From: Hal Fulton <hal9000@...>
Date: 2004-11-27T01:16:16+09:00
Subject: Re: YAML question (hi _why!)

why the lucky stiff wrote:
> Hal Fulton wrote:
> 
>> 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.
>>
> 
> Hi, Hal.  Good for you, bringing this up again.  We didn't finish this 
> discussion at all and the clock has struck loudly here: time to continue.

Bonjour and thanks for the reply!

Are you near an irc client, btw, since we seem to be talking in near-
realtime?

> In Hobix, I am currently using a mixin for classes which need the above. 
>  This mixin relies upon the presence of a property_map method within 
> those classes, which defines the full set of instance variables which I 
> want to output in YAML, along with whether those fields are optional or 
> not.

This property_map isn't clear to me yet. Why not class-level data instead
of instance?

I hadn't thought about the issue of required vs. optional. I would have
just said: If it doesn't have a default, and it's not there, default it
to nil.

Ordering is also interesting; I hadn't thought of that either.

> As you can see, the mixin simply defines a to_yaml_properties method, 
> which is a method understood by the YAML library.  The 
> to_yaml_properties method should return an Array of instance variable 
> names, in the order they are output.

Fine, clear enough.

> As you can see, the mixin also defines 'initialize', since you'll 
> probably want to apply the defaults even if the object is created 
> programmatically.

This confuses me greatly. What if a class already has an #initialize (as
most do?).

> You'll also need to hook yourself a YAML type for each class.  In the 
> case of the Hobix::Entry class, I'm hooking !hobix.com,2004/entry.

Argh. If I had to do this stuff every time, I'd almost rather keep using
the hack I'm using.

But I'll look at your examples. Maybe I just haven't seen the light yet.


Thanks much,
Hal