On Thu, Feb 09, 2006 at 07:33:53AM +0900, Jim Weirich wrote:
> I'm prototyping a DSL and came across a situtation where I have a lambda 
> that I would like to give to instance_eval, but the lambda takes 
> arguments.  Instance_eval will not supply any arguments when evaluating 
> the lambda and I don't see a straightforward way around this.
> 
> Any suggestions?

This is what 1.9's Object#instance_exec does:
 http://eigenclass.org/hiki.rb?Changes+in+Ruby+1.9#l10

One way to implement it in 1.8:


# The following script was annotated with my xmp filter
#  http://eigenclass.org/hiki.rb?Enhanced+xmp+code+evaluation+and+annotation
#
class Object
  def instance_exec(*args, &block)
    mname = "__instance_exec_#{Thread.current.object_id.abs}"
    class << self; self end.class_eval{ define_method(mname, &block) }
    begin
      ret = send(mname, *args)
    ensure
      class << self; self end.class_eval{ undef_method(mname) } rescue nil
    end
    ret
  end
end
    
# your test case
class Dummy
  def f
    :dummy_value
  end
end

def instance_eval_with_args(obj, *args, &block)
  # Magic goes here to evaluate +block+ in the scope of
  # +obj+, yet pass a list of argument values to the block.
  obj.instance_exec(*args, &block)
end

require 'test/unit'
class TestInstanceEvalWithArgs < Test::Unit::TestCase
  def test_instance_eval_with_args
    # Create a block that returns the value of an argument and a value
    # of a method call to +self+.  This is the basic functionality I need.
    block = lambda { |a| [a, f] }

    assert_equal [:arg_value, :dummy_value],
      instance_eval_with_args(Dummy.new, :arg_value, &block)
  end
end
__END__
# >> Loaded suite -
# >> Started
# >> .
# >> Finished in 0.000561 seconds.
# >> 
# >> 1 tests, 1 assertions, 0 failures, 0 errors

-- 
Mauricio Fernandez