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On ruby, what is the reason for include is private, while Object#extend is public?
316
In simple words, the difference between include and extend is that 'include' is for adding methods only to an instance of a class and 'extend' is for adding methods to the class but not to its instance.
What is a private method in Ruby? It's a type of method that you can ONLY call from inside the class where it's defined. This allows you to control access to your methods.
protected methods can be called by any instance of the defining class or its subclasses. private methods can be called only from within the calling object. You cannot access another instance's private methods directly.
The only way to have external access to a private method is to call it within a public method. Also, private methods can not be called with an explicit receiver, the receiver is always implicitly self. Think of private methods as internal helper methods.
Object#extend has to be public, otherwise you wouldn't be able to use it. After all, its purpose is to mix in a module into an object, so you'd generally call it like obj.extend(Foo), which isn't possible with private methods.
Module#include is usually only used inside a module body like so:
class Bar
include Foo
end
I.e. it is usually called without a receiver, so it doesn't have to be public. Of course, it doesn't have to be private either.
My guess is the reason why it is private is that it is more invasive, because it changes the behavior of every instance of Bar, whereas Object#extend only changes a single object. Therefore, Module#include is in some sense "more dangerous" and thus is made private.
I don't know whether that is the actual reason, but it is consistent with other similar methods like Module#define_method.
169
To be able to run Foo.include(Bar) at any point would most likely be a source of very nasty bugs.
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