MethodTypeCheckable-experimental.rb

module HasSize
  def size
  end
end

module MethodTypeCheckable
  def method_type_check(name, signature)
    origin = self.instance_method(name)
    define_method(name) do |*args, &block|
      ok = signature.each_with_index.map do |s,i|
        args[i].is_a?(s) || s.public_instance_methods.all? do |required_method_name|
          required_params = s.instance_method(required_method_name).parameters
          actual_params = args[i].method(required_method_name).parameters rescue nil
          required_params == actual_params
        end
      end.all?
      unless ok
        raise ArgumentError, "Type check failed"
      end
      origin.bind(self).call(*args, &block)
    end
  end
end

class MyService
  extend MethodTypeCheckable

  def initialize
    @map = {}
  end

  def set(key, value)
    @map[key] = value
  end
  method_type_check :set, [String, String]

  def lookup_common_prefix(key)
    key.size.times.map do |i|
      @map[key[0, i]]
    end
  end
  method_type_check :lookup_common_prefix, [HasSize]
end

class StringLike < String
end

sv = MyService.new
sv.set("type", "checking")   #=> ok
sv.set("type", StringLike.new("checking"))   #=> ok
sv.lookup_common_prefix("ty")   #=> ok
sv.lookup_common_prefix(Time.now)   #=> Type check failed (ArgumentError)