multimethod.rb

class Module
  def dispatch(*klasses, last)
    last_klass, mid = last.first
    klasses << last_klass
    __dispatch_list << [klasses, instance_method(mid)]
    define_method(mid, __dispatch_proc(__dispatch_list)) 
  end

  def __dispatch_list
    @__dispatch_list ||= []
  end

  def __dispatch_proc(dispatch_list)
    proc { |*args, &bk|
      _, meth = dispatch_list.find { |klasses, meth|
        args.size == klasses.size && args.zip(klasses).all? { |arg, klass| klass === arg }
      }
      raise TypeError, "bad argument types in method call" unless meth
      meth.bind(self).call(*args, &bk)
    }
  end
end

class Foo
  dispatch String, String => def bar(a, b)
    a + b
  end

  dispatch Integer, Integer => def bar(a, b)
    a * b
  end
end

puts Foo.new.bar("hello ", "world")
puts Foo.new.bar(9, 9)

Could you explain why am I getting TypeError but method definition works fine?