From: "Ara.T.Howard" <ahoward@...>
Date: 2003-12-06T02:12:05+09:00
Subject: Re: Attempted roadmap of future instance variables....

On Sat, 6 Dec 2003, ts wrote:

> Date: Sat, 6 Dec 2003 01:42:43 +0900
> From: ts <decoux@moulon.inra.fr>
> Newsgroups: comp.lang.ruby
> Subject: Re: Attempted roadmap of future instance variables....
> 
> >>>>> "A" == Austin Ziegler <austin@halostatue.ca> writes:
> 
> A> What is the problem that @_ variables are trying to solve? 
> 
>  Well, if I remember correctly the original proposition one of the problem
>  can be summarized with this stupid example.
> 
>  Imagine that someone define a module which implement a counter, he'll
>  write something like
> 
>    module A
>       def increment
>          @counter ||= 0
>          @counter += 1
>       end
> 
>       def get
>          @counter
>       end
>    end
> 
>  because the instance variable @counter is used only to remember some
>  internal state, this variable is never documented and in the documentation
>  there is only a reference to #increment and #get
> 
>  Now someone else use this nice module from RAA and write
> 
>    class B
>       include A
> 
>       def initialize
>          @counter = "counter" # he don't know that this variable is used
>        end
>    end
> 
>  He has completely broken the module A.
> 
>  You don't have this problem with class local variable because 
>   * when you call a method of A : @_counter is a fixnum
>   * when you call a method of B : @_counter is a string

so the problem is that there is no concept of 'private' instance variables in
ruby.  i finally understand - thanks guy.

this is a problem, but i also do not like the @_ syntax.  perhaps it would be
preferrable to be able to do this

module A
  private @counter
  protected @another_var
  def increment
    @counter ||= 0
    @counter += 1
  end
  def get
    @counter
  end
end

the default would obviously be public. this would not break existing code.
alternatively, if people prefer symbols we could have

  @var   # public instance var
  _@var  # protected instance var
  __@var # private instance var

this also would not break exiting (@_var) code.  maybe it would be too hard to
parse '_@' and '__@' as atomic token though?


-a
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