[ruby-core:106587] [Ruby master Feature#12084] `Class#instance`

Issue #12084 has been updated by Eregon (Benoit Daloze).


> For your information, `NilClass` is not a singleton class. It's a class with only an instance. This is side evidence of this method is confusing. Even the original proposer can misunderstand the concept.

Interesting, I think what confuses people (including myself) is `nil.singleton_class # => NilClass`.
And basically that's because `#singleton_class` has special behavior for true/false/nil (`special_singleton_class_of`).
Such behavior is not used for any other Ruby value (the rest is TypeError or create a new anonymous singleton class).

It's a bit funny too:
```ruby
irb(main):002:0> TrueClass.singleton_class?
=> false
irb(main):003:0> true.singleton_class.singleton_class?
=> false
```

It kind of makes sense given `TrueClass`, etc are defined as regular classes, and then creating an extra/separate singleton class would be confusing (methods could be defined on either).

I guess it would be possible to create TrueClass as a singleton class (with superclass `Object`) and name it, but that's another proposal and the pros/cons are unclear to me at this stage.

----------------------------------------
Feature #12084: `Class#instance`
https://bugs.ruby-lang.org/issues/12084#change-95251

* Author: sawa (Tsuyoshi Sawada)
* Status: Open
* Priority: Normal
----------------------------------------
For meta-programming/debugging purposes, I would like to request the inverse of `Object#singleton_class`. Namely, a method that is called on a class that is a singleton class, and returns the object it is a singleton of. Since the `Singleton` module in the standard library http://ruby-doc.org/stdlib-2.3.0/libdoc/singleton/rdoc/Singleton.html assigns the method name `instance` to such classes, I think `Class#instance` should be the name for such feature.

~~~RUBY
Array.singleton_class.instance # => Array
"foo".singleton_class.instance # => "foo"
~~~

When the receiver is a class but is not a singleton class, then it should raise an error.

~~~RUBY
Array.instance # => error
~~~



-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:[e-mail]?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>