From: Brian Candler <b.candler@...>
Date: 2010-06-21T19:41:01+09:00
Subject: Re: Module vs Binding Evaluation

Thomas Sawyer wrote:
> I am confused by the difference between using module_eval on a module,
> vs. using eval on a binding extracted from a module.
> 
> Have a look at this gist:
> 
>   http://gist.github.com/445263

This looks rather odd to me: (1) you are subclassing Module, (2) you are 
creating instances of [this subclass of] Module, and (3) you are 
extending each instance of this class/module with itself. These seem to 
be unusual things to do, although I'm sure you have a good reason.

If I rewrite the code to avoid this usage, it all looks OK:

# Sample 1

module M1
  def self.__binding__
    @binding ||= binding
  end
end

M1.module_eval <<-END
  X = 10
  def self.x; 10; end
END

raise unless M1.x  == 10 rescue puts "M1.x  " + $!
raise unless M1::X == 10 rescue puts "M1::X " + $!


# Sample 2

module M2
  def self.__binding__
    @binding ||= binding
  end
end

eval(<<-END, M2.__binding__)
  X = 10
  def self.x; 10; end
  def self.y; 20; end
END

raise unless M2.x  == 10 rescue puts "M2.x  " + $!
raise unless M2.y  == 20 rescue puts "M2.y  " + $!
raise unless M2::X == 10 rescue puts "M2::X " + $!


My simplistic understanding is that there are two sorts of object 
context in Ruby: the "current object", which is visible as 'self', and 
the "current class", which is pretty well hidden but it's where 'def' 
defines new instance methods, and is set by 'class' and 'class_eval'.


class C
end

C.send(:class_eval) do
  def foo
    puts "In foo"
  end
end

C.foo      rescue puts "foo is not a class method"
C.new.foo

C.send(:instance_eval) do
  def bar
    puts "In bar"
  end
end

C.bar
C.new.bar  rescue puts "bar is not an instance method"


However I don't understand this fully, because I can't see how your code 
manages to define singleton methods on s2, just by using eval and a 
binding. If I try a simpler example, it doesn't:


eval(<<EOS, C.send(:binding))
  def baz
    puts "In baz"
  end
EOS

baz
C.baz      rescue puts "baz is private global method"
C.new.baz  rescue puts "baz is not an instance method"
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