Class (re)definition in method body

UPDATE: modifed to use more idiomatic way of returning "***". See comments

Warning: This is really hacky and dirty. Writing the snippet here so I won't forget

Redefining existing classes in Ruby is really handy, and a breeze to do

puts 5     # prints "5"
 class Fixnum
  alias_method :old_to_s, :to_s
  def to_s
   "*" * self
  end
 end
 puts 5     # prints "*****"

But, sometimes you'd just want to limit such monkey patching, like within a method where you really really need the hack in place - and you promise to undo the hack right after you're done to return the world sanity (promise!)

class MyKlass
  def get_as_stars(value)
   # define funky behavior
   class ::Fixnum
    alias_method :old_to_s, :to_s
    def to_s
     "*" * self
    end
   end
 
   return value.to_s
  ensure
   # revert back to original
   class ::Fixnum
    alias_method :to_s, :old_to_s
   end
  end
 end

Unfortunately, that's invalid syntax 

SyntaxError: compile error
 : class definition in method body
 : class definition in method body

Well, what's a man to do? module_eval to the rescue, just replace your "class X" statement with "X.module_eval do"

class MyKlass
  def get_as_stars(value)
   # define funky behavior
   ::Fixnum.module_eval do
    alias_method :old_to_s, :to_s
    def to_s
     "*" * self
    end
   end
 
   return value.to_s
  ensure
   # revert back to original
   ::Fixnum.module_eval do
    alias_method :to_s, :old_to_s
   end
  end
 end
 
 k = MyKlass.new
 puts 3         # prints "3"
 puts k.get_as_stars(4) # prints "****"
 puts 5         # prints "5"

PS: Did this hack to get Time.now == Time.now working in a test. Grr