From: The Higgs bozo <higgs.bozo@...>
Date: 2010-01-24T23:36:46+09:00
Subject: Re: Writing proper getter in a Ruby way


I assume the usual thing is to assign @value = nil in initialize() and
call it a day.

But I think you are asking a higher-level question: How do we abstract
away this detail?

The attr_* methods define methods which create instance variables upon
first being called. But what if we want those instance variables to be
initialized beforehand?

We would need to override the attr_* methods. But those methods belong
to the class object--how are they going to initialize variables in the
instances? By inserting code into the initialize() ancestor chain.

module InitializeAttr
  [:attr_reader, :attr_writer, :attr_accessor].each do |method|
    define_method method do |*syms|
      super(*syms).tap do
        mod = Module.new do
          define_method :initialize do |*args, &block|
            super(*args, &block).tap do
              syms.each do |sym|
                instance_variable_set("@#{sym}", nil)
              end
            end
          end
        end
        include mod
      end
    end
  end
end

class Control
  attr_writer :a, :b
  def bar
    @a.to_i + 5
  end
end

control = Control.new
p control.instance_variables
#=> []
p control.bar
#=> warning: instance variable @a not initialized
#=> 5

class Experiment
  extend InitializeAttr
  attr_writer :a, :b
  def bar
    @a.to_i + 5
  end
end

exp = Experiment.new
p exp.instance_variables
#=> [:@a, :@b]
p exp.bar
#=> 5
# yay no warnings

Therein lies either an abstraction technique or an overkill technique.

A few things about define_method blocks,

+ The &block argument is unsupported in Ruby 1.8.6. Sell your soul to
eval() for a workaround.

+ Implicit super arguments are broken in 1.8.7. It passes the regular
arguments but forgets about the &block argument.

+ In Ruby 1.9 super *must* take explicit arguments. Don't know why.
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