Predicate method syntax

Question

I made a class with these methods:

def promotion_available?
  promotion.present?
end

def promotion
  @promotion ||= PromotionUser.user(@user.id).available.first
end

Then, a colleague removed the promotion method, and changed the predicate promotion_available? in this way:

def promotion_available?
  @promotion ||= PromotionUser.user(@user.id).available.first
end

Can I set an instance variable directly on a predicate method?
Can a predicate method return an entire object instead of true/false (I think no, but my colleague says the opposite)?

@sawa: [sic]? No, it's not sic, it's obviously a typo :) – Sergio Tulentsev Feb 06 '14 at 09:42 — Sergio Tulentsev, Feb 06 '14 at 09:42

score 3 · Answer 1 · answered Feb 06 '14 at 09:39

Can I set an instance variable directly on a predicate method?

Yes, there's nothing wrong with it. I do it often.

Can a predicate method return an entire object instead of tre/false (I think no, but my colleague says the opposite)

Yes, this is also common. But it only works if you use idiomatic ruby truthiness checks.

if obj.promotion_available? # GOOD
if obj.promotion_available? == true # BAD!

Remember, that only nil and false in ruby are falsey values. Everything else is true. This is why returning an object or nil works kinda like returning true or false.

The question is not clear at which level it is asking: Whether it is about syntactic correctness or about the convention. Your answer kind of inherits that ambiguity. But, +1. — sawa, Feb 06 '14 at 09:49

Stefan · Answer 2 · 2014-02-06T14:05:23.217

1

Although the change "works", it introduces a side effect: you have to call promotion_available? before you can actually use the promotion (assuming that there is a promotion method):

your_object.promotion             #=> nil

your_object.promotion_available?
your_object.promotion             #=> <#PromotionUser ...>

You original code doesn't have this dependency.

IMO it would be more intuitive to keep the promotion method and to remove promotion_available?. You could just write:

if obj.promotion
  # with promotion
else
  # without promotion
end

edited Feb 06 '14 at 14:05

answered Feb 06 '14 at 10:30

Stefan

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`promotion` method was removed, so he probably doesn't need to use it. :) – Sergio Tulentsev Feb 06 '14 at 10:44
@SergioTulentsev unless it's defined implicitly, e.g. via `attr_accessor` – Stefan Feb 06 '14 at 10:51
Yes, #promotion no longer exists, instead of it we calling @promotion defined on #promotion_available? – easteregg Feb 06 '14 at 11:08
@mirco if you are using `@promotion` somewhere else, you have to remember calling `promotion_available?` beforehand. – Stefan Feb 06 '14 at 11:25
1

just like that, and I do not think that having to call first #promotion_available? to arrange the instance @promotion is very intuitive and readable – easteregg Feb 06 '14 at 12:42
@mirco exactly, I would keep the `promotion` method instead, see my edit – Stefan Feb 06 '14 at 14:06

Predicate method syntax

2 Answers2