=begin
= preserved.rb
== SYNOPSIS

  require "preserved"

  class XXX
    def foo
      # ... should not be redefined ...
    end

    preserved :foo
  end

  class YYY < XXX
    def foo #########!! NameError raised here

== CLASS Module
=== METHODS
: preserved(name...)
  Prohibits redefining the methods specified by name. 
  We call these methods "preserved methods". 

: preserved_methods
  Returns an array which consists of preserved methods. 
=end

class Module
  PRESERVED_VERSION = "2001-07-20"

  def preserved(*meth)
    instance_eval %q{
      Preserved_methods = [] unless defined?(Preserved_methods)
      meth.each do |m|
	m = m.id2name if m.is_a? Symbol
	if ! m.is_a? String
	  raise ArgumentError, "#{m} is not a symbol"
	elsif ! instance_methods.include? m
	  raise NameError, "undefined method `#{m}' for class #{self}"
	end
	Preserved_methods.replace(Preserved_methods | [m])
      end
    }
  end

  def preserved_methods
    instance_eval %q{
      Preserved_methods = [] unless defined?(Preserved_methods)
      Preserved_methods.dup
    }
  end

  private

  def method_added(id)
    catch(:preserved!) do
      instance_eval %q{
	return if id == :method_added
	Preserved_methods = [] unless defined?(Preserved_methods)

	if Preserved_methods.include? id.id2name
	  throw :preserved!
	end
      }
      return
    end
    raise NameError, "preserved method `#{id}' redefined for class #{self}"
  end
end

if __FILE__ == $0
  _,demo,_=<<'=== EXAMPLE',<<"== AUTHORS",<<"=end";eval demo # Forget this ;-)
=begin
=== EXAMPLE

  class C
    def a; end
    p [1, C, preserved_methods] #=> [1, C, []]
    preserved :a
    p [2, C, preserved_methods] #=> [2, C, ["a"]]
  end

  class S < C
    p [3, S, preserved_methods] #=> [3, S, ["a"]]
    def a0; end
    preserved :a0
    p [4, S, preserved_methods] #=> [4, S, ["a", "a0"]]
  end

  class C
    p [5, C, preserved_methods] #=> [5, C, ["a"]]
  end

  class S
    def a; end                  #!! preserved method `a' redefinded for class S
  end

== AUTHORS

Gotoken

== HISTORY

  2001-07-20: Preserved_method would be modified by replace (kg)
  2000-10-17: First release (gotoken@notwork.org)

=end
end