From: Trans <transfire@...>
Date: 2007-11-28T20:48:20+09:00
Subject: Re: define_method and instance variable maddness



On Nov 28, 4:28 am, Sharon Phillips <phillip...@yahoo.co.uk> wrote:
> Hi,
>
> This problem has been driving me nuts, so I've finally given in and
> decided to ask for help.
>
> I'm just cutting my teeth on metaprogramming but am stuck referencing
> instance variables within define_method
>
> Here's the code snippet in question. It works, but is extremely ugly
> with its constant usage of instance_variable_get and set.
>
> child_list_name= ('@'+(child.to_s)+"_list")
> self_method= (self.name.downcase+'=').to_sym
> method_name= ('add_'+child.to_s).to_sym
>
> define_method method_name do |new_child|
>    instance_variable_set(child_list_name, []) if
> instance_variable_get(child_list_name).nil?
>    unless instance_variable_get(child_list_name).include? new_child
>      instance_variable_set(child_list_name,
> (instance_variable_get(child_list_name) << new_child))
>      if new_child.respond_to? self_method
>        new_child.send self_method, self
>      end
>    end
> end
>
> This is what I'd really like the new method to look like (for
> child== :personnel):
>
> def add_personnel (new_personnel)
>    unless (@personnel_list||=[]).include? new_personnel
>      @personnel_list << new_personnel
>      self_method= (self.name.downcase+'=').to_sym
>      if new_personnel.respond_to? self_method
>        new_personnel.send self_method, self
>      end
>    end
> end
>
> Any help appreciated.

I make two possible suggestions. Create yourself something shorter
than instance_variable_set/get. Eg.

    def __iv(name,value=Exception)
      if value==Exception
        instance_variable_get("@#{name}")
      else
        instance_variable_set("@#{name}", value)
      end
    end

That will make it easier to read:

  define_method method_name do |new_child|
    __iv(child_list_name, []) if __iv(child_list_name).nil?
    unless __iv(child_list_name).include? new_child
      __iv(child_list_name, (__iv(child_list_name) << new_child))
      if new_child.respond_to? self_method
        new_child.send self_method, self
      end
    end
  end

Secondly, use eval if it makes the code easier to maintain.

T.