create function(date,date,date,...)

Hi,

i am trying to declare function that gets six date arguments
and returns some status according actual date. It looks like
this:

CREATE FUNCTION datum_ok(DATE,DATE,DATE,DATE,DATE,DATE) RETURNS TEXT AS '
DECLARE
tests ALIAS FOR $1 ;
teste ALIAS FOR $2 ;
payments ALIAS FOR $3 ;
paymente ALIAS FOR $4 ;
interrupts ALIAS FOR $5 ;
interrupte ALIAS FOR $6 ;
today DATE ;
BEGIN
today := now()::date ;
.....

It happens that when some argument is NULL, than all argument get NULL.
It works correctly only when all arguments are not NULL.

SELECT
login,
datum_ok(
test_start, test_end,
payment_start, payment_end,
interrupt_start, interrupt_end )
FROM service ;

How can I mix NULL and DATE values in arguments?

Thanks

Radek Kanovsky, rk(at)uh(dot)cz