Wayward Predicates in Ruby

Recently I’ve been wondering how to name Ruby methods that have predicate looking names (e.g. foo?), but don’t behave like predicates - namely they return other values besides the canonical true and false.¹

Here are a few classic examples:

Float::INFINITY.infinite? # => 1

0.nonzero? # => nil
1.nonzero? # => 1

# funny enough...
0.zero? # => true
5.zero? # => false

Naming is hard and it took me a while to narrow my list of ideas to two options:

• wayward predicates
• deviant predicates

In the end I felt that “wayward predicates” was the way to go, as it sounds less deviant.²

Common vocabulary is important when discussing domain-specific topics and it makes it easier for people to refer to things within the domains. That’s why I’m writing this short article - perhaps it will be a first step to establishing a common name for a pretty common thing.

I’ve always been a believer that naming should be both informative and fun, and that’s certainly one fun name. Wayward predicates can surprise us unpleasantly every now and then, but they are also a classic example of the flexibility of Ruby and its continued resistance to sticking to narrowly defined rules and guidelines.

That’s all I have for you today. Keep hacking!

1. This came up in the context of RuboCop, in case someone’s wondering. ↩

2. See also https://docs.rubocop.org/rubocop/cops_naming.html#namingpredicatemethod for the first “official” usage of the term. ↩