Interfaces in Ruby

interface.rb

module Interface
  # For this module to be useful, it'll need to be prepended. For example, if
  # you create a CardInterface:
  #
  # module CardInterface
  #   extend Interface
  #   method :photo, returns: [Photo, NilClass]
  # end
  #
  # prepend it to the class:
  #
  # class MyClass
  #   prepend CardInterface
  # end
  #
  # Prepending ensures that the methods are executed in the correct order.

  def method(name, opts = {})
    define_method(name) do |*args|
      if respond_to?(name)
        Interface.validate_and_return(super(*args), opts[:returns])
      else
        raise Interface::NotImplementedError,
              "#{self.class} method #{name} not implemented"
      end
    end
  end

  def self.validate_and_return(value, return_type)
    return value unless return_type
    return_types = Array(return_type)
    if return_types.include?(value.class)
      value
    else
      raise Interface::WrongReturnTypeError,
            "Method did not return one of: #{return_type.map(&:to_s)}\n" \
            "Returned: #{value.class}"

    end
  end

  class NotImplementedError < NoMethodError
  end

  class WrongReturnTypeError < NoMethodError
  end
end

sample_interface.rb

module CardInterface
  # It's important to use this interace as:
  #   prepend CardInterface
  # so that the methods are executed in the correct order.
  extend Interface
  method :photo, returns: [Photo, NilClass]
  method :title, returns: [String, NilClass]
end

sample_model.rb

class Animal < ApplicationRecord
  prepend CardInterface
end