ExtFunCall.scala

import net.sf.saxon.functions.ExtensionFunctionCall
import net.sf.saxon.om.SequenceIterator
import net.sf.saxon.om.SingletonIterator.makeIterator
import net.sf.saxon.om.SingleNodeIterator
import net.sf.saxon.expr.XPathContext
import net.sf.saxon.om.StructuredQName
import net.sf.saxon.value.SequenceType.{SINGLE_STRING, SINGLE_ELEMENT_NODE}
import net.sf.saxon.value.StringValue

class ExtFunCall extends ExtensionFunctionCall {
    def call(args:Array[SequenceIterator], context:XPathContext) = makeIterator( new StringValue("hello") )
}

ExtFunDef.scala

import net.sf.saxon.functions.ExtensionFunctionDefinition
import net.sf.saxon.om.StructuredQName
import net.sf.saxon.value.SequenceType
import SequenceType.{SINGLE_STRING, SINGLE_ELEMENT_NODE}

class ExtFunDef extends ExtensionFunctionDefinition {
    val getFunctionQName = new StructuredQName("mp", "http://musicpath.org", "sparql")
    val getMinimumNumberOfArguments = 1
    //val getMaximumNumberOfArguments = 1
    val getArgumentTypes = List(SINGLE_STRING).toArray
    def getResultType(suppliedArgumentTypes:Array[SequenceType]) = SINGLE_ELEMENT_NODE
    def makeCallExpression = new ExtFunCall
}