From: Phrogz <gavin@...>
Date: 2007-04-05T04:55:13+09:00
Subject: Re: nil? and non-existent objects

On Apr 4, 1:25 pm, "Fran�ois Montel" <zeroh...@gmail.com> wrote:
> Why is it that the nil? method can sometimes be called on an object that
> doesn't exist, and other times will return an error. Under what
> circumstances can you call the nil? method on a non-existent object?
>
> Consider:
>
> >> puts x = 1 if x.nil?
> => 1
> >> puts 'z is not defined' if z.nil?
>
> => NameError: undefined local variable or method 'z'

Consider this:

  irb(main):001:0> puts x=1 if x
  => nil

  irb(main):002:0> puts y if y=1
  (irb):2: warning: found = in conditional, should be ==
  NameError: undefined local variable or method `y' for main:Object

  irb(main):003:0> if z=1 then puts z end
  (irb):3: warning: found = in conditional, should be ==
  1
  => nil

The difference between them is the order in which Ruby sees the
assignment *when preparing* (syntax parsing), not during execution.
The 'y' and 'z' cases above should run the same way, but due to the
way Ruby checks to determine if a variable exists, it needs to see an
assignment earlier in the file than a reference to the value.