", "a/b[@id='b_2']", "b2")
+
+    testNullAndErrorBehavior(testExpr)
+  }
+
+  test("xpath") {
+    def testExpr[T](xml: String, path: String, expected: Seq[String]): Unit = {
+      checkEvaluation(
+        XPathList(Literal.create(xml, StringType), Literal.create(path, StringType)),
+        expected)
+    }
+
+    testExpr("<a><b>b1</b><b>b2</b><b>b3</b><c>c1</c><c>c2</c></a>", "a/text()", Seq.empty[String])
+    testExpr("<a><b>b1</b><b>b2</b><b>b3</b><c>c1</c><c>c2</c></a>", "a/*/text()",
+      Seq("b1", "b2", "b3", "c1", "c2"))
+    testExpr("<a><b>b1</b><b>b2</b><b>b3</b><c>c1</c><c>c2</c></a>", "a/b/text()",
+      Seq("b1", "b2", "b3"))
+    testExpr("<a><b>b1</b><b>b2</b><b>b3</b><c>c1</c><c>c2</c></a>", "a/c/text()", Seq("c1", "c2"))
+    testExpr("<a><b class='bb'>b1</b><b>b2</b><b>b3</b><c class='bb'>c1</c><c>c2</c></a>",
+      "a/*[@class='bb']/text()", Seq("b1", "c1"))
+
+    testNullAndErrorBehavior(testExpr)
+  }
+
+  test("accept only literal path") {
+    def testExpr(exprCtor: (Expression, Expression) => Expression): Unit = {
+      // Validate that literal (technically this is foldable) paths are supported
+      val litPath = exprCtor(Literal("abcd"), Concat(Literal("/") :: Literal("/") :: Nil))
+      assert(litPath.checkInputDataTypes().isSuccess)
+
+      // Validate that non-foldable paths are not supported.
+      val nonLitPath = exprCtor(Literal("abcd"), NonFoldableLiteral("/"))
+      assert(nonLitPath.checkInputDataTypes().isFailure)
+    }
+
+    testExpr(XPathBoolean)
+    testExpr(XPathShort)
+    testExpr(XPathInt)
+    testExpr(XPathLong)
+    testExpr(XPathFloat)
+    testExpr(XPathDouble)
+    testExpr(XPathString)
   }
 }