From: Benoit Daloze <eregontp@...>
Date: 2010-01-25T00:51:02+09:00
Subject: Re: Writing proper getter in a Ruby way

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> I assume the usual thing is to assign @value = nil in initialize() and
> call it a day.
That's the first option, it just looks not cool at all, but it's very simple
an explicit, sure.

> 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.
Cool way to do that :) but overkill also...
It's very interesting anyway how we can change that.

> 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.

I don't have that:
($ ruby -v #=> ruby 1.9.2dev (2010-01-14 trunk 26319) [x86_64-darwin10.2.0])
class P
 def m(*args, &b)
   [args, b]
 end
end

class C < P
 def m(*args, &b)
   p [args, b]
   super.tap { |sup| p sup }
 end
end

C.new.m(:a, :b) { |e| e }
#=> [[:a, :b], #<Proc:...>]
#=> [[:a, :b], #<Proc:...>]
#=> [:a, :b]

So another way, probably less 'overkill' you made me think is:

class Test
 attr_reader :var
 def initialize(*args)
   if args.length > 0
     @var = :value
   end
 end

 alias :get_var :var
 def var
   get_var || 3
 end
end

p Test.new.var #=> 3
p Test.new(:arg1, :arg2).var #=> :value

Just aliasing the old accessor, too easy :) (and short, no line lost if we
compare to initialization, and we suppose to have already other attr_*)
I just forgot a moment name conflicts about methods don't exist in Ruby,
 because you can so easily copy(alias) the old method.

Thanks for your answer :)

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