[LRUG] Comparable Null Objects

[Michael Pavling]

On 15 Jul 2015 15:03, "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
>
>

Instead of including `Comparable`, set your `NoExpectedDate` class to
inherit from `Date` (keep the overloads of `.to_s` and `.<=>`)
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