What happens to instance variables in modules when they are extended into classes

[me]

Hi,

Please take a look at the code pasted at this URL (I will include the
code in the body of this message (below) for future readers, but the
rafb.net post will give readers for the next 24 hours something better
to look at): http://rafb.net/p/9MkY3H79.html

Why is it that an instance method of the module gets mixed into the
class, but instance variables of the module do not get mixed in?

Whats the best strategy for writing instance methods in a module that
depend on some state in the mixee?

Thanks very much,
Mike.

And now here is the code snippet:

module M
@module_instance_variable = 'miv'

def module_instance_method
@module_instance_variable
end
end


class C
extend M
end


puts "Is module_instance_method in the methods of the class?
#{C.methods.include?('module_instance_method')}"
puts "Is @module_instance_variable in the instance variables of the
class? #{C.instance_variables.include?('@module_instance_variable')}"
puts "Is @module_instance_variable in the class variables of the
class? #{C.class_variables.include?('@module_instance_variable')}"

 

[Brian Candler]

Why is it that an instance method of the module gets mixed into the
class, but instance variables of the module do not get mixed in?

Whats the best strategy for writing instance methods in a module that
depend on some state in the mixee?

Thanks very much,
Mike.

And now here is the code snippet:

module M
@module_instance_variable = 'miv'

def module_instance_method
@module_instance_variable
end
end


class C
extend M
end

Presumably you understand why the following code prints 'nil':

class C
@foo = "hello"

def bar
puts @foo
end
end
C.new.bar

So I'm not sure what you're trying to achieve with your code snippet. Do you
want @module_instance_variable to be an instance variable of module M, or an
instance variable of class C, or an instance variable of instances of class
C?

What you have at the moment is an instance variable of M, as shown by:

puts M.module_eval { @module_instance_variable }

If you want to set up instance variables of class C itself (remembering that
many different C's may each extend M), then one approach is

module M
def self.extended(obj)
puts "hello!"
obj.instance_eval { @miv = "miv" }
end
end

class C
extend M
end

puts C.instance_variables

HTH,

Brian.

 

[me]

Presumably you understand why the following code prints 'nil':

class C
@foo = "hello"

def bar
puts @foo
end
end
C.new.bar

So I'm not sure what you're trying to achieve with your code snippet. Do you
want @module_instance_variable to be an instance variable of module M, or an
instance variable of class C, or an instance variable of instances of class
C?

What you have at the moment is an instance variable of M, as shown by:

puts M.module_eval { @module_instance_variable }

If you want to set up instance variables of class C itself (remembering that
many different C's may each extend M), then one approach is

module M
def self.extended(obj)
puts "hello!"
obj.instance_eval { @miv = "miv" }
end
end

class C
extend M
end

puts C.instance_variables

HTH,

Brian.

Ok - yes thats twigged it for me. Thanks very much Brian.

Mike

 

[Giles Bowkett]

You know, there's a downside to your e-mail addy, (e-mail address removed). In
Gmail it says the thread was started by me and I go, oh, hmm, I don't
remember posting that. Was I in a hypnotic trance? Was I possessed by
demons? Did I post it a long long time ago?

(Fortunately the inevitable identity crisis resolves itself peacefully
and uneventfully.)