Variables are serious business. You can wink and nudge them whilst making inappropriate jokes, and sure, they’ll laugh along awkwardly, but at the end of the day they’ve much responsibility.

Because they are so serious, I will make this brief.

The first thing worth noting is indeed a common Ruby fact: undefined global and instance variables have a value of nil. I don’t know about you, but I (as a young Rubista) expected all undefined variable access attempts to raise an exception.

>> @what_it_is_jive_turkey
=> nil
>> what_it_is_jive_turkey
NameError: undefined local variable or method `what_it_is_jive_turkey'

Where this really becomes apparent is in Rails’ partials. You can throw around @counter + 1 if @counter all you want, but for local variables it’s a bit less succinct. Try counter + 1 if defined? counter.

There’s also this little gem:

>> not_defined
NameError: undefined local variable or method `not_defined'
>> not_defined = not_defined
=> nil
>> not_defined
=> nil

Ruby continues to amaze. The Pickaxe’s explanation of local variables vs methods helps unravel the mystery. You see, Ruby keeps track of which local variables have been assigned so far in scope so that it can tell the difference between local variables and methods without seeming silly. My guess is the above’s a perfect example of this: Ruby remembers that not_defined was assigned something when it hits the right side of the assignment operation. When it checks to see what not_defined’s value is, it finds nothing, so returns nil. Hence, not_defined is assigned nil. (I could be wrong.)

While we’re here, remember that blocks have strangeness in their scoping. They are greedy and, if they can, steal variables from the enclosing scope.

>> counter = 0
=> 0
>> 0.upto(100) { |counter| print counter + 1 }
12345678910...
=> 0
>> counter
=> 100

The block did not make its own scope-local variable counter but instead used the enclosing scope’s counter, assigning it a new integer each step of the way.

This also applies to instance and global variables.

>> @counter = 0
=> 0
>> 0.upto(100) { |@counter| print @counter + 1 }
12345678910...
=> 0
>> @counter
=> 100

Word is Matz is not happy with this part of Ruby and it will be removed in 2.0.

Of course, blocks refuse to add new variables to the enclosing scope. They take and they take, but, in the true spirit of Christmas, refuse to give:

>> counted
NameError: undefined local variable or method `counted'
>> 0.upto(100) { |counted| print counted + 1 }
12345678910...
=> 0
>> counted
NameError: undefined local variable or method `counted'

That’s it. Don’t forget.