From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2010-08-19T15:56:04+09:00
Subject: Re: Check existence of object and it's property at the same time

On Wed, Aug 18, 2010 at 11:47 PM, Cory Patterson <coryp@aquasun.com> wrote:
> David,
>
> You are completely correct.  I guess that was a bad example.  So the
> order of the conditions is important:
>>> if @member and @member.is_active?; end
> => nil
>>> if @member.is_active? && @member; end
> NoMethodError: undefined method `is_active?' for nil:NilClass
>

Yes, it's important. Ruby starts evaluating from left to right, and
will shortcircuit on the first value that would make the whole
expression false (or true if it's an "or").

irb(main):004:0> member = nil
=> nil
irb(main):005:0> if member or member.is_active?
irb(main):006:1> puts "one or both were true"
irb(main):007:1> end
NoMethodError: undefined method `is_active?' for nil:NilClass
	from (irb):5
	from :0
irb(main):008:0> member = "test"
=> "test"
irb(main):009:0> if member or member.is_active?
irb(main):010:1> puts "one or both were true"
irb(main):011:1> end
one or both were true

Here you have the opposite situation: when the first truthy expression
is found, the evaluation is shortcircuited to that value, as you can
see that the is_active? method is not called in the last example.

Jesus.