Dr Nic

Do you write code and smile?

Do spend two hours on a hack to save yourself one hour of time?

Is being a great coder important to you, even if you don’t know why its important?

Do you derive a sense of pride from a few lines of code you wrote?

I thought of all these questions whilst watching a great guitar clip… he’s gone beyond learning Pachelbel’s Canon in D Major, and overlayed his own code. Underneath the hat, I’m sure he’s smiling.

His pride for his guitar work makes me happy for my coding. You?

Many Rubist’s first explanation of metaprogramming is by why the lucky stiff (_why)’s Why’s (Poignant) Guide to Ruby, chapter 6, section 3.

You go on a dragon-hunting, adventure game using sexy Ruby syntax (a domain-specific language/DSL for adventure games?). Here is some sample syntax for defining a monster class:

 class Dragon < Creature
   life 1340     # tough scales
   strength 451  # bristling veins
   charisma 1020 # toothy smile
   weapon 939    # fire breath
 end

The life, strength, charisma and weapon class methods are generated by a traits class method called against the Creature class (read the chapter).

class Creature
  traits :life, :strength, :charisma, :weapon
end

Read this chapter section many times and admire the beauty of the idea (and amuse yourself with his writing style!).

But there is one small improvement that could be made: currently, after setting the trait methods (e.g. life 1340 sets the life trait to 1340), you cannot access the class’s trait values directly via their original trait method. That is, you cannot call Dragon.life to retrieve the value 1340.

This is due to a limitation of the define_method method being used. The relevant code from _why’s book is:

   def self.traits( *arr )
     # 2. Add a new class method to for each trait.
     arr.each do |a|
       metaclass.instance_eval do
         define_method( a ) do |val|
           @traits ||= {}
           @traits[a] = val
         end
       end
     end

The method creator define_method uses a block to define the generated method body. The parameters for the block (val in the example above) become the arguments of the method once its been added to the class. That is, if we call traits :life on our Creature class, then a class method will be generated that requires one argument - the value of the trait. That is, it will generate the following method:

class Creature
  def life(val)
    @traits ||= {}
    @traits[:life] = val
  end
end

Now, back to the problem. How do we support the syntax Dragon.life? To achieve this, the generated method would need to look like:

class Creature
  def life(val = nil)
    @traits ||= {}
    return @traits[:life] if not val
    @traits[:life] = val
  end
end

That is, we need a default value for our method argument. But… blocks don’t allow parameters to have default values. We cannot do the following:

         define_method( a ) do |val = nil|
           @traits ||= {}
           return @traits[a] if not val
           @traits[a] = val
         end

A pity, yes.

So, we need to generate our methods differently. The solution is as follows:

      metaclass.class_eval <<-EOS
        def #{a}(val=nil)
          @traits ||= {}
          return @traits[:#{a}] if not val
          @traits[:#{a}] = val
        end
      EOS

The eval methods allow a string to be passed to them. So, we shall pass it a string that defines a new method the old fashioned way: using the def method constructor. Thus it allows us to have default values for our arguments.

QED.

ALTERNATIVE from Chris @ Errtheblog.com

Use the splat! (*) to allow zero or more arguments. Pluck the first one off to represent the incoming argument or nil.

def self.traits( *arr )
  arr.each do |a|
    metaclass.instance_eval do
       define_method( a ) do |*val|
         val = val.first
         @traits ||= {}
         return @traits[a] if not val
         @traits[a] = val
       end
     end
  end
end

There are unanswered questions about its support for 2+ arguments (where we only need support for 0 or 1) that shall remain unanswered for the sake of this simple hack. But feel free to start throwing exceptions around if your users need protection from themselves.

Rails’ active_support library adds some wonderful functions into standard Ruby classes. Some we all use day-in-day out are attr_accessor and its class-level equivalent, cattr_accessor.

But cattr_accessor doesn’t work the way you (read, “me”) thought at first glance when you use subclasses. I thought if I declared a class accessor in the superclass, then I would have independent class attributes for all my subclasses. Apparently not…

>> class Parent; cattr_accessor :val; end
=> [:val]
>> class Child1 < Parent; end
=> nil
>> class Child2 < Parent; end
=> nil
>> Child1.val = 4
=> 4
>> Child2.val
=> 4
>> Child2.val = 5
=> 5
>> Child1.val
=> 5

Child1.val and Child2.val seem to be the same value. Not very independent at all. Internally, the classes share a common class attribute. This is useful in certain circumstances, but not what I was looking for.

Instead, I found class_inheritable_accessor.

>> class Parent; class_inheritable_accessor :val; end
=> [:val]
>> class Child1 < Parent; end
=> nil
>> class Child2 < Parent; end
=> nil
>> Child1.val = 4
=> 4
>> Child2.val = 4
=> 4
>> Child2.val = 5
=> 5
>> Child1.val
=> 4

Lovely. Each subclass will have an independent value once you’ve assigned it a value explicitly, else it will pick up the value from its superclass.

UPDATE: class_inheritable_accessor and co. actually clone the superclass’s inherited attributes, rather than just referencing them.