Using pl/pgsql or sql for in/out functions for types

I was bored, so I decided to attempt to create a new type under postgres.
I figured a type for a social security number would be easy.  Sure enough,
to_char and to_number make this extremely easy.

CREATE FUNCTION ssn_in (char) RETURNS numeric(9) AS '
   SELECT to_number($1, \'000 00 0000\')
  ' LANGUAGE 'sql';


CREATE FUNCTION ssn_out(numeric(9,0)) RETURNS char AS '
   SELECT to_char($1, \'000-00-0000\')::char
  ' LANGUAGE 'sql';
blah=> CREATE TYPE ssn  (INPUT = ssn_in, OUTPUT = ssn_out);
ERROR:  TypeCreate: function 'ssn_in(opaque)' does not exist
blah=>

Since sql functions can't have opaque arguments, I decided to attempt to
reimplement the functions in plpgsql...

CREATE FUNCTION ssn_out(opaque) RETURNS char AS '
BEGIN
   RETURN SELECT to_char($1, \'000-00-0000\')::char;
END;
' LANGUAGE 'plpgsql';

CREATE FUNCTION ssn_in (opaque) RETURNS numeric AS '
BEGIN
 RETURN SELECT to_number($1, \'000 00 0000\');
END;
' LANGUAGE 'plpgsql';

Creating the type works.

blah=> CREATE TYPE ssn ( INPUT = ssn_in, OUTPUT=ssn_out);
CREATE

And then a table is created...

blah=> CREATE TABLE foobar (id int, bigbrother ssn);
CREATE

Now, the fun part is when it comes time to insert some data.

blah=> INSERT INTO foobar  values (1, '123-45-5555');
NOTICE:  plpgsql: ERROR during compile of ssn_in near line 0
ERROR:  plpgsql functions cannot take type "opaque"

Is there anyway to do this without having to resort to writing the
functions in C or some other language?  Why doesn't CREATE FUNCTION
complain about plpgsql functions not being able to accept the opaque type
as an argument?

Regards,
John Havard

http://www.sevensages.org/