class ActionDispatch::PublicExceptions

will attempt to convert error information into the appropriate response type.
When a request with a content type other than HTML is made, this middleware
is not found it will fall back on /public/500.html.
the template in /public/500.<locale>.html. If an internationalized template
If an internationalized locale is set, this middleware will attempt to render
render the template found in /public/500.html.
directory. For example when this middleware receives a 500 response it will
response is expected it will render static error pages from the /public
When called, this middleware renders an error page. By default if an HTML

def call(env) 

def call(env)
  request      = ActionDispatch::Request.new(env)
  status       = request.path_info[1..-1].to_i
  begin
    content_type = request.formats.first
  rescue ActionDispatch::Http::MimeNegotiation::InvalidType
    content_type = Mime[:text]
  end
  body = { status: status, error: Rack::Utils::HTTP_STATUS_CODES.fetch(status, Rack::Utils::HTTP_STATUS_CODES[500]) }
  render(status, content_type, body)
end

def initialize(public_path) 

def initialize(public_path)
  @public_path = public_path
end

def render(status, content_type, body) 

def render(status, content_type, body)
  format = "to_#{content_type.to_sym}" if content_type
  if format && body.respond_to?(format)
    render_format(status, content_type, body.public_send(format))
  else
    render_html(status)
  end
end

def render_format(status, content_type, body) 

def render_format(status, content_type, body)
  [status, { "Content-Type" => "#{content_type}; charset=#{ActionDispatch::Response.default_charset}",
            "Content-Length" => body.bytesize.to_s }, [body]]
end

def render_html(status) 

def render_html(status)
  path = "#{public_path}/#{status}.#{I18n.locale}.html"
  path = "#{public_path}/#{status}.html" unless (found = File.exist?(path))
  if found || File.exist?(path)
    render_format(status, "text/html", File.read(path))
  else
    [404, { "X-Cascade" => "pass" }, []]
  end
end