Why can I assign an undefined variable to itself in Ruby and get nil? [duplicate]

Question

Possible Duplicate:
Why a = a is nil in Ruby?

There's a, shall we say, "odd phenomenon" in Ruby with using undefined variables. It's like this:

# irb session follows
#
foo        # undefined local variable or method 'foo'
bar        # same for 'bar'
foo = bar  # still same for 'bar'
foo = foo  # nil - HUH?
foo        # is now set to nil!?

Why can I assign an undefined variable to itself in Ruby and get nil?

Note that I'm using Ruby 1.9.3 here. I'm not sure what other versions this may be true in.

(Thanks to Gary Bernhardt for demonstrating this in his hilarious talk.)

Answer 1

The fact that bar is undefined is actually not the most interesting part, since the assignment does not even need to be attempted, for example

if false
  foo = 1
end

Sets foo to nil. As i understand it, local variable scope is determined statically in that it is determined without actually running the code, merely by analysing it. Ruby thinks the assignment could happen so it creates the local variable and sets it to nil. See http://ruby.runpaint.org/variables#local

Answer 2

Nil is magic in Ruby, because of the idea that everything is an object. There is actually a singleton nil object that gets assigned. When you did

foo = bar

the "foo" variable sprang into existence and took as value the magic nil object. Before you did the assignment, Ruby didn't have a way of "knowing" what foo is (is it a variable? a method call?), but once you do the assignment, it starts treating it as a variable.