From: Sean O'Halpin <sean.ohalpin@...>
Date: 2008-05-10T10:04:28+09:00
Subject: Re: Method lookup from a Lambda

On Fri, May 9, 2008 at 11:01 PM, John Conti <john@john-conti.com> wrote:
>>
>>          "Let's " + instance_eval(&grunt) + " it"
>
> Is there a way to pass arguments to &grunt during the invocation?  The
> call method allows arguments to be passed to the lambda's that expect
> them.  While my example does not show such a lambda, this is a feature I
> currently require.  If grunt did require arguments, how could I pass
> them?

Hi,

You need instance_exec, which is native in 1.9 but has to be
implemented in Ruby in 1.8.
The version below is the best one I have to hand. (There may be a
better one on Mauricio's blog but eigenclass.org is coming up with a
hiki error at the moment so I can't check).

Regards,
Sean

#
# instance_exec for ruby 1.8 by Mauricio Fernandez
# http://eigenclass.org/hiki.rb?bounded+space+instance_exec
# thread-safe and handles frozen objects in bounded space
#
class Object
  module InstanceExecHelper; end
  include InstanceExecHelper
  def instance_exec(*args, &block)
    begin
      old_critical, Thread.critical = Thread.critical, true
      n = 0
      methods = InstanceExecHelper.instance_methods
      # this in order to make the lookup O(1), and name generation O(n) on the
      # number of nested/concurrent instance_exec calls instead of O(n**2)
      table = Hash[*methods.zip(methods).flatten]
      n += 1 while table.has_key?(mname="__instance_exec#{n}")
    ensure
      Thread.critical = old_critical
    end
    InstanceExecHelper.module_eval{ define_method(mname, &block) }
    begin
      ret = send(mname, *args)
    ensure
      InstanceExecHelper.module_eval{ remove_method(mname) } rescue nil
    end
    ret
  end
end