class Steep::Subtyping::Check

def check_interface(relation)
  relation.interface!
  sub_interface, super_interface = relation
  method_pairs = super_interface.methods.each_with_object({}) do |(method_name, sup_method), hash| #$ Hash[Symbol, Relation[Interface::Shape::Entry]]
    if sub_method = sub_interface.methods[method_name]
      hash[method_name] = Relation.new(sub_type: sub_method, super_type: sup_method)
    else
      return Failure(relation) { Result::Failure::MethodMissingError.new(name: method_name) }
    end
  end
  All(relation) do |result|
    method_pairs.each do |method_name, method_relation|
      result.add(method_relation) do
        check_method(method_name, method_relation)
      end
    end
  end
end