Got completely stuck with using 'class <<' feature


#1

Hello!

I’m more than a bit confused with module inclusion and ‘class <<’ syntax
for
class definition, so I’m asking for some help.
Here’s a short program that produces an effect I cannot understand:

module One
def self.included mod
mod.extend Two
end

def one
‘one’
end
end

module Two
def two
‘two’
end
end

module Three
def self.included mod
class << mod
include One

  def three
    'three'
  end
end

end
end

class Klass1
include Three
end

class Klass2
include One
end

I get the following results after loading the .rb file:

irb(main):003:0> Klass1.methods.include? ‘one’
=> true
irb(main):004:0> Klass1.instance_methods.include? ‘one’
=> false
irb(main):005:0> Klass2.instance_methods.include? ‘one’
=> true
irb(main):006:0> Klass2.methods.include? ‘one’
=> false
irb(main):007:0> Klass1.methods.include? ‘three’
=> true
irb(main):008:0> Klass1.methods.include? ‘two’
=> false
irb(main):009:0> Klass1.instance_methods.include? ‘two’
=> false
irb(main):010:0> Klass2.methods.include? ‘two’
=> true
irb(main):011:0> Klass2.instance_methods.include? ‘two’
=> false

What I am asking for is a step by step explanation of what happens here,
as
I’m completely confused.
What I am trying to achieve is to make #two a class method of Klass1 - I
have no idea, why it doesn’t show up in Klass1.methods.

The complexity of this array of inclusions is actually explained by the
fact
that this sample is an oversimplified pattern extracted
from a project I’m working on now with modules One and Two being used by
client classes in my script. One uses extend to append
Two’s methods to a class’ list of class methods, thus extending both its
class and instance methods. However to work properly
methods form One and Two require some additional methods to be defined
in
classes that include them. I want my
classes (e.g. Klass1) to include both those necessary methods and module
One
by specifying just one include statement.
With that statement a utility module would be included (module Three)
that
both defines additional methods I mentioned
before and mixes One into a class it gets included into. Unfortunately I
didn’t succeed with this approach as methods
from Two wouldn’t mix as client class’ class methods. So I created a
small
model of this situation to trace possible problems
with inclusions, and the results got me completely confused, so I
decided to
post the code for small model in this mailing list
and ask for help and explanation.

Best regards,
Maksim B.


#2

Maksim B. schrieb:

Here’s a short program that produces an effect I cannot understand:

module Two
def two
‘two’
end
end

I show the effect of your program with boxes:

Two
±----+
| two |
±----+

This means, when you include module Two, you get an instance method
#two.

module One
def self.included mod
mod.extend Two
end

def one
‘one’
end
end

When you include module One, you get an instance method #one.
Additionally, you extend the including module/class with module Two,
which is the same as including module Two into the singleton class:

One
±--------+
| ±----+ |
| | two | |
| ±----+ |
| one |
±--------+

This means, when you include module One, you get an instance method #one
plus an instance method in the singleton class, or a class method .two.

end
Here, all you are doing is altering the singleton class of the including
module/class:

Three
±------------+
| ±--------+ |
| | ±----+ | |
| | | two | | |
| | ±----+ | |
| | one | |
| | three | |
| ±--------+ |
±------------+

You include module One into the singleton class, and you add an
additional class method .three. Note that “two” now is a method of the
singleton class’ singleton class. (You can nest them as long as you
want.)

class Klass1
include Three
end

class Klass2
include One
end

After this, the following results are correct:

irb(main):008:0> Klass1.methods.include? ‘two’
What I am trying to achieve is to make #two a class method of Klass1 - I
classes that include them. I want my
decided to
post the code for small model in this mailing list
and ask for help and explanation.

I’m not sure I understand you, but I think what you want is:

Three
±--------+
| ±----+ |
| | two | |
| ±----+ |
| one |
| three |
±--------+

For this you should try the following code:

module Three
def self.included mod
mod.instance_eval { include One }
end

 def three
   'three'
 end

end

Then you get:

Klass1.instance_methods - Object.instance_methods # => [“three”, “one”]
Klass1.methods - Object.methods # => [“two”]

which I think is what you want. Note that you can’t write

mod.include One

because #include is a private method, so you have to use #instance_eval.

Regards,
Pit