Feature #12059: `Array#single?`, `Hash#single?`

• Status: Open
• Assignee: -
• Target version: -

[sawa (Tsuyoshi Sawada)]

There are some use cases when one wants to check if an array or a hash has exactly one element. I propose Array#single? and Hash#single? that checks for such cases and returns either true or false. This is an analogy from the empty? method on the respective class.

• When creating an inflectional form out of an array:

a = ["object1", "object2"]
"There #{a.single ? "is" : "are"} #{a.length} #{a.single? ? "object" : "objects"}."
# => "There are 2 objects."

• When checking if all elements of the array are the same:

[1, 2, 2, 1].uniq.single? # => false
[1, 1, 1, 1].uniq.single? # => true

[sikachu (Prem Sichanugrist)]

I feel like the usage of this method wouldn't be generic enough, and could be accomplished easily by .size.one?

[mame (Yusuke Endoh)]

I'm neutral for the proposal itself. If it is accepted, I think "singleton?" is the best name.

In mathematics, a singleton ... is a set with exactly one element.

https://en.wikipedia.org/wiki/Singleton_(mathematics)

--
Yusuke Endoh [e-mail]

[danielpclark (Daniel P. Clark)]

Ruby currently supports the one? method on both Hash and Array.

[1].one?
# => true 
{a: 1}.one?
# => true 
[1].method(:one?).owner
# => Enumerable 

[nobu (Nobuyoshi Nakada)]

[1, false, nil].one? also returns true.

From ri Enumerable#one?:

---

enum.one? [{ |obj| block }]   -> true or false

---

Passes each element of the collection to the given block. The method returns
true if the block returns true exactly once. If the block is not given, one?
will return true only if exactly one of the collection members is true.

%w{ant bear cat}.one? { |word| word.length == 4 }  #=> true
%w{ant bear cat}.one? { |word| word.length > 4 }   #=> false
%w{ant bear cat}.one? { |word| word.length < 4 }   #=> false
[ nil, true, 99 ].one?                             #=> false
[ nil, true, false ].one?                          #=> true

[danielpclark (Daniel P. Clark)]

Right. Given the original examples by the OP Enumerable#one? works.

a = ["object1", "object2"]
"There #{a.one? ? "is" : "are"} #{a.length} #{a.one? ? "object" : "objects"}."
# => "There are 2 objects."
[1, 2, 2, 1].uniq.one?
# => false
[1, 1, 1, 1].uniq.one?
# => true