using functions to generate custom error messages

[I just sent this using the wrong e-mail address twice (oops !),
so assuming it won't show up, here it comes again...]

Hello,

Would it be considered good practice to use functions to
generate custom error messages ?

The only odd thing about it I can see is that constraints have
to be coded into the function, whether or not they've already
been included in the table itself.

Here's an example.

CREATE TABLE amelie (
    rating    int CHECK (rating <= 10)
);


The results of using just this :

hypatia=# INSERT INTO amelie VALUES ('11');
ERROR:  ExecAppend: rejected due to CHECK constraint amelie_rating


Here is the function with the constraint included again (phrased
differently):

CREATE FUNCTION maxten () RETURNS OPAQUE AS '
     BEGIN
         IF NEW.rating > 10 THEN
             RAISE EXCEPTION ''Alert ! Please enter a number
between 0 and 10.'';
         END IF;

         RETURN NEW;
     END;
' LANGUAGE 'plpgsql';


And the trigger:

CREATE TRIGGER trigtest BEFORE INSERT OR UPDATE
     ON amelie
     FOR EACH ROW
     EXECUTE PROCEDURE maxten();


The results of this:

hypatia=# INSERT INTO amelie VALUES ('11');
ERROR:  Alert ! Please enter a number between 0 and 10.

My goal with this is to stick with the principle of keeping
"business logic" in the DBMS, avoiding elaborate error checking
and other unnecessary verbosity in my Python CGI scripts. I can
easily insert the function generated ERROR message into an xhtml
template and display it in the web browser.

I do understand that there are other ways/places to do error
checking, in the CGI script or in JavaScript, but this is
how/where I'd like to do it.

BTW, does anyone know why \df doesn't find the function "maxten"
that I created ?


hypatia=# \df maxten

                List of functions
  Result data type | Name | Argument data types
------------------+------+---------------------
(0 rows)


Cheers,
Joel