From: Brian Candler <b.candler@...>
Date: 2010-03-01T03:47:40+09:00
Subject: Re: scope question

Nick Brown wrote:
> Consider the following Sinatra app:
> 
> require 'sinatra'
> vara = "foo"
> VARB = "bar"
> helpers do
>   def geta
>     vara
>   end
>   def getb
>     VARB
>   end
> end
> get '/a' do geta end
> get '/b' do getb end
> 
> When I run that and browse to /b, it displays "bar". When I browse to
> /a, however, it produces the error: "undefined local variable or method
> `vara'".
> 
> Why do these two variables differ in scope?

Which two variables?

vara is a local variable. Its scope extends to the end of the block 
where it's defined. However a method definition using 'def ... end' or a 
class definition using 'class ... end' block starts a fresh empty scope 
inside it.

VARB is not a variable at all, it's a constant. (Constants start with 
capital letters in Ruby).

> How can I declare a variable that has global scope but is not a
> constant?

$varc is a true global variable.

But usually you don't want these. Better is an instance variable of a 
class (remembering that a class is just an object)

class Foo
  @vard = "hello"
  def self.vard
    @vard
  end
  def self.vard=(v)
    @vard=v
  end
end

puts Foo.vard
Foo.vard = "world"
puts Foo.vard

Inside an instance method of foo you can use either 'Foo.vard' or 
'self.class.vard' to get at this value.
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