[LRUG] Comparable Null Objects

[Duncan Stuart]

Sorry that last code snippet should be:

def sort_by_expected_date
  sort do |a, b|
    if b.class = NoExpectedDate
      1
    else
      a <=> b
    end
  end
end

On 15 July 2015 at 13:03, Duncan Stuart <dgmstuart at gmail.com> wrote:

> 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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