Saxon

I have managed to integrate the CoffeeSacks library into my XML Workbench using Saxon-HE 11.3 Java in the backend.

While trying to use the extension functions provided from pure XPath without any namespace declarations present I found that Saxon raises no error on e.g. the expression

function-lookup(QName('http://nineml.com/ns/coffeesacks',  'grammar-string'), 1)

for the extension function being declared in https://github.com/nineml/coffeesacks/blob/main/src/main/java/org/nineml/coffeesacks/GrammarStringFunction.java#L31 as having one or two arguments

public class GrammarStringFunction extends CommonDefinition {
    private static final StructuredQName qName =
            new StructuredQName("", "http://nineml.com/ns/coffeesacks", "grammar-string");
    private URI baseURI = null;

    public GrammarStringFunction(Configuration config, ParserCache cache) {
        super(config, cache);
    }

    @Override
    public StructuredQName getFunctionQName() {
        return qName;
    }

    @Override
    public int getMinimumNumberOfArguments() {
        return 1;
    }

    @Override
    public int getMaximumNumberOfArguments() {
        return 2;
    }

    @Override
    public SequenceType[] getArgumentTypes() {
        return new SequenceType[]{SequenceType.SINGLE_STRING, SequenceType.OPTIONAL_ITEM};
    }

but in the end returns for that expression a function Q{http://nineml.com/ns/coffeesacks}grammar-string#2, i.e. a function expecting two arguments, and consequently raises an error when trying to call the function with only one argument e.g.

function-lookup(QName('http://nineml.com/ns/coffeesacks',  'grammar-string'), 1)("s = 'a', 'b', a'.")

gives

Error(s) during XPath evaluation: Number of arguments required for dynamic call to grammar-string is 2; number supplied = 1

Is that a restriction of function-lookup and dynamic function call with extension functions?

Or should function-lookup(QName('http://nineml.com/ns/coffeesacks', 'grammar-string'), 1) give me the one argument version?

Or does the extension function need a different implementation to allow the use with function-lookup for both the two and the one argument version?