Issue #13683 has been updated by Dan0042 (Daniel DeLorme).
> If collection is empty and a block was given, returns the block's return value:
I really think the block form should be like find/select
```ruby
[1,2,3].one{ _1.even? } #=> 2
[1,2,3,4].one{ _1.even? } #=> error
[1,2,3,4].one(nil){ _1.even? } #=> nil
```
> Having `Enumerable#find` take an optional keyword argument, say `exception:`, would make more sense, be useful, and have more generality.
I don't think so; find only returns the first element found. An argument that makes it continue searching and return an exception if it finds a second match... that alters the fundamental behavior too much imho.
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Feature #13683: Add strict Enumerable#single
https://bugs.ruby-lang.org/issues/13683#change-87280
* Author: dnagir (Dmytrii Nagirniak)
* Status: Feedback
* Priority: Normal
----------------------------------------
### 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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