From: Joel VanderWerf <vjoel@...>
Date: 2004-01-11T16:05:19+09:00
Subject: Re: How to reproduce the tk dialect?

shasckaw wrote:
> Hello,
> when using tk, you can initialize tk objects with a block, example:
> 
> TkLabel.new(root) {
>   text  'Hello, World!'
>   pack  { padx 15 ; pady 15; side 'left' }
> }

Here's how Tk might do it, though I haven't studied the source. (I don't 
think there's any way around using #instance_eval.)

   class TkLabel
     def initialize(obj, &block)
       instance_eval(&block) if block
     end
     def text(str)
       @text = str
     end
     def pack(&block)
       @packing = Packer.new(&block)
     end
     attr_reader :packing
     class Packer
       def initialize(&block)
         instance_eval(&block) if block
       end
       def padx(x)
         @padx = x
       end
       def pady(y)
         @pady = y
       end
       def side(str)
         @side = str
       end
     end
   end

   root = nil

   label = TkLabel.new(root) {
     text  'Hello, World!'
     pack  { padx 15 ; pady 15; side 'left' }
   }

   p label

There are some drawbacks:

- inside the block, all private methods and instance variables are 
accessible

- you can't access from within the block attributes that are defined in 
the context outside the block:

   @x = 1
   pack {
     padx @x # this is a different @x
   }

   instead, however, you can use local variables to "pass in" the value:

   temp_x = @x = 1
   pack {
     padx temp_x
   }

An alternative is:

     class Packer
       def initialize(&block)
         block.call(self) # or use yield
       end
       ...
     end

which keeps the "self" context the same in the block, avoids exposing 
internal stuff, and gives you an object (passed to the block) which you 
can use to do the same methods (padx etc.). But it doesn't look as 
elegant...

   pack { |packer|
     packer.padx 15
   }

There are times when I wish ruby had some kind of hygienic macro... but 
then I think about reading/debugging code written in somebody's personal 
macrofied language...