From: Robert Klemme <shortcutter@...>
Date: 2008-03-17T06:30:22+09:00
Subject: Re: Parametric module or injecting code via class method?

On 14.03.2008 00:42, ara howard wrote:
> On Mar 13, 2008, at 4:20 PM, Robert Klemme wrote:
> 
>> Using a Struct generated class as base class is a bad example IMHO
>> because those classes do already have the equation properties.
> 
> heh - yeah bad example.  i just used struct to avoid defining atts 'a'  
> and 'b' and an initializer - i think the example stands with that  
> though.
> 
>> At the moment the only advantage I see in using modules is avoidance  
>> of namespace pollution.  Personally I'd just have a method in class  
>> Module equate_on which defines methods as shown.
> 
> 
> well namspace pollution is no small thing!  the other huge advantage  
> is that running rdoc over something like
> 
>    class C
>      module ClassMethods
>      end
>    end
> 
> is about a billion times better than running it over
> 
>    class C
>      singleton_class.module_eval ....
> 
> not to mention grep et all.

The alternative - at least as far as I see it - is

class Module
   def equate_on(fields)
     ...
   end
end

class AnyClass
   attr_accessor :foo, :bar
   equate_on :foo
end

> the other huge advantage of using modules is subtle: if you start out  
> with code such as
> 
>    module M
> 
>      def self.included other
>        add_equate_method_to other
>      end
> 
>    end
> 
> and, over the course of your project end up with
> 
>     module  M
>       X = 4
>       Y = 2
>     end
> 
> 
> then the result of
> 
>    class C
>      include M
>    end
> 
> is that X and Y are dumped into C *even though the included block was  
> purposely setup to define what happens logically for module  
> inclusion*.  by carving out the target of inclusion one can do
> 
>    module M
>      module Methods
>      end
> 
>      def self.included other
>        other.extend Methods
>      end
>    end
> 
> and dump constants into M to your heart's content without  
> inadvertently dumping them into every class that wanted a little class  
> method.
> 
> this last bit i just recently have been using - but born out from long  
> debugging session ;-)

Yeah, but you can achieve the functionality without modules at all - at 
the small cost of defining a single instance method in class Module.  I 
guess you have some other code in mind (at least the one that you had to 
debug) but I am still not convinced that using modules is the best 
approach for such a basic thing as defining equivalence between 
instances of a class.  My .02 EUR...

Kind regards

	robert