Issue #13683 has been updated by matz (Yukihiro Matsumoto). I now agree with having a method with proposed behavior. We have to pick a name for it. I don't like `single`. I have no strong opinion for/against `sole`. But I don't want to introduce a new exception class dedicated to this sole method (as `#sole` in ActiveSupport does). The other candidates are: - one! - only - lone - singular AI strongly suggested `only`. I am still wondering. Any opinion? One possible solution is providing our version of `sole` that raises `ArgumentError`, only if AcriveSupport would define `SoleItemExpectedEror = ArgumentError` or something similar. Matz. ---------------------------------------- Feature #13683: Add strict Enumerable#single https://bugs.ruby-lang.org/issues/13683#change-116087 * Author: dnagir (Dmytrii Nagirniak) * Status: Feedback ---------------------------------------- ### Summary This is inspired by other languages and frameworks, such as LINQ's [Single](https://msdn.microsoft.com/en-us/library/bb155325%28v=vs.110%29.aspx) (pardon MSDN reference), which has very big distinction between `first` and `single` element of a collection. - `first` normally returns the top element, and the developer assumes there could be many; - `single` returns one and only one element, and it is an error if there are none or more than one. We, in Ruby world, very often write `fetch_by('something').first` assuming there's only one element that can be returned there. But in majority of the cases, we really want a `single` element. The problems with using `first` in this case: - developer needs to explicitly double check the result isn't `nil` - in case of corrupted data (more than one item returned), it will never be noticed `Enumerable#single` addresses those problems in a very strong and specific way that may save the world by simply switching from `first` to `single`. ### Other information - we may come with a better internal implementation (than `self.map`) - better name could be used, maybe `only` is better, or a bang version? - re-consider the "block" implementation in favour of a separate method (`single!`, `single_or { 'default' }`) The original implementation is on the ActiveSupport https://github.com/rails/rails/pull/26206 But it was suggested to discuss the possibility of adding it to Ruby which would be amazing. -- https://bugs.ruby-lang.org/