From: Stefano Crocco <stefano.crocco@...>
Date: 2009-11-30T06:15:16+09:00
Subject: Re: semantics of @x v. $x

On Sunday 29 November 2009, Ralph Shnelvar wrote:
> |SC> On Sunday 29 November 2009, Ralph Shnelvar wrote:
> |>> |Newbie here:
> |>> |
> |>> |Consider
> |>> |irb(main):001:0> @xyzzy = 5
> |>> |=> 5
> |>> |irb(main):002:0> defined? @xyzzy
> |>> |=> "instance-variable"
> |>> |irb(main):003:0> $xyzzy = 5
> |>> |=> 5
> |>> |irb(main):004:0> defined? $xyzzy
> |>> |
> |>> |
> |>> |What, semantically, is the difference between $x and @x when at top
> |>> |level?  (Am I at top level?)
> |>> |
> |>> |Do $xyzzy and @xyzzy have global scope at statement 5 and beyond?
> |
> |SC> @xyzzy = 5 defines an instance variable for the global object. That
> | instance SC> variable is accessible only from the top level. $xyzzy = 5,
> | instead, defines a SC> global variable, which can be accessed from
> | everywhere.
> |
> |SC> Here's an example:
> |
> |SC> @x = 1
> |SC> $y = 2
> |
> |SC> class C
> |SC>
> |SC>   def test
> |SC>     p defined?(@x)
> |SC>     p defined?($y)
> |SC>     p @x
> |SC>     p $y
> |SC>   end
> |SC>
> |SC> end
> |
> |SC> C.new.test
> |
> |SC> The output is:
> |
> |SC> nil
> |SC> "global-variable"
> |SC> nil
> |SC> 2
> |
> |SC> This shows that, while $y can be accessed even from instances of the C
> | class, SC> @x is accessible only from top level, that is only from the
> | main object.
> |
> |SC> By the way, I suspect you forgot to paste a last line of output in
> | your SC> example. defined?($xyzzy) should have given you
> | "global-variable".
> |
> |SC> I hope this helps
> |
> |SC> Stefano
> |
> |I thank you greatly for you explanation and your time.
> |
> |Is there a document that you know of that describes these scoping
> |rules for beginners?
> |

Well, these are among the basics of ruby, so any introduction to ruby should 
explain them. You can find the free version of Programming Ruby at 
http://www.ruby-doc.org/docs/ProgrammingRuby (it's a bit old, but is mostly 
still valid). Then, there are a number of other documentation for beginners 
mentioned here http://www.ruby-doc.org/docs/. I can't say which ones can be 
more useful, though.

Stefano