From: Joel VanderWerf <vjoel@...>
Date: 2003-06-03T16:16:55+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

And, a variant that allows nested contexts to be cumulative, at the cost 
of an additional outer wrapper around code that uses it.

   def with_context(params)
     finder, = catch(:context) { yield }
     finder.call(params)  if finder
   end

   def find_in_context(name)
     callcc do |again|
       throw(:context, [proc {|params|
         if params.has_key?(name)
           again.call(params[name])
         else
           again.call(find_in_context(name))
         end
       }, name])
     end
   end

   def use_dynamic_biding
     attempt, name = catch(:context) { yield }
     if attempt
       raise "Can't find context value for #{name}"
     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


   use_dynamic_biding do
     with_context(:name => 'dave', :color => 'red') do
       with_context(:foo => 'bar') do
         update_screen
       end
     end
   end