Re: Status of Fix Domain Casting TODO

Jim C. Nasby wrote:
> On Mon, Jan 01, 2007 at 06:30:40PM -0600, Andrew Dunstan wrote:
>
>> Tom Lane wrote:
>>
>>> "Jim C. Nasby" <jim(at)nasby(dot)net> writes:
>>>
>>>> FWIW, I'm running into this trying to create a 'raw' domain that would
>>>> automagically convert hex strings into actual binary data for storage in
>>>> a bytea.
>>>>
>>> I think you've got 0 chance of implementing that as a domain rather than
>>> an independent type. Without or without revisions in the casting rules,
>>> a domain has not got its own I/O functions, and never will.
>>>
>> This might be less of an issue if we allowed such IO functions to be
>> written in a loadable PL rather than in C.
>>
>
> I'm confused... couldn't I just write a cast function? Or is that what's
> meant by I/O functions?
>
> And yes, in this case I should be able to accomplish what I'm looking
> for just using encode() and decode().
>

The I/O functions are set up by the INPUT and OUTPUT params of the
CREATE TYPE statement. They convert to and from the type 'cstring'. If
you want to change the way a piece of data is read/produced (e.g.
automatically encode/decode the value) these are what you would need. A
domain is in effect a constrained type. But it inherits the I/O
functions of its base type. But constraints are not what you want - you
want to deal with representation, which is the property dealt with by
I/O functions - their fundamental purpose is to convert between external
and internal representation.

HTH

cheers

andrew