From: Tim Hunter <cyclists@...>
Date: 2006-03-21T22:34:00+09:00
Subject: Re: Initializing ivars from block without prefixing with self?

Dave Baldwin wrote:
> I am creating a DSL using Ruby.  A cut down version is below:
> 
> class _Box
>     attr_accessor        :width
>       
>      def initialize (&block)
>                self.width = 20            # some default value
>         instance_eval(&block) if block_given?
>     end
> end
> 
> # Helper function to avoid the DSL user having to use .new
> def Box(&block)
>     _Box.new(&block)
> end
> 
> I  want to do something like:
> 
>     Box {width = 10}
> 
> but the only way I can get it to work is
> 
>     Box  {self.width = 10}
> 
> Having to prefix the ivar with self.  makes the DSL look clunky and  
> wouldn't be acceptable to my users.
> 
> Is there anyway to avoid the self.?  Using {@width = 10} will work  but 
> prevents some necessary validation from taking place.
> 
> 
> I have an alternative where width is a method:
> 
> class _Box
>     def width (val = :no_param)
>         if val != :no_param
>             @width = val
>         end
>         @width
>     end
> end
> 
> so with this I can write
> 
>     Box {width 10}
> 
> which is acceptable from the DSL view point, but makes the getter  
> operations more expensive.  In the real version a getter operation  
> takes about 7 statements to allow for evaluation of block if the ivar  
> had been set to a Proc object and inheritance of values from its  parent 
> object (in a visual hierarchy).  The setter operation is often  only 
> done when the object is created but the getter is done very  frequently 
> so I want to move down the route of separate setters and  getters rather 
> than combining them in the one method as an optimization.
> 
> Dave.
> 
> 
> 
Good article on DSLs and addresses your question:
http://www.artima.com/rubycs/articles/ruby_as_dsl.html