From: Robert Klemme <shortcutter@...>
Date: 2008-05-24T21:59:51+09:00
Subject: Re: How to model something


Some random thougths...

On 24.05.2008 13:08, Tobias Weber wrote:
> my programm needs to store persons. They have a weight and a gender. You 
> are only allowed to ask males for their weight, females will throw 
> dishes/exceptions. There *is* a method to change someones gender.
> 
> I could have 'if gender' in every accessor or use the state pattern. But 
> that has no state, so I'm confused. I would have to access the variable 
> in the wrapper class.

I am not sure what you mean here.  Here's one way to do it:

class Person
   GENDERS = {
     :male => Class.new do
       def gender; :male; end
       def weight_check; end
     end.new,
     :female => Class.new do
       def gender; :female; end
       def weight_check; raise "Go away!" end
     end.new,
   }.freeze

   def initialize(gender)
     self.gender = gender
   end

   def gender=(g)
     @gender = GENDERS[g] or raise "Illegal gender #{g.inspect}"
   end

   def gender; @gender.gender; end

   def weight=(w)
     raise IllegalArgumentException, "negative" if w < 0
     @weight = w
   end

   def weight; @gender.weight_check; @weight; end
end

> The programm can also shoot people into space, in which case the 
> implementation of weight changes but the same restrictions apply.
> The proxy pattern will help here, probably by having weight first call 
> delegate.weight, and if that isn't NoMethodException multiplying with 
> gravity.

Well, in that case #weight isn't just a simple accessor but it needs an 
argument (gravity).  You probably want to have an attribute "mass" which 
stores the proper value.

> Any advice or implementations of similar problems I could look at?
> 
> (and in reality people are TV series with 6 genders and 5 other 
> attributes, and space is just but a remote place to store things)

What are the other four genders? :-))

Kind regards

	robert