Comments on: An introduction to modules : Part 2

By: Daniel Fischer

Daniel Fischer — Mon, 10 Sep 2007 18:11:18 +0000

Hey, that was wonderful! Thank you

By: BK

BK — Tue, 18 Sep 2007 13:24:38 +0000

So if I understand this correctly, it’s almost like a “preimplemented” interface… Interesting! Finally a plus for Ruby over dotnet!

By: Frank

Frank — Mon, 29 Oct 2007 18:54:51 +0000

@BK,

Exactly. I find the term “preimplemented” interface pretty accurate in fact! Thanks

By: Vivek Dandage

Vivek Dandage — Wed, 16 Jul 2008 09:14:40 +0000

Hey, really cool. Thank you.

By: jonny_noog

jonny_noog — Thu, 24 Jul 2008 07:00:46 +0000

“Finally a plus for Ruby over dotnet!”

…Finally?

By: How to display a collection grouped by an attribute value in Rails

How to display a collection grouped by an attribute value in Rails — Mon, 24 Nov 2008 11:36:43 +0000

[…] is a job for Enumerable#group_by (Enumerable is a module that is mixed in the Array […]

By: wdj

wdj — Tue, 21 Jul 2009 13:58:24 +0000

Alas….I understand Ruby Modules …Thanks

By: Pancho

Pancho — Tue, 24 Nov 2009 05:42:19 +0000

Thanks so much for such a clear and understandable explanation!

By: freenight

freenight — Tue, 24 Nov 2009 07:31:00 +0000

thanks Frank, got it. but what’s about the ‘instance variables’ in module??

By: freenight

freenight — Tue, 24 Nov 2009 07:32:13 +0000

wow … that’s not my avatar..

By: Frank

Frank — Fri, 27 Nov 2009 15:53:49 +0000

Thanks guys, I’m glad you liked the post!

freenight,this is a very good question.

Instance variables in modules can belong to either :

1: The module
2: The client class

If you need a variable that will exist at “module level” you will do it this way

module SomeModule
def self.set_instance_variable
#this method is not accessible for a client class
#in this case the term “instance” refers to the module itself
@x = “bla bla bla”
end

def self.get_instance_variable
@x
end
end

SomeModule.set_instance_variable
SomeModule.get_instance_variable #output : bla bla bla

Conclusion : When you use instance variables inside “module level” methods, these variables are only available in the module itself.

Now, if you want to use instance variables where an “instance” refers to the client class instance and NOT the module itself, you have to proceed this way.

module SomeModule
def set_instance_variable
#only difference is that I removed the ’self.’ part in the method definition. It means that this
#method will reside into the client class instance and NOT in the module itself
@x = “bla bla bla”
end

def get_instance_variable
@x
end
end

class SomeClass
include SomeModule
end

x=SomeClass.new

x.set_instance_variable
x.get_instance_variable #output is “bla bla bla”

By: freenight

freenight — Sat, 28 Nov 2009 05:16:06 +0000

got it Frank.

the first form:

module SomeModule
def self.talk
@x = ..

since module themselves are object, classes that get this module included have no access to it’s instance method, thus this @x is visible only in the module itself.

on the other hand, instance methods defined in modules becomes instance methods of object with which the class has got the module mixed in, thus this instance variable is.. well instance variable in
per-object basis,right Frank??

sorry for my pool english..

By: Frank

Frank — Sat, 28 Nov 2009 13:52:59 +0000

freenight, Exactly.

I’ll try to sum it up in a more concise way :

When you are in a module and define a “module method” (e.g. def self.my_module_method), the execution context is the module itself (which is an object as well). When you are in a module and define an “instance method” (e.g. def my_instance_method), the execution context is the class instance that includes the module. This is like you had pasted all the instance methods from the module in your class. This is why we can say that the instance variables are in the class