# Inclusion of module: differences

**URL:** <https://www.ruby-forum.com/t/inclusion-of-module-differences/138921>\
**Category:** Ruby\
**Created:** [May 29, 2008, 12:27am UTC](https://www.ruby-forum.com/t/inclusion-of-module-differences/138921 "2008-05-29T00:27:39Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![leonb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/22d042/32.png) [@leonb](https://www.ruby-forum.com/u/leonb)\
**Post date:** [May 29, 2008, 12:27am UTC](https://www.ruby-forum.com/t/inclusion-of-module-differences/138921/1 "2008-05-29T00:27:39Z")

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Hi all!

I have this code: [Parked at Loopia](http://pastie.caboo.se/205063)

And the output of that script is:

“Included from Array3”  
“Included from Array”  
“Included from Array2”  
“[]”  
“[]”  
“aaarghhhh…”  
nil

Obiously the difference in output is because I include the module in  
different ways. But why is it different? Am I doing something wrong?  
Should I use class\_eval or something?

Thanks in advance!

Leon

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**Author:** ![leonb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/22d042/32.png) [@leonb](https://www.ruby-forum.com/u/leonb)\
**Post date:** [May 29, 2008, 11:55am UTC](https://www.ruby-forum.com/t/inclusion-of-module-differences/138921/2 "2008-05-29T11:55:47Z")

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On Thu, May 29, 2008 at 12:27 AM, Leon B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> Hi all!
> 
> I have this code: [Parked at Loopia](http://pastie.caboo.se/205063)
> 
> And the output of that script is:
> 
> Obiously the difference in output is because I include the module in  
> different ways. But why is it different? Am I doing something wrong?  
> Should I use class\_eval or something?

Appending this to your test code may be enlightening:

p a.class.ancestors # =\> [Array, InstanceMethods, Enumerable, Object,  
Kernel]  
p a2.class.ancestors # =\> [Array2, Array, InstanceMethods,  
Enumerable, Object, Kernel]  
p a3.class.ancestors # =\> [Array3, InstanceMethods, Array,  
InstanceMethods, Enumerable, Object, Kernel]

This gives an indication of the search order for method lookup. For  
‘a’ which is an instance of Array, the inspect method is found in the  
Array class and included modules are thus not checked. Because you  
included InstanceMethods in array already, including it in Array2  
doesn’t do anything because Ruby tries to include a module only once  
in each class/module. Because the include in Array3 happens before the  
one in Array, it is actually performed because Ruby cannot foresee  
that you are about to include the same module in Array. So in case of  
Array3, InstanceMethods comes before Array in the method lookup path,  
in case of Array2 it doesn’t.

So basically, the order of includes in Ruby matters. Also, redefining  
methods of a class by including a module doesn’t always work because  
the module is put underneath the class, not on top of it. It only  
works if the method is defined in a superclass of the class where you  
include that module. (Still wishing Ruby had a preclude method…)

Peter

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**Author:** ![leonb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/22d042/32.png) [@leonb](https://www.ruby-forum.com/u/leonb)\
**Post date:** [May 29, 2008, 9:58pm UTC](https://www.ruby-forum.com/t/inclusion-of-module-differences/138921/3 "2008-05-29T21:58:55Z")

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Wow, thanks Peter! That was very enlightening!