From: Trans <transfire@...>
Date: 2008-08-30T03:17:26+09:00
Subject: Re: mysterious behavior of mixins



On Aug 29, 1:26 pm, i3dmaster <i3dmas...@gmail.com> wrote:
> I am trying to learn how mixins are behaved and encountered a strange
> behavior and got me even confused...
>
> irb(main):151:0> module A
> irb(main):152:1>   def self.showself
> irb(main):153:2>     puts "self is #{self.inspect}"
> irb(main):154:2>   end
> irb(main):155:1>   def showself
> irb(main):156:2>     puts "self is #{self.inspect}"
> irb(main):157:2>   end
> irb(main):158:1> end
> => nil
> irb(main):159:0> A.showself
> self is A
> => nil
> irb(main):160:0> class B
> irb(main):161:1>   include A
> irb(main):162:1> end
> => B
> irb(main):163:0> B.showself
> self is B

Are you sure about this? Ruby doesn't do that. (Which I've never been
completely happy about, but that's another story...)

> => nil
> irb(main):164:0> B.new.showself
> self is #<B:0x4ce78>
> => nil
> irb(main):165:0> class C
> irb(main):166:1>   def self.showself
> irb(main):167:2>     puts "in C's showself"
> irb(main):168:2>   end
> irb(main):169:1>
> irb(main):170:1* end
> => nil
> irb(main):171:0> class B < C
> irb(main):172:1>   include A
> irb(main):173:1> end
> TypeError: superclass mismatch for class B
>         from (irb):171
>         from :0
>
> Then I tried another several times and it always throws out TypeError,
> then I tried this:
>
> irb(main):022:0> class D
> irb(main):023:1>   include A
> irb(main):024:1> end
> => D
> irb(main):025:0> D.showself
> NoMethodError: undefined method `showself' for D:Class
>         from (irb):25
>         from :0
> irb(main):026:0> ^C
> irb(main):026:0> quit
>
> I was able to minxin A's class method (using class B) just a moment
> ago. What the heck is going on? Questions:
> Does mixins allow mixin class methods or not? If not, what's the
> common idiom to mixin class methods?
> Is there a concept of protected or private mixins? I see mixins be
> able to just mixed in with arbitrary classes. Is there possible to
> force some certian mixins only be able to be mixed in with a certain
> classes or other modules?

Look into Module#included and Module#append_features. Eg.

  module X
    def self.included(base)
      base.extend MetaX
    end

    module MetaX
      # class method here
    end

    extend MetaX
  end

T.