From: Michel Demazure <michel@...>
Date: 2009-08-22T02:58:14+09:00
Subject: Help : a scoping question

A meta-programming question. More precisely a scoping problem.

In short, is there a ruby way to execute a parameter-less block with a
given receiver ?

Here below my example : I am manipulating a pattern of rectangular boxes
in a plane, with a tree structure given by enclosure, and want to
describe them by a convenient DSL. I am keeping some parameters from the
real code to give a flavor of the example.


class Box

  HORIZ = 1
  VERT = 2
  FIXED = 4
  ADJUST = 8

  def initialize(enclosing, flag, columns = [], height = 0)
    @enclosing = enclosing
    enclosing << self if enclosing
    @enclosed = []
    ...
  end

  def <<(other)
    @enclosed << other
  end

  def box(params = {}, &block)
    newbox = Box.new(self, params[:opts], params[:colums] || [],
params[:height] || 0)
    block.call(newbox) if block
    newbox
  end

.....

end # class box

# duplication, part of the problem
def main_box(params ={}, &block)
  newbox = Box.new(nil, params[:opts], params[:colums] || [],
params[:height] || 0)
  block.call(newbox) if block
  newbox
end

big_box = main_box :opts => VERT do |s|
  s.box :opts => HORIZ, :colums => [1,2,3,4,5,6,7,8,9], :height => 10
  s.box :opts => HORIZ do |b|
    b.box :opts => VERT, :colums => [7,8,9], :height => 90
    b.box :opts => VERT do |z|
      z.box :opts => HORIZ | ADJUST, :colums => [1,2,3,4,5,6]
      z.box :opts => HORIZ do |y|
        y.box :opts => VERT | FIXED, :colums => [4,5,6], :height => 70
        y.box :opts => VERT do |x|
          x.box :opts => HORIZ| FIXED, :colums => [1,2,3], :height => 20
          x.box :opts => HORIZ | ADJUST,:colums => [1,2,3]
          x.box :opts => HORIZ | FIXED, :colums => [1,2,3], :height =>35
        end
      end
    end
  end
end

The local variables in the blocks could be all the same, and are useless
to the DSL, but they seem necessary to secure the scopes. My question is
: is there a way to write the method 'box' so that it accepts the
simplified DSL without variables :

... box params do
      box params do

Apologies for my English.

Michel Demazure
-- 
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