From: Austin Ziegler <austin@...>
Date: 2003-11-05T13:09:40+09:00
Subject: Re: Managing metadata about attribute types

On Wed, 5 Nov 2003 09:38:16 +0900, Simon Kitching wrote:
> I'm porting the Apache Jakarta Commons Digester (written in Java)
> to Ruby at the moment. This module processes xml in a rules-based
> manner. It is particularly useful for handling complex xml
> configuration files.

You may want to note xml-configfile in addition to XMLDigester that
James Britt mentioned.
  http://raa.ruby-lang.org/list.rhtml?name=xml-configfile

> Finding attributes on a Ruby class is simple (just look for
> "attr=" methods). Unfortunately, determining what object types it
> is valid to assign to that attribute is not so simple...

Aside from Ryan Pavlik's StrongTyping module, I'm not sure that this
is absolutely necessary. See below for a bit more information.

> I was wondering if there were any other Ruby projects which have
> faced this problem and come up with solutions? I would rather
> steal a solution than invent one :-)

I frankly don't see a reason to worry about this.

> Example of problem:
> 
>   Input xml is:
> <stock> 
>   <stock-item name="spanner" cost="12.50"/> 
>   <stock-item name="screwdriver" cost="3.80"/> 
> </stock> 
> 
>   // java
> class StockItem {
>   public void setName(String name) {....}
>   public void setCost(float cost) {....}
> }

>   # Ruby
> class StockItem
>   attr_accessor :name
>   attr_accessor :cost
> end


Why not do:

  class StockItem
    attr_accessor :name # Defaults to String
    attr_reader   :cost # Returns cost
    def cost=(x)
      @cost = x.to_f
    end

This way, you don't have to care what the appropriate type is --
your type worries about it.

-austin
--
austin ziegler    * austin@halostatue.ca * Toronto, ON, Canada
software designer * pragmatic programmer * 2003.11.04
                                         * 22.46.52