From: Mystifier <mystifier@...>
Date: 2005-01-15T00:04:39+09:00
Subject: Re: Mutable strings

Dear Austin,

Immutability as I understand is preserving state of the Object. State of
Object are instance variables in Ruby. My example does sot change class
variables, but the instance variable. Symbol class being immutable or not is
besides the point. I am talking about symbol instance, and ability to change
symbol instance variables makes them not immutable as such. Yes, the
sequence of chars they represent are immutable. Hence, I get an impression
that the Symbol is a immutable string class.

About learning things before writing VM, I am sure I have to. What I feel No
VM or Interpreter can exists with reasonable performance with have an
immutable string class. It is altogether a different matter what you call
them.



-----Original Message-----
From: Austin Ziegler [mailto:halostatue@gmail.com] 
Sent: Friday, January 14, 2005 7:59 PM
To: ruby-talk ML
Subject: Re: Mutable strings


On Fri, 14 Jan 2005 14:00:07 +0900, Mystifier <mystifier@users.berlios.de>
wrote:
> Dear Charles,
> 
> I have no intensions of changing the language APIs for it really does 
> not matter to a VM Implementation. All the components that inherently 
> affects the VM implementations are those that VM uses natively like 
> File, Strings, Numbers and Threads.
> 
> I surprisingly found Symbols are exactly not Immutable...., they are 
> just like any other class

Um. That's not the same at all. Symbols themselves are immutable. The Symbol
Class is not immutable.

Yes, this means that someone could override #to_s on Symbol to do something
else entirely, but Symbols themselves are immutable -- and they cannot have
virtual (singleton) classes:

  irb(main):001:0> foo = :foo
  => :foo
  irb(main):002:0> class << foo
  irb(main):003:1> end
  TypeError: no virtual class for Symbol
	  from (irb):2
  irb(main):004:0>

There is a significant difference between the two sorts of immutability that
you would be advised to understand before even thinking of attempting to
implement a VM to support Ruby.

-austin
-- 
Austin Ziegler * halostatue@gmail.com
               * Alternate: austin@halostatue.ca