Extend self?

Ruby
1

I am a newbie to Ruby. I’ve found some code in the file dependencies.rb
within Active Support:

module Dependencies
extend self
…

I would like to ask what does ‘extend self’ do here? What’s purpose? I
didn’t found any document mentioning such ‘self-extension’ case.

Cheers,
Donald

2

That’s quite simple, it saves you some typing :slight_smile:

module A
def a
p ‘a’
end
end

A.a # doesn’t work, because it is not written als def self.a - you
have to include the module in a class and initialize the class to
access it.

module A
extend self
def a
p ‘a’
end
end

A.a

=> ‘a’

voila, so now we can access the methods directly, otherwise you would
have to write

module A
def self.a
p ‘a’
end
def a
p ‘a’
end
end

or similar… makes things a bit simpler - oh, and this is no
scientific point of view of course, just wait for the multitude of
responses that will explain it in detail

( i count on you guys :slight_smile:

3

Really thanks to the quick response, but it seems that the two ways of
definitions are not really equivalent. Just did an experiment.

If I say

module A
def self.a
p ‘a’
end
end

class C
include A
end

C.a
NoMethodError: undefined method `a’ for C:Class

c = C.new
c.a
NoMethodError: undefined method `a’ for #<C:0x7e478>

While if I changed the definition of module A to

module A
extend self
def a
p ‘a’
end
end

…

C.a
NoMethodError: undefined method `a’ for C:Class
c = C.new
c.a
“a”

Anyone can explain this?

Cheers,
Donald

4

Donald Luo wrote:

C.a
NoMethodError: undefined method `a’ for C:Class

Anyone can explain this?

Module functions are not transfered by include by default. You could do
this:

class C
include A
extend A
end