[ruby-core:86554] [Ruby trunk Feature#13683] Add strict Enumerable#single

Issue #13683 has been updated by IotaSpencer (Ken Spencer).


matz (Yukihiro Matsumoto) wrote:
> Hmm, I don't like the name `single`. Besides that, I think it may be useful for database access, but I don't see the use-case of this method for generic Enumerable.
> 
> Matz.

I think of single as a method towards mutual exclusivity.
If an Array or Enumerable from another expression should only have a single element,
then this gives the process a much faster setup and possible rescue, as I currently have
one of my projects checking for the existence of 3 headers, `X-GitHub-Event`, `X-GitLab-Event`,
and `X-Gogs-Event`, and I found the easiest way was to use `one` from Enumerable, but I wanted it
to error out so that I could catch it with the rest of my raised exceptions from other errors that 
arise in the handling of the request.

How about these for suggestions.

`one_or_raise`
`one_or_nothing`

Part of my code for context.

~~~ ruby
      events = {'github' => github, 'gitlab' => gitlab, 'gogs' => gogs
      }
      events_m_e = events.values.one?
      case events_m_e
        when true
          event = 'push'
          service = events.select { |key, value| value }.keys.first
        when false
          halt 400, {'Content-Type' => 'application/json'}, {message: 'events are mutually exclusive', status: 'failure'
          }.to_json

        else halt 400, {'Content-Type' => 'application/json'}, {'status': 'failure', 'message': 'something weird happened'
        }
      end
~~~


----------------------------------------
Feature #13683: Add strict Enumerable#single
https://bugs.ruby-lang.org/issues/13683#change-71494

* Author: dnagir (Dmytrii Nagirniak)
* Status: Feedback
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
### 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.



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