Generally, when you specify a function the scale/precision/size of the return datatype is undefined.

For example, you say FUNCTION show_price RETURN NUMBER or FUNCTION show_name RETURN VARCHAR2.

You are not allowed to have FUNCTION show_price RETURN NUMBER(10,2) or FUNCTION show_name RETURN VARCHAR2(20), and the function return value is unrestricted. This is documented functionality.

Now, I get an precision error (ORA-01873) if I push 9999 hours (about 400 days) into the following. The limit is because the default days precision is 2

DECLARE
  v_int INTERVAL DAY (4) TO SECOND(0);
  FUNCTION hhmm_to_interval return INTERVAL DAY TO SECOND IS
    v_hhmm INTERVAL DAY (4) TO SECOND(0);
  BEGIN
    v_hhmm := to_dsinterval('PT9999H');
    RETURN v_hhmm;
    --
  END hhmm_to_interval;
BEGIN
  v_int := hhmm_to_interval;
end;
/

and it won't allow the precision to be specified directly as part of the datatype returned by the function.

DECLARE
  v_int INTERVAL DAY (4) TO SECOND(0);
  FUNCTION hhmm_to_interval return INTERVAL DAY (4) TO SECOND IS
    v_hhmm INTERVAL DAY (4) TO SECOND(0);
  BEGIN
    v_hhmm := to_dsinterval('PT9999H');
    RETURN v_hhmm;
    --
  END hhmm_to_interval;
BEGIN
  v_int := hhmm_to_interval;
end;
/

I can use a SUBTYPE

DECLARE
  subtype t_int is INTERVAL DAY (4) TO SECOND(0);
  v_int INTERVAL DAY (4) TO SECOND(0);
  FUNCTION hhmm_to_interval return t_int IS
    v_hhmm INTERVAL DAY (4) TO SECOND(0);
  BEGIN
    v_hhmm := to_dsinterval('PT9999H');
    RETURN v_hhmm;
    --
  END hhmm_to_interval;
BEGIN
  v_int := hhmm_to_interval;
end;
/

Any drawbacks to the subtype approach ?

Any alternatives (eg some place to change a default precision) ?

Working with 10gR2.