Issue #13683 has been updated by Eregon (Benoit Daloze). I think for the use case of "check if a single value" from @Dan0042 then `ary.first if ary.size == 1` or so is good enough. I agree the new method here is meant to assert and the usefulness of it comes from the exception if it unexpectedly doesn't hold. `value = ary.size == 1 ? ary.first : raise(SomeError)` is of course possible but that's quite verbose and I think a pattern worth capturing as a core method. ---------------------------------------- Feature #13683: Add strict Enumerable#single https://bugs.ruby-lang.org/issues/13683#change-116115 * 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/