[LRUG] Comparable Null Objects

[Duncan Stuart]

Hi LRUG - hopefully an interesting little problem:

I have an Event class which has an "expected_date" attribute.
Some events don't have an expected date, so like a good little OO
programmer i've created a Null object:

class NoExpectedDate
  def to_s(format=:default)
    "Unknown"
  end
end

This works great for printing the values, but when it comes to sorting the
list I of course get:
   ArgumentError: comparison of Date with NoExpectedDate failed

If I include Comparable and define <=> then one comparison works, but the
other doesn't :

class NoExpectedDate
  include Comparable
  def to_s(format=:default)
    "Unknown"
  end
  def <=>(other_date)
    1 # Treat it as after every other date
  end
end

$ NoExpectedDate.new > Date.today
=> true

$ Date.today > NoExpectedDate.new
ArgumentError: comparison of Date with NoExpectedDate failed

I think this is because Date's <=> method expects it's argument to be a
Date object or a number ("a numeric value as an astronomical Julian day
number"). I've tried defining to_i and to_r on NoExpectedDate, but no dice.

Can I get NoExpectedDate to pretend to be a Date (like SimpleDelegator lies
about it's class)? Is that evil?

I suppose I could always just define a method on Event to do this
particular sort, but that seems nasty for all sorts of reasons:

def sort_by_expected_date sort do |a, b|

    if b.class = NoExpectedDate

      1

    else

      a <=> b

    end

  end

end
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