commutativity.rb

# A possibly misguided attempt to introduce automatic operator commutativity to Ruby. Say
# I add a '+' method to Range so that for example:
#
#     (3..7) + 12 #=> (15..19)
#
# I'd expect to be able to call '12 + (3..7)' and get the same answer. This means I need
# to override Fixnum#+, meaning my new addition operator is expressed in two places. This
# module generates methods that automatically fill in the gaps so you only need to express
# the semantics of an operator in a single method, in a single class.
#
# A simple example:
#
#     class Range
#       def +(num)
#         (first + num)..(last + num)
#       end
#     end
#     
#     class Fixnum
#       include Commutativity
#     end
#     
#     4 + (1..15)
#     #=> (5..19)
#
# The module covers binary arithmetic, logic and comparison operators.
#
module Commutativity
  ALL_SYMBOLS = %w(** ~ + - * / % << >> & ^ | <= < > >= <=> == === =~)
  
  COMMUTATIVE = %w(+ * & ^ | == === =~)
  NEGATED     = %w(- <=>)
  INVERTED    = %w(/)
  OPPOSITES   = [%w(<< >>), %w(<= >), %w(< >=)]
  
  TRANSLATIONS = {
    COMMUTATIVE => lambda { |object, symbol, value| value.__send__(symbol, object) },
    NEGATED     => lambda { |object, symbol, value| -value.__send__(symbol, object) },
    INVERTED    => lambda { |object, symbol, value| 1.0 / value.__send__(symbol, object) }
  }
  
  def self.included(mod)
    mod.module_eval do
      
      TRANSLATIONS.each do |list, translation|
        list.each do |symbol|
          orig = instance_method(symbol) rescue nil
          define_method(symbol) do |value|
            begin
              orig.bind(self).call(value)
            rescue
              translation.call(self, symbol, value)
            end
          end
        end
      end
      
      OPPOSITES.each do |pair|
        pair.each_with_index do |symbol, i|
          orig = instance_method(symbol) rescue nil
          define_method(symbol) do |value|
            begin
              orig.bind(self).call(value)
            rescue
              not value.send(pair[(i+1)%2], self)
            end
          end
        end
      end
      
    end
  end
end