From: Robert Dober <robert.dober@...>
Date: 2007-05-20T06:22:53+09:00
Subject: Re: Block Trouble (Binding Issue?)

On 5/19/07, Joel VanderWerf <vjoel@path.berkeley.edu> wrote:
> Pieter V. wrote:
> > I'm presently trying to work on a little metaprogramming project, but
> > am finding my efforts a little frustrated by ... well, I'm not rightly
> > sure what by.  It seems to be a scoping issue, from what I can tell.
>
> Yup, it sure is. define_method(name, &block) changes the scope of block
> to the instance, but the yield still references a block in class Foo scope.
>
> A fix might be to replace yield with instance_eval(&b), as below, but
> that will preclude passing arguments to the block, in case you ever need
> to do that. Google for instance_exec for a discussion, hacks for the
> present, and hope for the future (1.9 has it)...

As we just had a discussion about it maybe you might be interested in
this, it comes from Mauricio Fernandez' Eigenclass. Hopefully he does
not mind that I post this slightly modified version. The original can
be found here http://eigenclass.org/hiki/bounded+space+instance_exec
there is very good stuff over there.
Especially as you are interested in metaprogramming.
All I added to Mauricio's code is to remove the temporary method
*before* calling it, thus avoiding a theoretical possibility of
endless recursion in the temporary method call.

class Object
 include InstanceExecHelper = Module.new
 def instance_exec(*args, &block)
   begin
     old_critical, Thread.critical = Thread.critical, true
     n = 0
     n += 1 while respond_to?(mname="__instance_exec#{n}")
     InstanceExecHelper.module_eval{ define_method(mname, &block) }
     mthd = method(mname)
     InstanceExecHelper.module_eval{ remove_method(mname) } rescue nil
   ensure
     Thread.critical = old_critical
   end
   mthd.call *args
 end
end
.
HTH
Robert

-- 
You see things; and you say Why?
But I dream things that never were; and I say Why not?
-- George Bernard Shaw