From: Mauricio Fernandez <mfp@...>
Date: 2006-07-09T17:15:01+09:00
Subject: Re: instance_exec

On Sun, Jul 09, 2006 at 03:09:52PM +0900, ara.t.howard@noaa.gov wrote:
> >Mauricio Fernandez's looks better though --think I'll have to rip it
> >for Facets ;-)
> 
> i just came across a bug in it (it's not re-entrant) - here's the fix:
> 
[#instance_exec implementation] 

The example you gave does work with my implementation too:

  RUBY_VERSION                                   # => "1.8.5"
  RUBY_RELEASE_DATE                              # => "2006-06-24"
  class Object
    module InstanceExecHelper; end
    include InstanceExecHelper
    def instance_exec(*args, &block)
      mname = "__instance_exec_#{Thread.current.object_id.abs}_#{object_id.abs}"
      InstanceExecHelper.module_eval{ define_method(mname, &block) }
      begin
        ret = send(mname, *args)
      ensure
        InstanceExecHelper.module_eval{ remove_method(mname) } rescue nil
                                                      #        ==========
                                                      #       thx to this
      end
      ret
    end
  end

  a = []

  a.instance_exec 42 do |x|
    instance_exec x do |y|
      push y
      p size
    end
  end
 
  a                                              # => [42]
  a.size                                         # => 1
# >> 1


I had actually a test for that (see test_instance_exec_nested below). Do you
have any additional assertions handy?
(So far test_instance_exec_with_frozen_obj fails in all the other
implementations I've seen.) 

  require 'test/unit'
  class TestInstanceEvalWithArgs < Test::Unit::TestCase
    def test_instance_exec
      # Create a block that returns the value of an argument and a value
      # of a method call to +self+.  
      block = lambda { |a| [a, f] }
  
      assert_equal [:arg_value, :dummy_value], 
        Dummy.new.instance_exec(:arg_value, &block)
    end
  
    def test_instance_exec_with_frozen_obj
      block = lambda { |a| [a, f] }
  
      obj = Dummy.new
      obj.freeze
      assert_equal [:arg_value, :dummy_value],
        obj.instance_exec(:arg_value, &block)
    end
  
    def test_instance_exec_nested
      i = 0
      obj = Dummy.new
      block = lambda do |arg|
        [arg] + instance_exec(1){|a| [f, a] }
      end
  
      # the following assertion expanded by the xmp filter automagically from:
      # obj.instance_exec(:arg_value, &block) #=>
      assert_equal([:arg_value, :dummy_value, 1], obj.instance_exec(:arg_value, &block))
    end
  end


-- 
Mauricio Fernandez  -   http://eigenclass.org   -  singular Ruby