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comp.lang.ruby

What does "extend self" do?

Jeff

1/4/2007 8:09:00 PM

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
8 Answers

Stefano Crocco

1/4/2007 8:16:00 PM

0

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

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

dblack

1/4/2007 8:17:00 PM

0

James Gray

1/4/2007 8:17:00 PM

0

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

> 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...?

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 Gray II

Xavier Noria

1/4/2007 8:24:00 PM

0

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

> 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...?

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

Trans

1/4/2007 9:06:00 PM

0


Xavier Noria wrote:
> On Jan 4, 2007, at 9:09 PM, Jeff wrote:
>
> > 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...?
>
> 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.

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.

Xavier Noria

1/4/2007 9:18:00 PM

0

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

Jeff

1/4/2007 10:03:00 PM

0

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

Jeff

1/5/2007 12:02:00 AM

0


dblack@wobblini.net 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