Getting the typename of a polymorphic function's magical $0 variable

Hello,

I've written the following function:

CREATE OR REPLACE FUNCTION indifferent_cast(literal_value text,
type_specification anyelement) RETURNS anyelement AS
$function_body$
-- This function is used when writing migrating scripts and the like.
-- It attempts to cast to the datatype specified by
"type_specification", but, in the event of the cast
-- being unsuccessfuly, it swallows the error and returns NULL

-- example of usage:
-- select indifferent_cast('5391502794050'::text, NULL::gtin)

-- It's useful for migrating data from legacy systems without
appropriate integrity constraints, where a small
-- minority of a particularly field of data will not cast to a desired
datatype (particularly a domain with a check constraint),
-- and you're quite happy to lose this data rather than relax your
constraints or manually correct the legacy data, or you cannot
-- be reasonably expected to correct the malformed data ( as, perhaps,
in the case of a malformed GTIN barcode)

-- It's called indifferent_cast for a reason

-- Clearly, creating a regular cast through CREATE CAST isn't
appropriate for this

DECLARE

BEGIN
    $0 := $1;
    return $0;

EXCEPTION
            WHEN others THEN
                RAISE NOTICE 'Failed to perform indifferent_cast';
                RETURN NULL;

END;

$function_body$
LANGUAGE 'plpgsql' IMMUTABLE

I would like to be able to RAISE a more appropriate, business domain
level notice, such as 'could not validate barcode' or 'could not
validate e-mail address', based on a CASE statement that checks the
dynamic type of $0 against some likely candidates for my application.

Is it possible to do this? How?

Thanks,
Peter Geoghegan