ruby-on-rails - Ruby 变量如何以及何时实例化

Question

从 Rails 控制台:

development environment (Rails 3.2.9)
1.9.2p320 :001 > defined?(kol)
 => nil 
1.9.2p320 :002 > if 1==2
1.9.2p320 :003?>   kol = 'mess'
1.9.2p320 :004?>   end
 => nil 
1.9.2p320 :005 > defined?(kol)
 => "local-variable" 
1.9.2p320 :006 > kol
 => nil 

我的问题是，为什么即使条件 (1==2) 失败，变量 kol 也会被实例化为 nil？

Answer 1

这与 Ruby 解释器读取代码的方式有关。

The assignment to the variable doesn't have to be executed; the Ruby interpreter just needs to have seen that the variable exists on the left side of an assignment. (Programming Ruby 1.9 & 2.0)

a = "never used" if false
[99].each do |i|
  a = i # this sets the variable in the outer scope
end
a # => 99

"Ruby interpreter creates the variable even though the assignment isn't actually executed." http://www.jacopretorius.net/2012/01/block-variable-scope-in-ruby.html