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 
irb(main):002:0> unless defined?(fooo) ; fooo = false ; end
=> false 

thx

Answer 1

Apparently, ruby creates local variable at parse time setting them to nilso it is defined and this is done whether the code is executed or not.

When the code is evaluated at your first line, it doesn't execute the assignment part since foo is set to nil. In the second line, because fooo has not been parsed yet, defined?returns nil letting the code inside the block execute and assign fooo.

As an example you can try this:

if false  
  foo = 43  
end  
defined? foo  
=> "local-variable"

This is taken from a forum post at ruby-forum.

Answer 2

Let's start with something simpler:

# z is not yet defined
irb(main):001:0> defined?(z)
=> nil

# Even though the assignment won't execute,
# the mere presence of the assignment statement
# causes z to come to life.
irb(main):002:0> z = 123 if false
=> nil
irb(main):003:0> defined?(z)
=> "local-variable"
irb(main):004:0> z
=> nil

Now we can figure out your first example.

foo = true unless defined?(foo)

Is foo defined? Before we press ENTER in irb, no. However, the presence of the assignment statement causes foo to come to life. That means the assignment statement won't be executed, leaving foo in existence but having nil as its value. And what is the last expression evaluated in the irb line? It is unless defined?(foo), which evaluates to nil.

For more info on how assignments (even those that do not get executed) cause variables to exist, see this discussion of Variable/Method Ambiguity.

In your second example, there is nothing mysterious at all: fooo is not defined, so the code in the block executes, setting fooo to false. That assignment is the last expression evaluated, so false is the return value of our block.

Answer 3

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.