Optional argument in PL/pgSQL function

I am trying to write a PL/pgSQL function with optional arguments. It performs a query based on a filtered set of records (if specified), otherwise performs a query on the entire data set in a table.

For example (PSEUDO CODE):

CREATE OR REPLACE FUNCTION foofunc(param1 integer, param2 date, param2 date, optional_list_of_ids=[]) RETURNS SETOF RECORD AS $$
    IF len(optional_list_of_ids) > 0 THEN
        RETURN QUERY (SELECT * from foobar where f1=param1 AND f2=param2 AND id in optional_list_of_ids);
    ELSE
        RETURN QUERY (SELECT * from foobar where f1=param1 AND f2=param2);
    ENDIF
$$ LANGUAGE SQL;

What would be the correct way to implement this function?

As an aside, I would like to know how I could call such a function in another outer function. This is how I would do it - is it correct, or is there a better way?

CREATE FUNCTION foofuncwrapper(param1 integer, param2 date, param2 date) RETURNS SETOF RECORD AS $$
BEGIN
   CREATE TABLE ids AS SELECT id from foobar where id < 100;
   RETURN QUERY (SELECT * FROM foofunc(param1, param2, ids));
END
$$ LANGUAGE SQL