From: "David A. Black" <dblack@...>
Date: 2007-09-26T07:31:06+09:00
Subject: Re: Using a block to configure/initialize a class

Hi --

On Wed, 26 Sep 2007, Emmanuel Oga wrote:

> Hi! i need a solution for this, it's pretty simple but i can't get it to
> work...
>
> suppose:
>
> class Options
>
>  def self.define_an_option
>    # ...don't-know-how-implementation :(
>  end
>
>  define_an_option :option1, String
>  define_an_option :option2, Array
> end
>
> Then, i want to do something like this:
>
> opt= Options.new
> opt.option1 => ""
> opt.option2 => []
>
> opt.option1 do |o|
>  o= "Hello"
> end

That's just a re-assignment to the variable 'o'. It's not going to
have any impact on opt.option1.

> opt.option2 do |o|
>  o << 10
> end
>
> options.option1 => "Hello"
> options.option2 => [10]
>
> Thats all! Seems pretty simple, but i tried with a lot of variants and I
> don't get it to work. Any ideas?

The block syntax seems a little odd. Is there any reason not to just
use attr_accessor? If you want it to default to a new instance of some
specific class, you could do something like:

class Options

   def self.define_an_option(name, klass)
     attr_writer name
     iv = "@#{name}"
     define_method(name) do
       unless instance_variables.include?(iv)
         instance_variable_set(iv, klass.new)
       end
       val = instance_variable_get(iv)
       yield val if block_given?       # if you really need this
       val
     end
   end

   define_an_option :option1, String
   define_an_option :option2, Array
end

(and I suspect there are other versions of attr_reader with a default
value out there somewhere).


David

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