Adds default messages to Exception subclasses.

example.rb

require "./with_message.rb"

MyError = StandardError.with_message "This is my error message"

raise MyError # the raised error's message is "This is my error message"
raise MyError, "Something else" # the raised error's message is "Something else"

with_message.rb

#!/usr/bin/env ruby

class Exception
  alias_method :initialize_original, :initialize
  
  class << self
    attr_accessor :default_message
    
    def with_message msg
      @global_message ||= nil
      e = Class.new self
      e.default_message = msg
      e
    end
  end
  
  def initialize msg=nil
    initialize_original msg || self.class.default_message
  end
end