domain casting?

Hello

I try casting function for domains. I can do it, but it doesn't work.

betal=# create domain mydomain text;
CREATE DOMAIN
betal=#
betal=# create or replace function mydomain_todate(mydomain)
betal-# returns date as $$
betal$# select to_date($1, 'YYYYMMDD');
betal$# $$ language sql;
CREATE FUNCTION
betal=#
betal=# create cast (mydomain as date)
betal-# with function mydomain_todate(mydomain);
CREATE CAST
betal=#
betal=# select mydomain_todate('20070715777'::mydomain);
mydomain_todate
-----------------
2007-07-15
(1 row)

betal=#
betal=# select '20070715777'::mydomain::date;
ERROR: invalid input syntax for type date: "20070715777"
betal=#

Is possible casting domain?

Regards
Pavel Stehule

betal=# select version();
version
-------------------------------------------------------------------------------------------------------
PostgreSQL 8.3devel on i686-pc-linux-gnu, compiled by GCC gcc (GCC)
4.1.2 20070502 (Red Hat 4.1.2-12)
(1 row)