What does "extend self" do?

Ruby
1

I happened to be reading dependencies.rb in the Rails source, and it
starts like this:

require ‘set’
require File.dirname(FILE) + ‘/core_ext/module/attribute_accessors’
require File.dirname(FILE) + ‘/core_ext/load_error’
require File.dirname(FILE) + ‘/core_ext/kernel’

module Dependencies #:nodoc:
extend self
…

What is the “extend self” doing? I thought at the top a module, ‘self’
was pretty much an empty context at that point… but I guess not,
since the writer obviously thinks self contains something worth
extending…?

Jeff

2

On Jan 4, 2007, at 9:09 PM, Jeff wrote:

extend self
…

What is the “extend self” doing? I thought at the top a module,
‘self’
was pretty much an empty context at that point… but I guess not,
since the writer obviously thinks self contains something worth
extending…?

It extends the very module object. That’s one-liner to add all the
module instance methods as module functions. That is

module Foo
extend self
def foo
‘foo’
end
end

allows the call Foo.foo.

To do it by hand you’d add a

module_function :method

for each method.

– fxn

3

Alle 21:09, giovedì 4 gennaio 2007, Jeff ha scritto:

…

What is the “extend self” doing? I thought at the top a module, ‘self’
was pretty much an empty context at that point… but I guess not,
since the writer obviously thinks self contains something worth
extending…?

Jeff

In a module definition, outside methods, self refers to the module
itself, as
it happens inside the definition of a class. For instance, the following
code

module MyModule
puts self.class
puts self.name
end

gives

Module
MyModule

4

On Jan 4, 2007, at 2:09 PM, Jeff wrote:

extend self
…

What is the “extend self” doing? I thought at the top a module,
‘self’
was pretty much an empty context at that point… but I guess not,
since the writer obviously thinks self contains something worth
extending…?

Well, like any method call in Ruby, there is a receiver here. In
this case, it’s just the implicit self, so the call is actually
self.extend(self). Self, in that context, is the module
Dependancies. Dependancies.extend(Dependancies) means, duplicate all
the instance methods as module methods.

Does that make sense?

James Edward G. II

5

Google seems to have lost my previous 2 attempts to reply to this
thread, here we go again…

Awesome, thanks for the help everyone! For some reason when I saw the
“extend” at the top of the class, instead of at the bottom, I didn’t
see how it know about the instance methods that follow. Doh!

Thanks again,
Jeff

6

Xavier N. wrote:

allows the call Foo.foo.

To do it by hand you’d add a

module_function :method

for each method.

Not quite the same however. Using module_function actually creates a
new method that is a copy of the first. extend_self OTOH adds the
module to it’s own metaclass’ inheritance chain, so in that case they
are the same method.

T.

7

Hi –

On Fri, 5 Jan 2007, Jeff wrote:

…

What is the “extend self” doing? I thought at the top a module, ‘self’
was pretty much an empty context at that point… but I guess not,
since the writer obviously thinks self contains something worth
extending…?

self is never empty; it’s always something. At the top of a module
definition, it’s the module object itself:

module M
p self
end

will print:

M

So what extend self does is it extends the module object by making the
instance methods in the module available to it:

module M
def greet
puts “hi”
end
extend self
end

M.greet # => hi

Now the object M has access to the instance methods defined in M –
which it also happens to be :slight_smile:

David

8

On Jan 4, 2007, at 10:05 PM, Trans wrote:

To do it by hand you’d add a

module_function :method

for each method.

Not quite the same however. Using module_function actually creates a
new method that is a copy of the first. extend_self OTOH adds the
module to it’s own metaclass’ inheritance chain, so in that case they
are the same method.

Right, I just meant them to be conceptually similar to add some
redundancy to the explanation. But it wasn’t exact.

Thank you!

– fxn

9

[email protected] wrote:

So what extend self does is it extends the module object by making the
instance methods in the module available to it:

(and to all the others who said similar things)

Thanks everyone! For some reason I thought that order was important,
and having it appear before defining the instance methods would be
futile. As soon as I started reading all the replies I realized I was
being silly.

Thanks again.
Jeff