From: i3dmaster <i3dmaster@...>
Date: 2008-08-30T02:26:05+09:00
Subject: mysterious behavior of mixins

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
=> 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?