From: John Croisant <jacius@...>
Date: 2010-08-02T07:48:57+09:00
Subject: Re: how to check if some object is included in some module?

On Sun, Aug 1, 2010 at 2:20 PM, Jarmo Pertman <jarmo.p@gmail.com> wrote:
> I know that there are methods #kind_of?/#is_a? to check if one object
> is a same type or a subclass of another object. But what if i want to
> check if one object is in some module? For example:
>
> module MyModule
>  class MyClass
>  end
> end
>
> m = MyModule::MyClass.new
> m.kind_of?(MyModule) # => false
>
> I would be happy if the statement above would return true since
> MyClass is in module MyModule. Or why wouldn't it (except that it
> hasn't meant to with current implementation)?
>
> Anyway, i was hoping to find some other method for this kind of
> checks, but was unable to find any from Class, Module, Object, Kernel
> classes/modules. So i made one example myself: http://gist.github.com/503646
>
> I still think that there should be some easier built-in way to perform
> this kind of checks. Maybe i have just missed it...
>
> Jarmo Pertman

I would do it this way:

  MyModule.constants.include?(:MyClass) # note the ":", it's a symbol

(It's also possible to define a general method which takes a class,
gets the class's name, converts it to a symbol, and then tests whether
the module has that constant. But I'll leave that an an excercise for
the reader.)

Another possible way to do it would be this:

  MyModule.const_get(:MyClass)

This will return MyModule::MyClass if it exists (which will be
considered true in an "if" statement), or raise NameError if it
doesn't exist. Because it raises an error in the negative case, it's
not convenient to use it in an "if" statement, which is why I consider
it less desirable than the first way.

HTH,

- John

P.S. I second David's advice not to modify Object#descendant?. The
relationship you are testing for is not ancestry/inheritance, so it
doesn't make much sense for the #descendant? method to do it.