Evaluate XPATH 2.0 expression with SAXON - saxon

Question

I struggeled to find a working SAXON example to perform a simple task like evaluating the following XPATH 2.0 expression:
abs(-a)
Evaluating an expression like
3 + 4
works quite fine, but I didn't manage to provide SAXON a value for the variable a.
I tried stuff like:
final Processor proc = new Processor(false);
final XPathCompiler xpath = proc.newXPathCompiler();
QName a = new QName("a");
xpath.declareVariable(a);
System.out.println(xpath.evaluate("abs(-a)", null));
But obviously this doesn't work since I didn't provide a value for a.
How can I declare a value for a?
Any help would be highly appreciated!
Edit: This doesn't work either:
final Processor proc = new Processor(false);
final XPathCompiler xpath = proc.newXPathCompiler();
XPathSelector exp = xpath.compile("abs(-$a)").load();
exp.setVariable(new QName("a"), new XdmAtomicValue(17));
XdmItem result = exp.evaluateSingle();
This throws an exception:
net.sf.saxon.s9api.SaxonApiException: Undeclared variable in XPath expression: $a
at net.sf.saxon.s9api.XPathCompiler.internalCompile(XPathCompiler.java:500)
at net.sf.saxon.s9api.XPathCompiler.compile(XPathCompiler.java:472)
at xpathtest.SaxonTest.test(SaxonTest.java:26)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)
Caused by: net.sf.saxon.trans.XPathException: Undeclared variable in XPath expression: $a
at net.sf.saxon.sxpath.IndependentContext.bindVariable(IndependentContext.java:458)
at net.sf.saxon.expr.parser.XPathParser.parseVariableReference(XPathParser.java:1905)
at net.sf.saxon.expr.parser.XPathParser.parseBasicStep(XPathParser.java:1703)
at net.sf.saxon.expr.parser.XPathParser.parseStepExpression(XPathParser.java:1638)
at net.sf.saxon.expr.parser.XPathParser.parseRelativePath(XPathParser.java:1564)
at net.sf.saxon.expr.parser.XPathParser.parsePathExpression(XPathParser.java:1529)
at net.sf.saxon.expr.parser.XPathParser.parseSimpleMappingExpression(XPathParser.java:1541)
at net.sf.saxon.expr.parser.XPathParser.parseUnaryExpression(XPathParser.java:1419)
at net.sf.saxon.expr.parser.XPathParser.parseUnaryExpression(XPathParser.java:1390)
at net.sf.saxon.expr.parser.XPathParser.parseExprSingle(XPathParser.java:511)
at net.sf.saxon.expr.parser.XPathParser.parseFunctionArgument(XPathParser.java:2629)
at net.sf.saxon.expr.parser.XPathParser.parseFunctionCall(XPathParser.java:2371)
at net.sf.saxon.expr.parser.XPathParser.parseBasicStep(XPathParser.java:1723)
at net.sf.saxon.expr.parser.XPathParser.parseStepExpression(XPathParser.java:1638)
at net.sf.saxon.expr.parser.XPathParser.parseRelativePath(XPathParser.java:1564)
at net.sf.saxon.expr.parser.XPathParser.parsePathExpression(XPathParser.java:1529)
at net.sf.saxon.expr.parser.XPathParser.parseSimpleMappingExpression(XPathParser.java:1541)
at net.sf.saxon.expr.parser.XPathParser.parseUnaryExpression(XPathParser.java:1419)
at net.sf.saxon.expr.parser.XPathParser.parseExprSingle(XPathParser.java:511)
at net.sf.saxon.expr.parser.XPathParser.parseExpression(XPathParser.java:450)
at net.sf.saxon.expr.parser.XPathParser.parse(XPathParser.java:386)
at net.sf.saxon.expr.parser.ExpressionTool.make(ExpressionTool.java:102)
at net.sf.saxon.sxpath.XPathEvaluator.createExpression(XPathEvaluator.java:153)
at net.sf.saxon.s9api.XPathCompiler.internalCompile(XPathCompiler.java:497)
... 25 more

Answer 1

Firstly, it should be abs(-$a). Note the $.
To provide parameters you can't use the "short-cut" method XPathCompiler.evaluate(). You have to do it the long way:
XPathSelector exp = xpath.compile("abs(-$a)").load();
exp.setVariable(new QName("a"), new XdmAtomicValue(17));
XdmItem result = exp.evaluateSingle();

Answer 2

I know this thread is old but I ran into this problem recently, and spent almost 2 hours searching for answers. Finally, I got this working with Saxon-HE 9.4. I kept getting an error from compile, in spite of declaring the external variable until I also called:
xpath.setAllowUndeclaredVariables(true);