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Fixing the limitations in traditional Module#include

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README.markdown

Include Complete

(c) John Mair (banisterfiend) 2010 MIT license

Removes the shackles from Module#include

Use Module#include_complete to bring in singleton classes from modules. No more ugly ClassMethods and included() hook hacks.

example: include_complete():

Using include_complete the class methods of the module will be mixed in along with the instance methods:

module M
  # class method
  def self.hello
    puts "hello!"
  end

  # instance method
  def bye
    puts "bye!"
  end
end

class A
  include_complete M
end

# invoke class method
A.hello #=> hello!

# invoke instance method
A.new.bye #=> bye!

Motivation

When a class inherits from another class it inherits both the instance methods and class methods from its superclass.

Module inclusion does not work this way, only the module's instance methods are mixed into the receiver's ancestor chain. This shortcoming necessitates the ClassMethods included-hook-hack.

In my opinion this behaviour of modules violates the principle of least surprise, though I'm aware not everyone agrees with this.

include_complete was written to make module inclusion work more like class inheritance.

The extend_complete method

For completeness the extend_complete method has also been implemented. Like traditional extend it mixes the module's instance methods into the singleton class of the receiver. But where do the singleton methods on the module end up? On the singleton class of the singleton class of the receiver ;)

module M
  def self.hello
    :hello
  end
end

class C
  extend_complete M

  class << self
    hello #=> :hello
  end
end

As a result of this, it is unlikely extend_complete will be of much use to anyone :)

How does it work?

include_complete is a C extension that implements a highly modified rb_include_module() function. Traditional module inclusion uses the class pointer of the Included Module to point to the original module; include_complete instead uses the class pointer to point to a wrapped version of the singleton class of the module and stores the original module in a hidden __module__ instance variable. This wrapped singleton class is then injected into the ancestor chain of the receiver's singleton class.

Limitations

include_complete uses a recursive function to generate the Included Modules, and the base case of this recursion is reached when the singleton class of Module is encountered. In the case where the module has a meta-meta class the recursive function will not terminate and the program will hang.

It is highly unlikely and, as far as I know, next to useless for a module to possess any higher order metaclasses so this limitation is unlikely to be a problem in practice.

Criticisms

It may be argued that the current behaviour of modules is desirable and that you do not in fact want module singleton classes to be mixed in during inclusion. There are reasonably good arguments to support this case which range from the obvious: That you do not want your class to access hook methods on the module such as included and extended. To the more subtle arguments: That a singleton class is not a module and so cannot be mixed into an ancestor chain dynamically.

Nonetheless, in my opinion, the advantages of include_complete behaviour outweigh these considerations - It brings a nice symmetry and consistency in behaviour to module inclusion and class inheritance, it obviates the need for the included hook hack, and it (in my opinion) correlates more closely with expectation and satisfies the principle of least surprise.

Have a play, and decide for yourself :)

Companion Libraries

include_complete is one of a series of experimental libraries that mess with the internals of Ruby to bring new and interesting functionality to the language, see also:

  • Remix - Makes ancestor chains fully read/write.
  • Object2module - Convert Classes and Objects to Modules so they can be extended/included
  • Prepend - Prepends modules in front of a class; so method lookup starts with the module
  • GenEval - A strange new breed of instance_eval

Contact

Problems or questions contact me at github

Real Include

This project was previously called real_include

Dedication

For Dahrkael

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