Question

When it is in fact not defined, it gets the value nil just because it was “touched”:

$ irb

ruby-1.9.2-p0 > foo = true if !defined? foo
 => nil 
ruby-1.9.2-p0 > foo
 => nil 

ruby-1.9.2-p0 > if !defined? bar
ruby-1.9.2-p0 ?>  bar = true
ruby-1.9.2-p0 ?>  end
 => true 
ruby-1.9.2-p0 > bar
 => true 

so the if … end works as expected, but foo = true if ... doesn’t.

Answer 1

Ruby defines a local variable just before executing a line containing an assignment, so defined?(foo) will always be true for the one-liner.

Another example showing that local variables are defined before any part of the line are executed:

defined? foo # => false
foo = foo    # => foo is now nil