./ruby/mixin_class_methods_global_state.txt

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23:58 < multi_io> what I want is a mixin that provides class methods that share 
                  some global state


module M

  @@cv = 42

  def self.append_features(base)
    super
    base.extend(ClassMethods)
  end

  module ClassMethods
    def cm2
      puts @@cv
    end
  end

end


class C
  include M
  cm2  # => should print 42, but:
       #   in `cm': uninitialized class variable @@cv in M::ClassMethods (NameError)
end


Day changed to 28 Feb 2005

00:01 < shyouhei> what about a cnostant of Array?
00:01 < shyouhei> CV = [42]
00:02 < shyouhei> def cv; CV[0]; end

i.e.:

module M

  CV = [42]

  def self.append_features(base)
    super
    base.extend(ClassMethods)
  end

  module ClassMethods
    def cv
      CV[0]
    end

    def cv=(x)
      CV[0]=x
    end
  end

end


class C
  include M

  puts cv  # 42
  self.cv=23
  puts cv  # 23
end


class C2
  include M

  puts self.cv  # 23
end


class C3
  include M
end


puts C3.cv  # 23

C3.class_eval "self.cv=38"

puts C3.cv  # 38
puts C.cv   # 38



00:03 < shyouhei> is a good old trick -- implementation of a class variable on 
                  ruby 1.4;  before Matz introduced @@cv


00:13 < multi_io> thanks
00:14 < multi_io> yep, that works


00:10 < shyouhei> an alternative [use Module.new{block} (>1.8)]

i.e.:


module M
  @@cv = 42
  def self.append_features(base)
    super
    base.extend(ClassMethods)
  end
 
  ClassMethods = Module.new do
    def cv
      @@cv
    end

    def cv=(x)
      @@cv=x
    end
  end
end


class C
  include M

  puts cv  # 42
  self.cv=23
  puts cv  # 23
end


class C2
  include M

  puts self.cv  # 23
end


class C3
  include M
end


puts C3.cv  # 23

C3.class_eval "self.cv=38"

puts C3.cv  # 38
puts C.cv   # 38




0:27 < multi_io> shyouhei: your alternative works too, but I think I don't 
                  fully understand why...
00:27 < multi_io> albeit...
00:27 < multi_io> maybe I do
00:29 < multi_io> is it because the block is a closure and thus accesses a 
                  different @@cv?
00:30 < shyouhei> multi_io: exactly. blocks can "see" variables outside, while 
                  module statements cannot.
00:32 < multi_io> shyouhei: are there any other implications when using your 
                  alternative? I.e. the ClassMethods behaving differently?
00:34 < shyouhei> No.  The only difference is that @@cv (or any other variables 
                  if you use) can be seen or not.

(he's right I think -- at least all my unit tests kept working)

00:38 < shyouhei> ,... maybe. Some parts of ruby are difficult than what human 
                  beging can understand :-)

00:43 < multi_io> hmm.. there appears to be one glitch: when using rdoc to 
                  generate documentation for this, the ClassMethods's methods 
                  are documented as if they belonged to the root namespace?C
00:44 < multi_io> (this appears to be an rdoc problem...)
00:44 < multi_io> (actually I'm surprised that rdoc generated documentation for 
                  those methods at all...)

(so I chose the 1st alternative for the time being)
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