In Ruby why won't `foo = true unless defined?(foo)` make the assignment?

Question

What\'s going on here? What is the subtle difference between the two forms of \"unless\"?

> irb(main):001:0> foo = true unless defined?(foo)
=> nil         


        

Answer 1

irb(main)> foo = true unless defined?(Integer)
=> nil 
irb(main)> foo = true unless defined?(thisIsUndefined)
=> true

Your first block is returning nil because the way it's written leaves 2 options:

• foo is not defined --> assign true
• foo is defined --> do nothing

Here, foo must be defined when the line is evaluated. Thus, nothing happens and nil is returned.

irb(main)> unless defined?(Integer) ; fooo = false ; end
=> nil
irb(main)> unless defined?(thisIsUndefined) ; fooo = false ; end
=> false 

Your second block operates the same way your first one does. If fooo is not defined, the block is entered and fooo is set to false. The result of the last line of the block is the return value of the block, thus the false you are seeing. If fooo does exist, then the block is skipped over and nothing happens, therefore there is nothing to return, therefore the nil.

Based on your code, I would say that foo was defined when this code was run and fooo was not (test code shown was generated in Ruby 1.8.6). If you did not define either of these before running this code, then you may have something called foo that is defined by default (do defined?(foo) by itself to check). Try using a different name and see if you get the same results.

Edit:

irb(main)> defined?(bar)
=> nil
irb(main)> bar = true unless defined?(bar)
=> nil
irb(main)> defined?(bar)
=> "local-variable"

Apparently, defined?() is returning true since it has already seen bar (at the beginning of the line), even though you are still in the process of defining it.