# Module Definition Idioms

**URL:** <https://www.ruby-forum.com/t/module-definition-idioms/98361>\
**Category:** Ruby\
**Created:** [July 14, 2006, 3:19am UTC](https://www.ruby-forum.com/t/module-definition-idioms/98361 "2006-07-14T03:19:30Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![xmg-eric](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/x/f04885/32.png) [@xmg-eric](https://www.ruby-forum.com/u/xmg-eric)\
**Post date:** [July 14, 2006, 3:19am UTC](https://www.ruby-forum.com/t/module-definition-idioms/98361/1 "2006-07-14T03:19:30Z")

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Looking at HTree, I see a few more things  
that seem weird to me, but which are  
probably perfectly normal Ruby:

## file: parse.rb

module HTree  
def HTree.parse  
…  
def

def HTree.parse\_xml  
…  
end

…

def Text.parse\_cdata\_section(raw\_string)  
…  
end

def Comment.parse(raw\_string)  
…  
end

## end # module

This is all very strange and new to me.  
There are a dozen files, each of which adds  
to the “HTree” module, and in each module,  
methods are added to several classes.

What makes that a desirable way to structure  
things? In the Java world, each class is  
in its own file. What are the advantages of  
doing things this way, instead?

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<div class="post-metadata">

**Author:** ![xmg-eric](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/x/f04885/32.png) [@xmg-eric](https://www.ruby-forum.com/u/xmg-eric)\
**Post date:** [July 14, 2006, 6:26am UTC](https://www.ruby-forum.com/t/module-definition-idioms/98361/2 "2006-07-14T06:26:00Z")

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On 7/13/06, Eric A. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> def Comment.parse(raw\_string)
> 
> What makes that a desirable way to structure  
> things? In the Java world, each class is  
> in its own file. What are the advantages of  
> doing things this way, instead?

Probably to define the module HTree as the top level namespace.  
“Text” and “Comment” are pretty common names for classes/modules, so  
putting them inside an HTree module puts them in their own namespace  
and keeps everything organized.

- Rob

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<div class="post-metadata">

**Author:** ![xmg-eric](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/x/f04885/32.png) [@xmg-eric](https://www.ruby-forum.com/u/xmg-eric)\
**Post date:** [July 14, 2006, 8:53am UTC](https://www.ruby-forum.com/t/module-definition-idioms/98361/3 "2006-07-14T08:53:28Z")

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Rob S. wrote:

> > ```
> > ...
> > 
> > ```
> 
> putting them inside an HTree module puts them in their own namespace  
> and keeps everything organized.

Aha. Module as namespace. That rings a bell.  
Thanks.

---

<div class="post-metadata">

**Author:** ![xmg-eric](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/x/f04885/32.png) [@xmg-eric](https://www.ruby-forum.com/u/xmg-eric)\
**Post date:** [July 14, 2006, 9:00am UTC](https://www.ruby-forum.com/t/module-definition-idioms/98361/4 "2006-07-14T09:00:13Z")

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Ok. I get that using a module lets you establish  
a namespace. Now for the other part of the question:

Why is the HTree module split up among half  
a dozen files: parse.rb, and others??

And come to think of it, is it possible that  
the Comment and HTree classes are intuited by  
Ruby, since they’re not explicitly defined,  
in this file at least?

Rob S. wrote:

> > ```
> > ...
> > 
> > ```
> > 
> > end
> > 
> > ## end # module
> > 
> > What makes that a desirable way to structure  
> > things?
> 
> Probably to define the module HTree as the top level namespace.

---

<div class="post-metadata">

**Author:** ![xmg-eric](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/x/f04885/32.png) [@xmg-eric](https://www.ruby-forum.com/u/xmg-eric)\
**Post date:** [July 14, 2006, 10:49am UTC](https://www.ruby-forum.com/t/module-definition-idioms/98361/5 "2006-07-14T10:49:57Z")

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On Jul 14, 2006, at 7:57, Eric A. wrote:

> Ok. I get that using a module lets you establish  
> a namespace. Now for the other part of the question:
> 
> Why is the HTree module split up among half  
> a dozen files: parse.rb, and others??

This question sounds to me an awful lot like “why organise your code  
at all? why not just use one big file?” For large projects (or even  
small-to-medium projects that you expect other people to use), some  
sort of functional organisation is necessary.

