Feature #12084: `Class#instance`

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

[sawa (Tsuyoshi Sawada)]

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.

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.

Array.instance # => error

[justcolin (Colin Fulton)]

This feature would solve a lot of problems I had while doing what should have been simple meta-programming (if there is such a thing as "simple" meta-programming...).

Something to consider is what happens with nested singleton classes. Let's say you have this code:

Array.singleton_class
     .singleton_class
     .instance

Does that return Array or the first singleton class? I would think the former is more useful, but less obvious.