一些未声明的变量是 nil,一些会抛出错误。怎么来的?
$ irb
1.9.3p0 :001 > asdf # local
NameError: undefined local variable or method `asdf' for main:Object
from (irb):1
from /Users/saizai/.rvm/rubies/ruby-1.9.3-p0/bin/irb:16:in `<main>'
1.9.3p0 :002 >@asdf # instance
=> nil
1.9.3p0 :003 >@@asdf # class
NameError: uninitialized class variable @@asdf in Object
from (irb):3
from /Users/saizai/.rvm/rubies/ruby-1.9.3-p0/bin/irb:16:in `<main>'
1.9.3p0 :004 > $asdf # global
=> nil