From: "Ara.T.Howard" <Ara.T.Howard@...>
Date: 2005-06-02T23:28:58+09:00
Subject: how to create a variable with eval - solved! [WAS] Re: binding, ObjectSpace._id2ref [WAS] Re: [ANN] parseargs-0.0.0

On Thu, 2 Jun 2005, Zev Blut wrote:

> Hello,
>
> On Thu, 02 Jun 2005 05:30:28 +0900, Ara.T.Howard <Ara.T.Howard@noaa.gov> 
> wrote:
>
>> the problem here is that the variables will only be __available__ through 
>> eval
>> either.  eg. you'd have to
>> 
>>    p 'eval a'
>> 
>> and could never
>> 
>>    p a
>> 
>> i seem to recall a way of working around this but can't remember the 
>> thread...
>> if i can figure it out i'll add this feature.

> Wow I am quite surprised at this!

everyone is!  ;-)

> It is quite odd that adding a variable  through the use of eval, makes the
> variable appear in the local_variables method, but that you cannot access
> the variable unless you use eval... Below is a simpler example showing how
> using eval modifies the the local_variables response.
>
> ----------------------------------------------------------------------
> puts "[" + local_variables.join(" , ") + "]"
> #=> []
> eval 'a = 42'
> puts "[" + local_variables.join(" , ") + "]"
> #=> [a]
> puts a
> #=> undefined local variable or method `a' for main:Object (NameError)
> ----------------------------------------------------------------------
>
> If you cannot access a in your binding, why would eval add a to the
> local_variables?  Can anyone explain this?  I would like to know.

i didn't know about that, but it suggests a way to rig method_missing in
Object to make it __seem__ like a variable has been created in scope:

for example:

   harp:~ > cat a.rb
   require 'binding_of_caller'

   class Object
     def create_var name, env = nil
       Binding::of_caller do |bnd|
         eval("#{ name }=nil", env || bnd)
       end
     end
     def set_var name, value, env = nil
       Binding::of_caller do |bnd|
         eval("#{ name }=ObjectSpace::_id2ref(#{ value.object_id })", env || bnd)
       end
     end
     def get_var name, env = nil
       Binding::of_caller do |bnd|
         eval("#{ name }", env || bnd)
       end
     end
     alias __method_missing__ method_missing
     def method_missing(m,*a,&b)
       Binding::of_caller do |bnd|
         lv = eval 'local_variables', bnd
         name = m.to_s
         if lv.include? name
           return(get_var(name, bnd))
         else
           __method_missing__(m,*a,&b)
         end
       end
     end
   end

   set_var 'a', 42
   p a

   harp:~ > ruby a.rb
   42


wow!  it __can__ be done.  now someone try to break it please!  ;-)

-a
-- 
===============================================================================
| email :: ara [dot] t [dot] howard [at] noaa [dot] gov
| phone :: 303.497.6469
| My religion is very simple.  My religion is kindness.
| --Tenzin Gyatso
===============================================================================