Bind a column default value to a function in SQL 2005

DOC_ID  DOC_Order DOC_Name
   1       1        aaa
   2       3        bbb
   3       2        ccc

CREATE FUNCTION [dbo].[NEWDOC_Order]
(
)
RETURNS int
AS
BEGIN

RETURN (SELECT MAX(DOC_ORDER) + 1 FROM DOC_Documents)

END

alter table DOC_Order
add constraint
df_DOC_Order
default([dbo].[NEWDOC_Order]())
for DOC_Order

Create FUNCTION [dbo].[NEWDOC_Order]
(
)
RETURNS int
AS
BEGIN

RETURN (SELECT ISNULL(MAX(DOC_ORDER),0) + 1 FROM DOC_Documents)

END

[dbo].[NEWDOC_Order]()