I have code that looks like this:
module A def b(a) a+1 end end class B include A end I would like to write a method in the class B that looks sort of like this
class B def b(a) if a==2 # are you sure? same result as the old method 3 else A.b(a) end end end How do I go about doing this in Ruby?
You want the super function, which invokes the 'previous' definition of the function:
module A def b(a) p 'A.b' end end class B include A def b(a) if a == 2 p 'B.b' else super(a) # Or even just `super` end end end b = B.new b.b(2) # "B.b" b.b(5) # "A.b" 
(a) to super. Just super without any arguments (but no empty parentheses) passes all arguments and any block to the superclass method.include, so generally speaking, they behave like superclasses when used as mixins.
class B alias old_b b # one way to memorize the old method b , using super below is also possible def b(a) if a==2 '3' # returning a string here, so you can see that it works else old_b(a) # or call super here end end end ruby-1.9.2-p0 > x = B.new => #<B:0x00000001029c88> ruby-1.9.2-p0 > ruby-1.9.2-p0 > x.b(1) => 2 ruby-1.9.2-p0 > x.b(2) => "3" ruby-1.9.2-p0 > x.b(3) => 4 