extension function using saxon s9api - saxon

Question

I am trying to add an extension function, but is failing with :
Caused by: net.sf.saxon.trans.XPathException: Unknown system function follow()
at net.sf.saxon.expr.parser.XPathParser.grumble(XPathParser.java:282)
I see (in debug) that function registered with the integrated library. I was expecting saxon to look for the function in the integrated library but it is searching in system functions and throwing error. What is causing this function to be represented as a system function.
I am using the following :
<dependency>
<groupId>net.sf.saxon</groupId>
<artifactId>Saxon-HE</artifactId>
<version>9.7.0-14</version>
</dependency>
Thank you
my code is
import net.sf.saxon.expr.XPathContext;
import net.sf.saxon.lib.ExtensionFunctionCall;
import net.sf.saxon.lib.ExtensionFunctionDefinition;
import net.sf.saxon.om.Sequence;
import net.sf.saxon.om.StructuredQName;
import net.sf.saxon.s9api.Processor;
import net.sf.saxon.s9api.XPathCompiler;
import net.sf.saxon.s9api.XPathExecutable;
import net.sf.saxon.trans.XPathException;
import net.sf.saxon.value.SequenceType;
public class FollowTest {
public static void main(String[] args) throws Exception {
new FollowTest().test();
}
private void test () throws Exception {
Processor proc = new Processor(false);
proc.registerExtensionFunction(new Follow());
XPathCompiler xx = proc.newXPathCompiler();
XPathExecutable x = xx.compile("follow(/a/b/c)/type='xyz'");
}
public class Follow extends ExtensionFunctionDefinition {
#Override
public StructuredQName getFunctionQName() {
return new StructuredQName("", "http://example.com/saxon-extension", "follow");
}
#Override
public int getMinimumNumberOfArguments() {
return 1;
}
#Override
public int getMaximumNumberOfArguments() {
return 1;
}
#Override
public SequenceType[] getArgumentTypes() {
return new net.sf.saxon.value.SequenceType[] {SequenceType.SINGLE_STRING,};
}
#Override
public SequenceType getResultType(SequenceType[] suppliedArgumentTypes) {
return SequenceType.NODE_SEQUENCE;
}
#Override
public boolean trustResultType() {
return true;
}
#Override
public boolean dependsOnFocus() {
return false;
}
#Override
public boolean hasSideEffects() {
return false;
}
#Override
public ExtensionFunctionCall makeCallExpression() {
return null;
}
private class followCall extends ExtensionFunctionCall {
#Override
public Sequence call(XPathContext context, Sequence[] arguments) throws XPathException {
return null;
}
}
}
}

Answer 1

In the XPath expression you have written
follow(/a/b/c)
A function name with no namespace prefix is assumed to be in the default namespace for functions, which by default is the system function namespace http://www.w3.org/2005/xpath-functions. You need to use a prefix that's bound to the URI appearing in the extension function definition, namely http://example.com/saxon-extension