From: Steve Klabnik <steve@...>
Date: 2012-03-07T04:23:10+09:00
Subject: Re: Getting the class of an object.

methods that end in ? may not return a boolean, but they will return
something that's 'truthy' or 'falsy':

$ irb
1.9.3p125 :001 > defined?(foo)
 => nil
1.9.3p125 :002 > if defined?(foo)
1.9.3p125 :003?>   puts "Defined!"
1.9.3p125 :004?>   end
 => nil
1.9.3p125 :005 > foo = "hello"
 => "hello"
1.9.3p125 :006 > defined?(foo)
 => "local-variable"
1.9.3p125 :007 > if defined?(foo)
1.9.3p125 :008?>   puts "Defined!"
1.9.3p125 :009?>   end
Defined!
 => nil