gistfile1.rb

class Foo
  def initialize
    yield(self) if block_given?
  end
end

# Shouldn't these be equivalent?  Why isn't thing1 created?

f1 = Foo.new { |me| @thing1 = 'this is thing1' }
f1.instance_variable_get("@thing1") #=> nil

f1.instance_eval { @thing2 = 'this is thing2' }
f1.instance_variable_get("@thing2") #=> 'this is thing2'