From: Simon Kitching <simon@...>
Date: 2003-11-05T09:38:16+09:00
Subject: Managing metadata about attribute types

Hi,

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.

However some of the very nice features of this module depend upon being
able to introspect a class to find what attributes it has, and what
their datatypes are.

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...

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 :-)

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

In the java version, when the "cost" attribute is encountered in the xml
input, it is seen that the target class has a setCost(float) method, so
the string "12.50" is converted to a float before invoking the setCost
method.

I want to achieve the same effect in the Ruby version. I do *not* want
to effectively invoke this in ruby:
  stock_item.cost=('12.50') # string passed


Anyone have any references to "pre-existing art"???

Thanks,

Simon