Multimethods in Ruby

multimethod.rb

class Multimethod
  attr_accessor :dispatch, :methods, :heirarchy, :default_method
  class NoMatchingMethodError < StandardError
  end

  def initialize(&dispatch)
    @dispatch = dispatch
    @methods = []
    @heirarchy = {}
  end

  def add_method(dispatch_value, &method)
    if (dispatch_value == :default)
      @default_method = method
    else
      @methods << [dispatch_value, method]
    end
  end

  def derive(from,to)
    # TODO
  end

  def invoke(*args)
    dispatch_value = @dispatch[*args]
    @methods.each do |(val, meth)|
      if (val == dispatch_value)
        return meth[*args]
      end
    end
    
    return @default_method[*args] if @default_method

    raise NoMatchingMethodError, "No method found for value #{dispatch_value}."
  end
end

m = Multimethod.new(&:class)

m.add_method Array do |arr|
  "I'm an array with #{arr.count} things in it"
end

m.add_method String do |s|
  "I'm a string that uppercased is: #{s.upcase}"
end

m.add_method :default do |obj|
  "I'm a #{obj.class}, which is dumb"
end

puts m.invoke([1,2,3])
puts m.invoke("some string")
puts m.invoke(123)

####################

class Hero < String; end
class Monster < String; end

fight = Multimethod.new do |*players|
  players.map(&:class)
end

fight.add_method [Hero, Hero] do |hero1, hero2|
  "#{hero1} high-fives #{hero2}."
end

fight.add_method [Monster, Hero] do |monster, hero|
  "#{monster} eats #{hero}."
end

fight.add_method [Hero, Monster] do |hero, monster|
  "#{hero} slays #{monster}."
end

fight.add_method [Monster, Monster] do |monster1, monster2|
  "#{monster1} plots with #{monster2}."
end

puts fight.invoke(Hero.new("Thor"), Hero.new("Batman"))
puts fight.invoke(Monster.new("Nessy"), Hero.new("Wonderwoman"))
puts fight.invoke(Monster.new("Hitler"), Monster.new("Mussolini"))

# Later on...

class Dragon < String; end

# This doesn't work yet:
# puts fight.invoke(Dragon.new("Puff"), Hero.new("Harry Potter"))
# -> defmulti.rb:34:in `invoke': No method found for value [Dragon, Hero]

# Make it work:
fight.add_method [Dragon, Hero] do |dragon, hero|
  "#{dragon} toasts #{hero}."
end

puts fight.invoke(Dragon.new("Puff"), Hero.new("Harry Potter"))