> And come to think of it, is it possible that  
> the Comment and HTree classes are intuited by  
> Ruby, since they’re not explicitly defined,  
> in this file at least?

I don’t believe so. Class names are constants, and as such have to  
be initialized somewhere before they’re they’re used. Try this example:

module Foo  
def Bar.bar  
puts “hello world”  
end  
end

-:2: uninitialized constant Foo::Bar (NameError)

HTree::Comment and HTree::Text will be defined somewhere in one of  
the files 'require’d into parse.rb (this includes the whole chain of  
a-requires-b-requires-c-etc).

I’ll throw in my bit on the original question as well:

Rob S. wrote:

> On 7/13/06, Eric A. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:
> 
> > [HTree parse.rb example]  
> > What makes that a desirable way to structure  
> > things?  
> > Probably to define the module HTree as the top level namespace.

In this case it also seems to encapsulate similar behaviour within a  
single file: all the parsing methods for the various classes are  
defined in a single location. More generally, if Java is “one class,  
one file,” this approach would be “one behaviour, one file.”

> In the Java world, each class is  
> in its own file. What are the advantages of  
> doing things this way, instead?

Quite similar to the Java model, I’d say, just substitute ‘behaviour’  
for ‘class’ in all the points. For example, if, in Java, you’d say  
that an error would be easy to localise because you know where the  
class is defined, here you might say it’s easy to localise because  
you know where that behaviour is defined.

Which one is more appropriate likely depends both on the code and the  
coder. I imagine that for certain class structures where there are  
many similarities in behaviour across the classes(e.g., there’s an  
A#a, B#a, C#a, D#a, etc. that all do similar things), but little  
interaction within classes (e.g., A#a and A#b don’t particularly care  
about each other), grouping by behaviour makes a lot of sense, at  
least to me.

matthew smillie.

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<div class="post-metadata">

**Author:** ![xmg-eric](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/x/f04885/32.png) [@xmg-eric](https://www.ruby-forum.com/u/xmg-eric)\
**Post date:** [July 15, 2006, 1:40am UTC](https://www.ruby-forum.com/t/module-definition-idioms/98361/6 "2006-07-15T01:40:09Z")

</div>

Matthew S. wrote:

> small-to-medium projects that you expect other people to use), some sort  
> of functional organisation is necessary.

You are kidding, right? I am just not grokking this  
particular style of organization. Everything I’ve  
seen about modules explains the syntax. But the  
syntax is being used in ways I never expected.

So far, I’ve seen:

- classes inside of modules (expected)
- modules inside of classes (why?)
- big files with multiple modules in them (why?)
- modules split up among multiple files,  
adding methods to different classes  
(explained below. Thanks!)

Understanding the syntax of “module” and “class”  
doesn’t begin to help understand the philosophy  
behind those organizational styles, or help me  
understand the mental model that produced them.

So I’m here, hoping to expand my comprehension.

> Class names are constants, and as such have to be  
> initialized somewhere before they’re they’re used.  
> Try this example:
> 
> module Foo  
> def Bar.bar  
> puts “hello world”  
> end  
> end
> 
> -:2: uninitialized constant Foo::Bar (NameError)

Right. I should have tried that experiment.

> HTree::Comment and HTree::Text will be defined somewhere in one of the  
> files 'require’d into parse.rb (this includes the whole chain of  
> a-requires-b-requires-c-etc).

Makes sense.

> single file: all the parsing methods for the various classes are defined  
> in a single location. More generally, if Java is “one class, one file,”  
> this approach would be “one behaviour, one file.”

AHA! That is a /really/ interesting observation.  
That seems useful, somehow. I like it immediately.

> Which one is more appropriate likely depends both on the code and the  
> coder. I imagine that for certain class structures where there are many  
> similarities in behaviour across the classes(e.g., there’s an A#a, B#a,  
> C#a, D#a, etc. that all do similar things), but little interaction  
> within classes (e.g., A#a and A#b don’t particularly care about each  
> other), grouping by behaviour makes a lot of sense, at least to me.

Thanks much. Grouping by behavior is a very interesting  
concept. When I learn something that changes one bit of  
parsing code, there is a good chance that I will need to  
modify other bits, as well. That organization could make  
it easier to replicate changes.

So that covers