From: Joel VanderWerf <vjoel@...>
Date: 2003-06-03T16:04:08+09:00
Subject: Re: Dynamically scoped variables

Dave Thomas wrote:
> Just as a bit of fun I posted a piece on implementing dynamically scoped 
> variables in Ruby at
> 
>    http://pragprog.com/pragdave/Tech/Random/StackContext.rdoc,v

Very cool!

Here's a variant that lets you access local variables from nested 
dynamic scopes, though I'm not sure that's a good idea :)

   def with_current_context
     finder = catch(:context) { yield }
     finder.call(Proc.new.binding) if finder
       ## ? Is there an easier way to get caller's binding?
   end

   def find_in_context(name)
     callcc do |again|
       throw(:context,
       proc {|b|
         begin
           val = eval(name, b)
           again.call(val)
         rescue NameError
           raise "Can't find context value for #{name}"
         end
       })
     end
   end

   def update_widget
     name = find_in_context("name")
     color = find_in_context("color")
     puts "<#{color}>#{name}</#{color}>"
   end

   def update_screen
     update_widget
   end

   name = 'dave'; color = 'red'
   with_current_context do
     update_screen
   end