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I’ve noticed that some rubyists like to use indentation to delineate method visibility.

The first time I noticed this was when Marcel Molina Jr. used it during the charity testing workshop at the Ruby Hoedown.. I just encountered it again in at least one piece of sample code from Rob Orsini’s “Rails Cookbook.”

During the testing workshop, Chad Fowler expressed displeasure with this style, and I’ve got to agree.

Here’s an example of this style:

class Gadget
  def framilize
    razzlitize if options[:secret_ingredient_6X]
  end

  private
    def razzlitize
    end
end

The problem is that the indentation is “artificial”, there really is no nesting of evaluation scope here. The way that Module#private and it’s kin work when they are called without arguments is to set a marker onto Ruby’s eval stack which gets popped when the current evaluation context is exitted. So the effects of the private method remain in effect until either the end of the current context, or one of the other methods like public or protected, whichever comes first. This means that if we extend the above example:

class Gadget
  #...
  private
    def razzlitize
    end

  def dazzlitize
  end
end

The indentation can trick us into thinking that dazzlitize is public when it’s really private.

I’d always found the nature of methods like private without arguments to be a minor annoyance with Ruby syntax. In my humble opinion they should have been able to take a block which delineated their effect. What if we could write:

class Gadget
  #...
  private do
    def razzlitize
    end
  end

  def dazzlitize
  end
end

Well, actually we can write that, but it doesn’t work as expected, since the block is silently ignored, and the method doesn’t even get defined.

So I started doing a little metaprogramming, to fix this. It seemed obvious how to do this. You alias_method Module#private so you can extend it, redefine the method to check for a block and if none is given, call the original method, otherwise evaluate the block in a way which makes any methods defined private. And of course you use test driven design to do this.

I made some progress but ran into some problems:

1. The obvious way to evaluate the block:

   class module
     alias :old_private, :private
     def private(*args, &blk)
       if blk_given?
          module_eval do
            old_private
            blk.call
          end
       else
         old_private(*args)
       end
   end

   Doesn’t seem to work. The method isn’t made private. I had to resort to getting a list of the methods before the block, then calling old_private with the difference between the new methods after the block is evaluated, and those before.

   The module_eval now looked something like this:

   module_eval do
     existing = instance_methods(false)
     blk.call
     new_methods = instance_methods(false)-existing
     old_private(new_methods.map {|m| m.to_sym}

   There are some obvious shortcomings in this code, but at this point, the goal was to do the simplest thing which could work for the test cases at hands, the complications would be dealt with later.

2. To add insult to injury, calling old_private from within the new definition doesn’t work either, the evaluation context gets marked as described above, but it gets popped off when we exit the context of the new definition. At this point I decided to punt, temporarily at least, and just define a new method Module#with_private which took a block, and leave the existing Module#private alone.
3. Then I decided that I should test changing the visibility of an existing method, in other words a test case like this:

   class Test
     protected
     def meth;end
     with_private do
        def meth;end
     end
   end

   Test#meth should end up private, but my simple diff failed to notice the definition, things were starting to get more complicated. It looked like I’d need to hook Module#method_added and probably others. I began to wonder whether it was worth it.

Then I started to ponder the fact that the effect of private/protected/public gets popped when the evaluation context exits. So I said to myself, “Is there a way to take advantage of that?” Well, Virginia, yes there is.

class Gadget
  #...
  class_eval do
    private

    def razzlitize
    end

  end

  def dazzlitize
  end
end

This seems to work as expected, here’s a link to a test case file.

Now I still prefer being able to give a block to Module#private and it’s ilk, and I might still work on that, but in the meantime, class_eval and module_eval seem to provide a way to actually do what I want.