Comment by bashkiddie

>Ruby has a concept of mixins (Golang interfaces), these are not objects.

Ruby "mixins" are the affordance of sharing instance methods from a module via a keyword. Modules are objects, and are instances of class Module:

  module ProvidesFooMixin
    def foo = "foo"
  end

  class Bar
    include ProvidesFooMixin
  end

  Bar.new.foo
  # => "foo"

  puts(ProvidesFooMixin.class)
  # => Module

  ProvidesFooMixin.object_id
  # => (some integer value)

Like all methods, `puts` is an object:

  method(:puts)
  # => #<Method: Object(Kernel)#puts(*)>

  method(:puts).class
  # => Method

  method(:puts).object_id
  # => (some integer value)

>What object does the method `puts` belong to?

It belongs to the object you are calling it from within. You don't need to call `puts` with a receiver because it is an instance method, just like you don't need to call an instance method `foo` via `self.foo`. But you could choose to use a receiver, since the `puts` you know and love is just another instance method. You can try `self.puts` for yourself in some context!

Your classes (and their instances) inherit this `self.puts` instance method from the Object class, which includes the Kernel module, which provides `Kernel#puts`. So the only reason you can send it as a message without a receiver is because it is just another instance method (again, the same as calling instance method `#foo` without using `self.foo`).

Caveat: You can build an "alternate universe" object hierarchy by inheriting from BasicObject, and in your alternate universe, you can choose to not `include Kernel`, and you will see that instances of your new objects do not have access to `puts` in their instance scope.

`puts` is just a method of the Kernel module: https://ruby-doc.org/3.4.1/Kernel.html#method-i-puts, just like `p` and many others. Kernel is included in the Object class that is the root of the class hierarchy, so its methods are available in every ruby object.

Mixins are just modules, which are objects, which you can call methods on. (Or rather, send messages to) You can easily verify this in irb calling a method on (for example) the Enumerable module:

    irb(main):001> Enumerable.class
    => Module

Well, “puts” is a method defined in the Kernel module, which is included in the Object class, which means it is available in all contexts (since every object is a subclass of the ‘Object’ class). So, to answer your question, the puts method belongs to the Kernel module object, and the Kernel module object is an instance of a Module object.

Answerable by a few minutes of googling. Sort of:

puts is a method which has a class: the Method class:

  irb(main):001:0> method(:puts)
  => #<Method: main.puts>
  irb(main):002:0> method(:puts).class
  => Method

  irb(main):003:0> 3.puts()
  Traceback (most recent call last):
          2: from /usr/bin/irb:11:in `<main>'
          1: from (irb):3
  NoMethodError (private method `puts' called for 3:Integer)

  irb(main):004:0> method(:puts).class.methods
  => [:allocate, :superclass, :<=>, :<=, :>=, :==, :===, :autoload?, :autoload, :included_modules, :include?, :name, :ancestors, :attr, :attr_reader, :attr_writer, :attr_accessor, :instance_methods, :public_instance_methods, :protected_instance_methods, :private_instance_methods, :constants, :const_get, :const_set, :const_defined?, :class_variables, :remove_class_variable, :class_variable_get, :class_variable_set, :class_variable_defined?, :public_constant, :freeze, :inspect, :deprecate_constant, :private_constant, :const_missing, :singleton_class?, :prepend, :class_exec, :module_eval, :class_eval, :include, :<, :>, :remove_method, :undef_method, :alias_method, :protected_method_defined?, :module_exec, :method_defined?, :public_method_defined?, :to_s, :public_class_method, :public_instance_method, :define_method, :private_method_defined?, :private_class_method, :instance_method, :instance_variable_set, :instance_variable_defined?, :remove_instance_variable, :instance_of?, :kind_of?, :is_a?, :tap, :instance_variable_get, :instance_variables, :method, :public_method, :singleton_method, :define_singleton_method, :public_send, :extend, :to_enum, :enum_for, :pp, :=~, :!~, :eql?, :respond_to?, :object_id, :send, :display, :nil?, :hash, :class, :singleton_class, :clone, :dup, :itself, :yield_self, :taint, :tainted?, :untrust, :untaint, :trust, :untrusted?, :methods, :frozen?, :protected_methods, :singleton_methods, :public_methods, :private_methods, :!, :equal?, :instance_eval, :instance_exec, :!=, :__send__, :__id__]

Modules (mixins) are objects, classes are also objects (of type Class).

The Kernel module is included in the Object class, which means its methods are available to every Ruby object and can be accessed from virtually any scope, including the top-level (global) context where self is an instance of Object called "main."

> I beg to differ. [...] Why do you not call puts with its objects name?

In the first place, I'd say what you're asking for goes beyond "everything is an object".

But I think your questions can be answered in a way that affirms that "everything is an object" in Ruby anyway.

> Why do you not call puts with its objects name?

Because it belongs to whatever object you're working in already; `puts` is identical to `self.puts`. And yes, you're always working in an object: https://bparanj.gitbooks.io/ruby-basics/content/chapter1.htm...

> What object does the method `puts` belong to?

As indicated above, it belongs to the object `self`. It gets added object via the mixing-in Kernel module into Object. Kernel is itself an instance of class Module: https://docs.ruby-lang.org/en/3.4/Module.html

The `puts` in Kernel delegates to `puts` from IO, which is likewise an instance method belonging to the object you can refer to by the name `$stdout`: https://docs.ruby-lang.org/en/3.4/IO.html

> Ruby has a concept of mixins [and], these are not objects.

Sure they are. Mixins themselves inherit from Module, and Modules are also objects (just like classes are).

Some highlights from Chapter 27 ("Library Reference: The Class Model") of the recent edition of the pickaxe book (emphasis mine):

> The Kernel module is included by class Object, so its methods are available in every Ruby object. One of the reasons for the Kernel module is to allow methods like `puts` and `gets` to be available everywhere and even to look like global commands. Kernel methods allow Ruby to still maintain an "everything is an object semantics".

and regarding mixins:

> The Module class is the class of any module you declare with the `module` keyword. Each module is an instance of class Module.

on Object:

> Object is the parent class of (almost) all classes in Ruby unless a class explicitly inherits from. BasicObject. [...] Object mixes in the Kernel module, making the built-in functions globally accessible.

tl;dr: mixins in Ruby are instances of class Module, and their methods end up bound to instances of class Object. Abstract module methods that don't belong to a concrete instance of some class that mixed in their module belong to the Object that is the module itself (the instance of class Module). (The same kind of thing is how class methods work.)