From: "Chr. Rippel" <chr_news@...>
Date: 2002-01-26T20:32:55+09:00
Subject: TypeError verus NoMethodError

I wanted to point out an inconistency (IMO as usual) that build
in singleton/immutable classes NilClass, TrueClass FalseClass,
Symbols and Numeric instances follow the old style Exception
scheme of raising a TypeError when attempting to clone or dup -
instead of following the new style NoMethodError exception.

I also take this as an opportunity to send in a fix for my revisited
version of singleton.rb - (I just realized that I removed the wrong
dup ...).  Hm, maybe we should stick with the old version
anyway ...

/Christoph

PS. A patch is pointless it's just about as long ...

----
# The Singleton module implements the Singleton pattern - i.e.
#
# class Klass
#    include Singleton
#    # ...
# end
#
# *  ensures that only one instance of Klass called ``the instance''
#    can be created.
#
#    a,b  = Klass.instance, Klass.instance
#    a == b   # => true
#    a.new     #  NoMethodError - new is private ...
#
# *  ``The instance'' is created at instanciation time,
#    in other words the first call of Klass.instance(), thus
#
#    class OtherKlass
#        include Singleton
#        # ...
#    end
#    p "#{ObjectSpace.each_object(OtherKlass) {}}" # => 0
#
# *  This behavior is preserved under inheritance.
#
#
# This is achieved by marking
# *  Klass.new and Klass.allocate - as private
# *  removing #clone and #dup and modifying
# *  Klass.inherited(sub_klass)- to ensure that the
#    Singleton pattern is properly inherited.
#
# In addition Klass is providing the additional class methods
# *  Klass.instance()  - returning ``the instance''
# *  Klass._load(str) - returning ``the instance''
# *  Klass._wait()    - a hook method putting a second (or n-th)
#    thread calling Klass.instance on a waiting loop if the first call
#    to Klass.instance is still in progress.
#
# The sole instance method of Singleton is
# *  _dump(depth) - returning the empty string.
#    The default Marshalling strategy is to strip all state information,
#    i.e. instance variables and taint state etc. from ``the instance''.
#    Providing custom _dump(depth) and _load(str) method allows
#    the (partial) resurrection of a previous state of ``the instance''.
#    Also see the last example.

module Singleton
    private

    def _dump(depth=-1) '' end

    class << self
        undef_method :extend_object
        private
        def append_features(mod)
            unless  mod.instance_of? (Class)
                raise TypeError.new "Inclusion of the OO-Singleton module in
module #{mod}"
            end
            unless   (class << mod; self end) <= (class << Object; self end)
                raise TypeError.new "Inclusion of the OO-Singleton module in
singleton type"
            end
            super
        end

        def included (klass)
            klass.class_eval do
                undef_method :clone
                undef_method :dup
            end

            class << klass
                private  :new, :allocate
                define_method (:instance, Singleton::FirstInstanceCall)

                private
                def _wait
                    sleep(0.1)  while false.equal?(@__instance__)
                end
                def _load(str) instance end
                def inherited(sub_klass)
                    super
                    class << sub_klass
                        define_method (:instance, Singleton::FirstInstanceCall)
                    end
                    sub_klass.instance_eval do @__instance__ = nil end
                end
            end
            klass.instance_eval do @__instance__ = nil end
        end
    end

    FirstInstanceCall = proc do
        # @__instance__ takes on one of the following values
        # * nil     -  before and after a failed creation
        # * false  -  during creation
        # * sub_class instance  -  after a successful creation
        _wait()  unless @__instance__.nil?
        Thread.critical = true
        unless @__instance__
            @__instance__  = false
            Thread.critical = false
            begin
                @__instance__ = new
            ensure
                if @__instance__
                    def self.instance() @__instance__ end
                else
                    # failed instance creation
                    @__instance__ = nil
                end
            end
        else
            Thread.critical = false
        end
        @__instance__
    end
end

if __FILE__ == $0

#basic example
class SomeSingletonClass
    include Singleton
end
a = SomeSingletonClass.instance
b = SomeSingletonClass.instance # a and b are same object
p a == b # => true
begin
    SomeSingletonClass.new
rescue  NoMethodError => mes
    puts mes
end


# threaded example with exception and customized hook #_wait method
Thread.abort_on_exception = false
def num_of_instances(mod)
    "#{ObjectSpace.each_object(mod){}} #{mod} instance"
end


class Ups < SomeSingletonClass
    def initialize
        type.__sleep
        puts "initialize called by thread ##{Thread.current[:i]}"
    end
    class << self
        def _wait
            @enter.push Thread.current[:i]
            sleep 0.02 while false.equal?(@__instance__)
            @leave.push Thread.current[:i]
        end
        def __sleep
            sleep (rand(0.1))
        end
        def allocate
            __sleep
            def self.allocate; __sleep; super() end
            raise  "allocation in thread ##{Thread.current[:i]} aborted"
        end
        def instanciate_all
            @enter = []
            @leave = []
            1.upto(9) do |i|
                Thread.new do
                    begin
                        Thread.current[:i] = i
                        __sleep
                        instance
                    rescue RuntimeError => mes
                        puts mes
                    end
                end
            end
            puts "Before there were #{num_of_instances(Ups)}s"
            sleep 6
            puts "Now there is #{num_of_instances(Ups)}"
            puts "#{@enter.join "; "} was the order of threads entering the
waiting loop"
            puts "#{@leave.join "; "} was the order of threads leaving the
waiting loop"
        end
    end
end

Ups.instanciate_all
#  results in message like
#  Before there were 0 Ups instances
#  boom - allocation in thread #8 aborted
#  initialize called by thread #3
#  Now there is 1 Ups instance
#  2; 3; 6; 1; 7; 5; 9; 4 was the order of threads entering the waiting loop
#  3; 2; 1; 7; 6; 5; 4; 9 was the order of threads leaving the waiting loop


# Customized marshalling
class A
    include Singleton
    attr_accessor :persist, :die
    def _dump(depth)
        # this strips the @die information from the instance
        Marshal.dump(@persist,depth)
    end
end
def A._load(str)
    instance.persist = Marshal.load(str)
    instance
end

a = A.instance
a.persist = ["persist"]
a.die = "die"

stored_state = Marshal.dump(a)
# change state
a.persist = nil
a.die = nil
b = Marshal.load(stored_state)
p a == b  #  => true
p a.persist  #  => ["persist"]
p a.die     #  => nil

end
-----