What is the difference between include and extend in Ruby?

extend - adds the specified module's methods and constants to the target's metaclass (i.e. the singleton class) e.g.

• if you call Klazz.extend(Mod), now Klazz has Mod's methods (as class methods)
• if you call obj.extend(Mod), now obj has Mod's methods (as instance methods), but no other instance of of obj.class has those methods added.
• extend is a public method

include - By default, it mixes in the specified module's methods as instance methods in the target module/class. e.g.

• if you call class Klazz; include Mod; end;, now all instances of Klazz have access to Mod's methods (as instance methods)
• include is a private method, because it's intended to be called from within the container class/module.

However, modules very often override include's behavior by monkey-patching the included method. This is very prominent in legacy Rails code. more details from Yehuda Katz.

Further details about include, with its default behavior, assuming you've run the following code

class Klazz  include Modend

• If Mod is already included in Klazz, or one of its ancestors, the include statement has no effect
• It also includes Mod's constants in Klazz, as long as they don't clash
• It gives Klazz access to Mod's module variables, e.g. @@foo or @@bar
• raises ArgumentError if there are cyclic includes
• Attaches the module as the caller's immediate ancestor (i.e. It adds Mod to Klazz.ancestors, but Mod is not added to the chain of Klazz.superclass.superclass.superclass. So, calling super in Klazz#foo will check for Mod#foo before checking to Klazz's real superclass's foo method. See the RubySpec for details.).

Of course, the ruby core documentation is always the best place to go for these things. The RubySpec project was also a fantastic resource, because they documented the functionality precisely.

• #include RubySpec rubydoc
• #included RubySpec rubydoc
• #extend RubySpec rubydoc
• #extended RubySpec rubydoc
• #extend_object RubySpec rubydoc
• #append_features RubySpec rubydoc

What you have said is correct. However, there is more to it than that.

If you have a class Klazz and module Mod, including Mod in Klazz gives instances of Klazz access to Mod's methods. Or you can extend Klazz with Mod giving the class Klazz access to Mod's methods. But you can also extend an arbitrary object with o.extend Mod. In this case the individual object gets Mod's methods even though all other objects with the same class as o do not.

That's correct.

Behind the scenes, include is actually an alias for append_features, which (from the docs):

Ruby's default implementation is to add the constants, methods, and module variables of this module to aModule if this module has not already been added to aModule or one of its ancestors.