From: Dave Baldwin <dave.baldwin@3dlabs.com>
Date: 2006-03-21T20:51:10+09:00
Subject: Initializing ivars from block without prefixing with self?

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.