[LRUG] Comparable Null Objects

[Duncan Stuart]

Thanks John

* I want to sort things of the same type (instances of the Event class).
* I want to sort them *by* an attribute containing things which Quack
enough like dates to be sorted against each other
* I want to do this so that they're displayed in date order in an email
* Yes, it's an Array. I could create an EmailEventCollection object which I
would tell "give me the things for my email" and it would respond with an
array which was (amongst other things) sorted, but I don't think that would
resolve the issue at hand: it hides the mess in a different way, but
internally I'd still be doing the same sort, no?

On 15 July 2015 at 15:09, <jc at panagile.com> wrote:

>
> | Some events don't have an expected date, so like a good little OO
> programmer i've created a Null object:
>
> Yay! Good OO programmer.
>
> | but when it comes to sorting the list
>
> wait, wat? Less good OO programmer.
>
> I can see three things going on here, pretty much simultaneously.
>
> 1) You want to sort a list of different types of things. Like when you try
> to sort a list of Silverback Gorillas and instances of the number 37 it’s
> hard because it doesn’t really make sense.
>
> 2) You don’t have a list. You have an Array object. Is this the best place
> to send the ‘sort’ message to? Should it own the behaviour of sorting these
> particular disparate types of things?
>
> 3) You switched from clean OO thinking (we need an object to encapsulate
> this state) to imperative thinking (we have a list and we need to sort it).
>
> I blame Ruby for all of this. It makes you think you’re doing OO
> programming but throws in many non-OO ideas to trip you up.
>
> It’s not clear that an OO approach is the best approach here, but if you
> want to follow it try to think less about objects and more about the
> messages between objects. E.g., instead of ‘I need to sort a list’ think ‘I
> need to send a message somewhere telling it to do something (in this case
> to… what? To sort some objects? But why do you care?)’.
>
> Hope this helps or at least makes you think or at the very least persuades
> you to rage quit programming and go and live in a forest,
> John
>
>
> On Wednesday, Jul 15, 2015 at 1:03 pm, 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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