Dynamic binding in plpgsql function

Hi,

I would like to write a generic plpgsql function with a text parameter being a callback function name so that my general function can call this callback function. e.g.:

CREATE OR REPLACE FUNCTION ST_MyCallbackFunction(y int)
RETURNS int AS $$
DECLARE
BEGIN
RETURN someCalculationBasedOnY;
END;
$$ LANGUAGE 'plpgsql';

CREATE OR REPLACE FUNCTION ST_MyGeneralFunction(callback text)
RETURNS SETOF geomval AS $$
DECLARE
x integer;
y integer;
BEGIN
y := somecalculation;
x := 'callback'(y); --This is what I need
RETURN x;
END;
$$ LANGUAGE 'plpgsql';

I don't want to do an EXECUTE statement since I have no table to put after the FROM clause. I want to assign the resulting value directly to a variable like in my example.

Can I/How can I achieve this?

Thanks,

Pierre