From: "Mauricio Fernández" <batsman.geo@...>
Date: 2002-10-23T23:55:14+09:00
Subject: Re: Things That Newcomers to Ruby Should Know (10/16/02)

On Wed, Oct 23, 2002 at 09:35:34PM +0900, Gavin Sinclair wrote:
> > Would it be possible for Ruby to know whether an object is referenced by
> > several variables so += does '<<' or '... = ... + ...' when appropriate?
> >
> > Perhaps a bit in each String indicating whether it is shared or
> > not would make it... and GC issues could be ignored at first.
> 
> I don't think so.  += is syntax sugar for you-know-what.  Imagine trying to
> explain to a newbie "... but except in this case ..."
> 
> Side-effects are a part of life in Ruby (and not only in Ruby).  Programmers
> have to deal with them.  They're not even a bad thing.

[Disclaimer: I'm not proposing a modification of the language, see [52838]
 Even though there's a subtle change in the semantics, no code should
 break, and this is only an optimization, and nothing else]

a = "bla bla"
a += " --- wtf?"
p a   => "bla bla --- wft?" 

In this stupid example, we sure don't care whether Ruby is creating a
new object or not, and the programmer doesn't mind having Ruby do
  a << " --- wtf?"
instead of 
 a = a + " --- wtf?"

Now we certainly don't want Ruby to use '<<' internally in
b = a
b += " the object pointed to by a should not change!"

I consider the use of '<<' when possible to be nothing but a slight
optimization which might (or not) prove to make Ruby faster.
You can do a "incomplete RC" (in fact I think something like that was
already used in Ruby, somewhere), for the common case where an object is
referenced only once. If another variable references it, set a "shared
bit" to 1, and that's it. so you don't have to do complete RC which
would be expensive.

So we'd have
 a = "a"
 b = "b"
 a += "a"   => a == "aa", using '<<'
 b = a    => b == "aa"
 b += "b" => b == "aab", used ' = ... + ..."
# simple RC in action now
 a += "a" => a == "aaa", using " = ... + ..."
# cause the 'shared bit' wasn't reset, as we're not doing real RC 

This is not a language modification (unless somebody really needs a new
object to be created *always* when doing +=), only a (possible) speed
optimization.


PS: I'm getting a computer of my own next week, and as I'm gonna have a
lot of free time, I may try this and other different tricks on Ruby's
implementation, following my new policy on how to improve Ruby [52838]
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/52838

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