From: "ara.t.howard" <ara.t.howard@...>
Date: 2008-10-26T13:41:52+09:00
Subject: Re: Object#instance_eval but better


On Oct 24, 2008, at 11:55 AM, Richard Crowley wrote:

>  have a bit of code here that I'd love to make better:
>
> class Foo
>    def initialize(path)
>        self.instance_eval(File.read(path))
>    end
> end
>
> I don't want to continue down the path of reading an entire file  
> into a string and then eval'ing that string just to get the contents  
> of the file executed as if it were the body of a method in a class.   
> The file passed to the constructor above might contain something like:
>
> @foo = 'This will become an instance variable when you do  
> Foo.new(path)'
>
> I've tried all manner of hacks to make Kernel#load and friends  
> execute code as if it were in the calling scope.  In every case I've  
> seen the loaded file leave no lasting effect on the instance  
> variables of the calling method.  My goal is to be able to do  
> exactly this:


reverse your logic: don't force the code to eval inside a context -  
provide the context.




cfp:~ > cat a.rb
class Foo
   class << self
     attr_accessor :current

     def initializing &block
       caller = eval 'self', block

       if caller.is_a?(Foo)
         previous = current
         begin
           Foo.current = caller
           block.call
         ensure
           Foo.current = previous
         end
       else
         current.instance_eval &block
       end
     end
   end

   def initialize path
     Foo.initializing{ load path }
   end
end

foo = Foo.new('b.rb')
p foo.instance_variables




cfp:~ > cat b.rb
Foo.initializing do
   @foo = 42
end



cfp:~ > ruby a.rb
["@foo"]





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