From the blog post "Error Handling with Either Monads in Ruby"

endpoint.rb

patch '/users/:id/' do
  result = UserUpdater.new(User, config.mailer).call(params).to_result
  
  # a simple, "unambitious" version
  if result.success?
    result.sucess.to_json
  else
    {error: result.failure}.to_json
  end
end