From: Nicholas Van Weerdenburg <nick@...>
Date: 2004-11-26T05:54:47+09:00
Subject: Re: YAML question (hi _why!)

Hal,

Would this be appropriate for the YAML equivalent of DTDs?

In XML this can be handled by DTDs (and I assume schemas and Relax NG, 
but I'm not familiar with those). Oddly, I don't think it's a required 
part of the spec, so different parsers would or would not display the 
default attributes, say when rendering to a web page. I think IE showed 
them,  but Mozilla didn't which bummed me out.

It was very nice for hand-editing, saving a lot of typing, but sometimes 
awkward when I hadn't looked at the XML for a while, since it was hidden 
in the DTD, and not explicit in the document I was looking at.  An 
editor that showed defaults in a ghost-like overlay would have been 
helpful there. But overally, the saving of typing vastly outweighed the 
lack of explicitness, since I was authoring a lot of XML by hand. Plus 
the resulting common-case files were much simpler and easy to read.

If it specifically about Ruby serialization, it may make more sense to 
have a ruby library that hacks yaml, like you do. I'd hate to have to 
manage a dtd just for defaults- why not add it to the code? Also, then 
it would possible provide a common mechanism for other persistence 
mechanisms- XML, etc. (though maybe that's over generalizing). I suppose 
the downside of this is that yaml is not self-documenting then, which is 
an important/nice aspect, especially if you want to use the YAML in 
downsteam applications (other languages, style-sheet renderings, etc.). 
On the other hand, the code should be self-documenting, ideally.  But a 
dtd-type doc provides the documentation outside of the ruby context, 
such as for handediting. I guess that's the problem with multiple 
demands- it stresses the design :).

One thought...

require "yaml"
require "persistence/defaults"  # hacks YAML to insert/remove defaults

class Document
    default_value :author => "Anonymous Coward"   # explicit initial 
value doesn't easily equal a wanted default.
    ....
end

Nick

Hal Fulton wrote:

> _why,
>
> Just saw you post and was reminded to ask: How are we coming on a
> "good" solution to my default values problem?
>
> For those curious, what I want to do is establish defaults for
> fields in an object, such that:
>
>   1. When I dump an object, fields that still have the default
>      values will be left unspecified.
>   2. When I load an object, unspecified fields will be given their
>      default values.
>
> Right now I'm using an initialize_copy hack and some other stuff.
>
> Something like this can greatly compress the YAML, I've found.
> Very convenient when you read or hand-edit the file.
>
>
> Cheers,
> Hal
>
>