No One Thing

Let’s say you have a class Foo:

class Foo
  THE_CONST = 'parent_const'

  def get_val
    'parent_val'
  end

  def show_const
    puts THE_CONST
  end

  def show_val
    puts get_val
  end
end

Further, let’s say you have a class Bar, derived from Foo, that redefines a couple of things:

class Bar < Foo
  THE_CONST = 'child_const'

  def get_val
    'child_val'
  end
end

It seems kind of obvious that:

f = Foo.new
f.show_const  # => parent_const
f.show_val    # => parent_val

But would you have guessed that:

b = Bar.new
b.show_const  # => parent_const WTF!?!?!
b.show_val    # => child_val

This caused me no end of headaches during a weekend bout of refactoring. Pushing default constants up to the parent class completely hosed me up in those child classes that tried to override them. Overriding dumb getter methods that just return constants, though, works fine.

I’m sure there’s some computer-sciencey reason for this that Matz could explain to me, but I had really expected the every-call-is-a-message nature of Ruby to handle those two cases more consistently.