What is Replace Conditional with Polymorphism Refactoring? How is it implemented in Ruby?

Question

I recently came across the Replace Conditional with Polymorphism Refactoring while asking for elimination of if..else conditional in ruby.the l

Answer 1

I think I would format the bolded phrase a little differently, i.e.: Refactor your code to replace the conditional with polymorphism.

If that is indeed what the comment is supposed to mean, then Yehuda Katz has a great post giving an example in ruby: http://yehudakatz.com/2009/10/04/emulating-smalltalks-conditionals-in-ruby/

Basically, the argument is that an if/else statement exists to execute different code based on the value of a boolean. It requires special syntax, and is limited to only the types TrueClass/FalseClass (or Object/NilClass if you're being lax about truthiness). Dynamic dispatch on the other hand performs the same operation of selecting branches of code to execute based on the value of an object, but it does not rely on specialized syntax, nor is it limited to any specific group of types.

Answer 2

The Replace Conditional with Polymorphism Refactoring is rather simple and it is pretty much exactly what it sounds like. You have a method with a conditional like this:

def speed
  case @type
  when :european       then base_speed
  when :african        then base_speed - load_factor * @number_of_coconuts
  when :norwegian_blue then if nailed? then 0 else base_speed(@voltage) end
end

and you replace it with polymorphism like this:

class European
  def speed
    base_speed
  end
end

class African
  def speed
    base_speed - load_factor * @number_coconuts
  end
end

class NorwegianBlue
  def speed
    if nailed? then 0 else base_speed(@voltage)
  end
end

You can apply the Refactoring again to NorwegianBlue#speed by creating a subclass of NorwegianBlue:

class NorwegianBlue
  def speed
    base_speed(@voltage)
  end
end

class NailedNorwegianBlue < NorwegianBlue
  def speed
    0
  end
end

Voilà, all your conditionals are gone.

You might ask yourself: does this always work? Can I always replace an if with message dispatch? And the answer is: yes, you can! In fact, if you didn't have if, you can implement it yourself using nothing but message dispatch. (This is, in fact, how Smalltalk does it, there are no conditionals in Smalltalk.)

class TrueClass
  def iff(thn:, els: ->{})
    thn.()
  end

  def &
    yield
  end

  def |
    self
  end

  def !
    false
  end
end

class FalseClass
  def iff(thn:, els: ->{})
    els.()
  end

  def &
    self
  end

  def |
    yield
  end

  def !
    true
  end
end

(3 > 4).iff(thn: ->{ 'three is bigger than four' }, els: ->{ 'four is bigger than three' } )
# => 'four is bigger than three'

true.& { puts 'Hello' }
# Hello

true.| { puts 'Hello' }
# => true

Also relevant: the Anti-IF Campaign