From: brabuhr@...
Date: 2012-01-28T09:02:50+09:00
Subject: Re: Why is a = a nil instead of giving NameError

On Fri, Jan 27, 2012 at 6:40 PM, Navindra Umanee <navindra@cs.mcgill.ca> wrote:
> Could someone help me understand what is happening here?
>
> irb(main):005:0> a = a
> => nil
>
> a is undefined, so why does "a = a" work instead of giving a NameError
> like "b = c" does?

https://github.com/michaeledgar/ripper-plus
"Local variables are created immediately when the Ruby parser parses a
left-hand-side local variable. Before the right-hand-side is even
parsed, the local variable has been created. This also results in an
infamous anomaly:

  def say_what?
    x = 'hello' unless defined?(x)
    puts x
  end

say_what? prints a blank line! This is because as soon as the x on the
LHS is parsed, x is a local variable with value nil. By the time
defined?(x) is executed, x has long since been a local variable!"