From: Joel VanderWerf <joelvanderwerf@...>
Date: 2010-09-15T05:45:55+09:00
Subject: Re: Declarative relations between object attributes

On 09/13/2010 11:46 PM, Knut Franke wrote:
> Thanks for the link; I managed to miss that one when trying to google
> existing solutions. :-) I've taken the liberty to include some of the
> ideas in my code.

:)

> class Model
>     cell :name, :email, :role, :caption
>     def initialize
>        self.name = "Your Name"
>        self.email = "user@example.com"
>        self.role = "To"
>        calculate(:caption) { "#{self.role}: #{self.name}<#{self.email}>"
> }
>     end
> end

I was going to say: why not just do

     def self.caption
       "#{self.role}: #{self.name} <#{self.email}>"
     end

but the calculate method makes it possible to observe(:caption), right?

How does #observe know that caption depends on #role, #name, and #email? 
It seems difficult to determine (efficiently) whether to call the 
caption observers when one of the cells changes. The fact that caption 
depends on role, name, and email (but no others) is embedded opaquely in 
a proc. Maybe I should just read your code ;)