Question

I have a Module named Coordinated that provides distance related methods to any model that implements the latitude and longitude methods.

module Coordinated
  ...
  def crow_flies_to(place)
    raise ArgumentError, "place does not implement latitude and longitude" unless is_coordinated?(place)
    Math.sqrt((latitude - place.latitude).abs**2 + (longitude - place.longitude).abs**2)
  end

  def is_coordinated?(place)
    place.respond_to?(:latitude) && place.respond_to?(:longitude)
  end
  ...
end

I’d like to test that classes into which Coordinated is included implement the required methods when the class is loaded. How do I do that?

Answer 1

You can use the included hook that runs when a module is included:

module M
  def self.included(klass)
    raise "Must implement 'foo'" unless klass.instance_methods.include?(:foo)
  end
end

class C; include M; end
#=> RuntimeError

class C
  def foo; end
  include M
end
#=> C

This has the drawback that the inclusion must happen after the methods are defined, since they must already exist when the hook runs.