From: Paul Lutus <nospam@...>
Date: 2006-12-06T01:45:05+09:00
Subject: Re: Unexpected scope range

Andreas Warberg wrote:

> I am new to Ruby but have a strong java background.
> 
> So far I have been enjoying Ruby a great deal.
> 
> But I have been wondering about why the scope of variables works as it
> does. It seems that very often, I need to prefix my variables with
> either @ or $ in order to access them in method calls.
> I did some searching for an explanation but did not find any.
> 
> In java variables can be defined and used like this:
> 
> Object x = myX;
> 
> public void printx(){
>     System.out.println(x);
> }
> 
> x is available when the method is called. This is not the case with
> Ruby:
> 
> x = myX
> 
> def printx
>    puts x
> end
> 
> Running the code will produce an error: "NameError: undefined local
> variable or method `x' for main:Object".
> 
> In order to reveal x to the method one must, for instance, declare it
> global by prefixing the $. This requires more typing (which, in many
> other respects, ruby tries to avoid).
> 
> How come the scope works in this way? I expected the visibility of
> variables to flow down into the code tree (but not up, of course).

In Ruby, variables used within a method are marked this way:

example   # method local
@example  # class instance variable
@@example # class variable
$example  # global variable

The term "class variable" above refers to a variable that is common to the
entire class, and "instance variable" refers to a variable that is unique
to an instance of the class.

This means you can use @this_syntax to refer to a variable you want to share
between method within a class, without using global variables, something
one wants to avoid unless necessary.

-- 
Paul Lutus
http://www.arachnoid.com