From: Brian Candler <lists@...>
Date: 2012-04-19T02:46:48+09:00
Subject: Re: Why must I know whether I extend a class or a module?

Marc Heiler wrote in post #1057208:
> Hi.
>
> I have one module and one class.
>
> module Foo
>   def test
>     puts 'This is method test() from module Foo.'
>   end
> end
>
> class Bar
>   def test
>     puts 'This is method test() from class Bar.'
>   end
> end
>
>
> If I want to modify that at a later time, I need
> to do something like:
>
> module Foo
>   def test
>     puts 'Yes, still testing'.
>   end
> end
>
> Or
>
> class Bar
>   # etc..
>
> My question is:
>
> - Why do I need to KNOW that it is a module or a class?
>
> Can't I simply say "I do not care what it is a class or a module, simply
> add this method to it."?

Yes you can:

>> class Foo; end
=> nil
>> module Bar; end
=> nil
>> Foo.module_eval { def xxx; puts "xxx"; end }
=> nil
>> Bar.module_eval { def yyy; puts "yyy"; end }
=> nil
>> Foo.new.xxx
xxx
=> nil
>> Object.new.extend(Bar).yyy
yyy
=> nil

Note that module_eval and class_eval both exist, but I have not been 
able to find a case where they behave differently; as far as I know they 
are interchangeable.

> I don't understand why I need to explicitely know that one is a module
> and the other is a class.

I guess it's just a syntax choice, and a safety feature - you don't 
accidentally add methods to a module when you meant a class, or vice 
versa.

After all, when you write

    module Foo ...

then you are talking about a single thing, "Foo", so you almost 
certainly know what kind of thing it is. If you want to do it 
dynamically (e.g. the module or class is in a variable) then you need a 
different syntax anyway:

var = Foo
var.module_eval { ... }

or:

var = :Foo
m = Object.const_get(var)
m.module_eval { ... }

Regards,

Brian.

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