[LRUG] Comparable Null Objects

[Frederick Cheung]

On 15 July 2015 at 13:03:21, Duncan Stuart (dgmstuart at gmail.com) wrote:

$ 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 think this is what coercion is for (e.g. http://www.mutuallyhuman.com/blog/2011/01/25/class-coercion-in-ruby/ )


Fred

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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