Create Type with typmod_in

I'm trying to create a custom type (PostgreSQL 9.0) which will essentially auto truncate a string to a certain length.

The type (auto_trunc_char) will be used as follows:

Create table blah (
Some_name auto_trunc_char(40)
)

Can this be done purely in plpgsql?
If so, how?

I know I have to declare the following (at least) but I'm not sure how to flesh out the functions:

create type auto_trunc_char (
INTERNALLENGTH = VARIABLE,
INPUT = auto_trunc_char_in_function,
OUTPUT = auto_trunc_char_out_function,
TYPMOD_IN = auto_trunc_char_modifier_in
);

Regards,

Russell Keane

INPS

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