Roda errors handler in separate file

app.rb

require_relative 'errors'

class App < Roda
  plugin :error_handler

  include Errors

  route do |r|
    r.root do
      raise ArgumentError
      'root'
    end
  end
end

errors.rb

module Errors
  def self.included(base)
    base.error do |e|
      handle_errors(e)
    end
  end

  private

  def handle_errors(e)
    case e
    when ArgumentError
      'argument error'
    else
      'runtime error'
    end
  end
end