From: Joel VanderWerf <vjoel@...>
Date: 2008-03-11T15:23:08+09:00
Subject: Re: setting local variables via eval

ara howard wrote:
>   http://drawohara.com/post/28514698

Nice hack, as usual :)

Isn't

   class Object
     def scope
       lambda{  }
     end
   end

enough? You don't need to return binding from the lambda, AFAICT. The 
lambda never gets called.

Also, an alternative to mucking with ObjectSpace:

         Thread.current[:val] = v
         definition = "#{ var } = Thread.current[:val]"

in place of

         value = "ObjectSpace._id2ref #{ v.object_id }"
         definition = "#{ var } = #{ value }"

Of course, the value of Thread.current[:val] should be saved and 
restored. Maybe this makes the hack jruby friendly.

There is one case in which the double_quoted_heredoc is not hygienic, 
when a line of @template begins with __template__:

   view2 = View.new '__template__'
   view3 = View.new  view2.render
   puts view3.render

But I don't see any hygienic solution (indenting and using <<- doesn't 
work, because the interpolated strings might have line breaks). Maybe 
ruby needs a heredoc variant that runs to eof.

With these changes, your code becomes:

   class Object
     def scope
       lambda{  }
     end
   end

   class View
     def initialize template
       @template = template
     end

     def render locals = {}
       old_val = Thread.current[:val]
       context = scope
       locals.each do |k, v|
         var = k
         Thread.current[:val] = v
         definition = "#{ var } = Thread.current[:val]"
         eval definition, context
       end
       double_quoted_heredoc = ['<<__template__', @template, 
'__template__'].join("\n")
       eval double_quoted_heredoc, context
     ensure
       Thread.current[:val] = old_val
     end
   end

   view = View.new '<body> #{ x + y } </body>'

   puts view.render(:x => 40, :y => 2)  #=> <body> 42 </body>


-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407