Bug #11064: #singleton_methods for objects with special singleton_class returns an empty array

• Status: Open
• Priority: Normal
• Assignee: -
• Target version: -
• ruby -v: ruby 2.2.1p85 (2015-02-26 revision 49769) [x86_64-linux]
• Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN
• Tags: core, maybenotbug

[rbjl (Jan Lelis)]

def nil.bla
  42  
end

# works
nil.bla #=> 42
nil.singleton_method(:bla) #=> #<Method: NilClass#bla>
NilClass.instance_methods.include? :bla #=> true

# does not work
nil.singleton_methods #=> []

[matz (Yukihiro Matsumoto)]

nil does not have its singleton class, but NilClass plays the role of the singleton class (since nil is the only instance of NilClass).
So a singleton method definition defines a method in NilClass. It's a kind of special treatment of singleton methods. Same for true and false.
We have several options:

• keep the behavior as it is, since nil does not have a singleton class
• nil.singleton_methods should return methods defined in NilClass (same for TrueClass and FalseClass)

Both have their own trade-offs. Either is OK for me but I slightly prefer the former, since it's easier.

Matz.

[sawa (Tsuyoshi Sawada)]

matz (Yukihiro Matsumoto) wrote in #note-1:

nil does not have its singleton class, but NilClass plays the role of the singleton class

I would appreciate it if @matz (Yukihiro Matsumoto) could kindly explain what the difference is between "being a singleton class" and "playing the role of a singleton class".

At the moment, I cannot understand at all how they are different. I am actually puzzled by a related comment by matz: https://bugs.ruby-lang.org/issues/12084#note-6. Why can we not simply say "NilClass is the singleton class of nil"?

[Eregon (Benoit Daloze)]

• Related to Bug #11063: Special singleton class should return true for singleton_class? test added