From: Ross Bamford <rossrt@...>
Date: 2006-03-29T23:38:44+09:00
Subject: Re: Why do some methods names which replace the content in	placedoesn't have the "!" sign?

On Wed, 2006-03-29 at 21:48 +0900, dblack@wobblini.net wrote:
> Hi --
> 
> On Wed, 29 Mar 2006, Gregory Seidman wrote:
> 
> > On Wed, Mar 29, 2006 at 12:56:25AM +0900, dblack@wobblini.net wrote:
> > [...]
> > } I'm talking about Ruby, not C.  In Ruby, you can modify a receiver
> > } without changing its instance variables -- in fact, you can modify a
> > } receiver that doesn't *have* any instance variables.
> > }
> > } irb(main):001:0> s = ""
> > } => ""
> > } irb(main):002:0> s.replace("abc")
> > } => "abc"
> > } irb(main):003:0> s.instance_variables
> > } => []
> > }
> > } The term "instance variable" has a specific and unambiguous meaning in
> > } Ruby.
> >
> > I repeat, it's a technicality. A string's internal state is held in
> > variables tied to the particular instance. They are not visible to the Ruby
> > runtime, however, so they are not listed in the array returned by the
> > object's instance_variables method. Do you really believe that methods,
> > which modify those variables, do not change the instance variables simply
> > because the runtime is unaware of the variables? No, I don't think so. You
> > are arguing a technicality.
> 
> I'm not arguing at all.  Someone asked a question about instance
> variables (meaning Ruby instance variables, I assumed), and I answered
> it.  I assume, by default, that questions about Ruby are questions
> about Ruby itself, rather than the C implementation.  It's possible
> that that's not what the OP meant, but in that case it's a good chance
> for him to learn the customary use of the terminology.
> 
> Obviously *something* changes when you change the contents of a string
> or array.  But calling it, without qualification, an "instance
> variable" of the object is almost certainly a recipe for unclarity and
> confusion down the road.

If I can just chime in here, I agree with what I think is David's point
that instance variables and 'state' are separate (though often
overlapping) things. Should this be a bang method?:

  class PoolStr < String
    def suffixes
      @suffixes ||= find_suffixes
    end
  end

Here, 'suffixes' uses an instance variable to cache the result, but I
wouldn't say this modifies the object's state as far as the user of the
API is concerned, since were it removed the class would work exactly the
same, just more slowly. I think the distinction is between internal and
external state (both of which may or may not be maintained in instance
variables) - exactly what constitutes which of those is probably down to
the guy who codes it up.

-- 
Ross Bamford - rosco@roscopeco.REMOVE.co.uk