cursor return null

Hello,
in postgres function (id bigint ),the following code not return
any value with artNums ,But when i do select statement ony it's output the
values with out include cursor

i.e,cursor problem ..?

please let me know what should i do to getvalues from cursor, but i dont
where i missing

function (id bigint )

DECLARE
PartNums varchar (10000);
artNums CURSOR for
SELECT p.PART_NUM part_num
FROM lineitem sol, part p
WHERE sol.ORDER_ID = id AND p.PART_ID = sol.PART_ID;

BEGIN
FOR cPart IN artNums LOOP
BEGIN
PartNums := PartNums || cPart.part_num || ', ';
END;