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# How Lucky renders errors

When an error occurs, the Lucky::ErrorHandler calls the auto-generated Errors::Show action in src/actions/errors/

The Errors::Show action has 3 key methods:

  • render(error)
  • default_render(error)
  • report(error)

# render

First, the Errors::Show action will try to find a matching render method. The render methods take the error as an argument and uses method overloading to find a match. If one matches, the error will be rendered.

For example, if MyCustomError is raised, and there is a method for handling it, that method will be used and default_render will not be called:

def render(error : MyCustomError)
  error_html message: "Super Custom", status: 418

error_html is an automatically generated method on Errors::Show that shows an HTML page. You can customize it however you want. You can learn more about how to customize how errors are displayed later in the guide.

# default_render

If no render method matches then the default_render method will be used. This method will send a 500 HTTP status code and either an HTML page or a JSON response depending on the client’s desired format. If your app is using API mode, it will only send JSON and not an HTML page.

You can learn more about how to customize how errors are displayed later in the guide.

# report

This method handles reporting the error. We’ll talk about this more in the section on reporting errors

# Error handling in development

When using a browser with Lucky in development mode, Lucky uses the ExceptionPage shard to display a helpful page with your stack trace, and exception message.

When using JSON, Lucky will render errors as JSON whether in development or production.

# Seeing the error page your users will see

Sometimes in development you want to see the page your users will see instead of the debug page.

To do so, change the the settings.show_debug_output option to false:

# config/
Lucky::ErrorHandler.configure do |settings|
  settings.show_debug_output = false

Remember to change it back once you’re done so you can see the debug page.

# Customizing how errors are displayed

Let’s say you have an error class MyCustomError in your app. When this error is raised, you want to show a custom error to your users. Open up the Errors::Show in src/actions/errors/, and add your render method like this.

def render(error : MyCustomError)
  if html?
    error_html message: "Custom error message.", status: 418
    error_json message: "Custom error", status: 418

If there is no render for the exception, it will fallback to the default one that is generated with every Lucky project: default_render(error : Exception). You can customize that method in the same way by changing the message or status codes:

def default_render(error : Exception)
  error_json "Something went very very wrong", status: 500

# Customizing errors for just one format

You can customize an error for just one format if you’d like:

def render(error : ThisIsOnlyImportantForJsonError)
  if json?
    error_json "Something for JSON clients", status: 418

If the client wants JSON back, it will get this error message, otherwise the method will return nil and Lucky will fall back to using the default_render method.

# Customizing the error page/JSON

Lucky generates error_json and error_html methods in Errors::Show. These methods and the pages/serializers they call can be customized.

For example, error_html renders Errors::ShowPage. You can change that page’s styles and content in src/pages/errors/

# Errors that Lucky handles out of the box

Lucky will also handle a few errors out of the box. For example, Lucky::RouteNotFoundError will return a 404:

def render(error : Lucky::RouteNotFoundError)
  if html?
    error_html "Sorry, we couldn't find that page.", status: 404
    error_json "Not found", status: 404

If you open src/actions/errors/, you’ll see the other errors that Lucky handles by default.

One of special note is the Lucky::RenderableError. We’ll talk about these more in the section on renderable errors

# Using Errors for Control Flow

You may need to use custom errors for your control flow like manually rendering a 404 page when a user shouldn’t see certain pages, for example.

Since Lucky will pass all exceptions to the Errors::Show action for you, you can raise specific errors to display the error page you want.

get "/profiles/:slug" do
  profile = ProfileQuery.find_by_id_or_slug(slug)

  if current_user.is_allowed_to_view?(profile)
    # Return a 202 with the appropriate page
    html ShowPage, profile: profile
    # raising this error will render the 404 page for you

For rending non 200 status pages without raising errors, see rendering HTML with non 200 status

# Renderable errors

In general this should be a last resort or for libraries that want to provide default behavior for errors. Usually you should use render methods in Errors::Show because it is more customizable and simpler to work with.

Lucky comes with a Lucky::RenderableError module that can be included in errors so that Lucky knows what the status code and message should be. Errors with Lucky::RenderableError must have a renderable_status and renderable_message method defined.

By default, Lucky::RenderableErrors are handled with the render(error : Lucky::RenderableError) method included in all new Lucky projects.

It looks something like this:

def render(error : Lucky::RenderableError)
  if html?
    error_html DEFAULT_MESSAGE, status: error.renderable_status
    error_json error.renderable_message, status: error.renderable_status

If you want to make it so your error is rendered with this method, you can do this:

# Define your custom exception
class NotAuthorizedError < Exception
  include Lucky::RenderableError

  def renderable_status

  def renderable_message
    "Not authorized"

When NotAuthorizedError is raised, Lucky will use the defined status code and message, unless you have a render method for the error (render(error : NotAuthorizedError)).

# Reporting errors

In your src/actions/errors/ file, there is a report method. By default this method is empty, but you can change it to report the error however you want. You can send an email, send the error to one or more services, or anything else you want.

# Example of reporting to Sentry

Let’s use the Raven shard to send an error report to Sentry:

# src/actions/errors/
def report(error : Exception)

This will send the error report to Sentry. See the Raven README to learn more about installing and how to customize error reporting with Sentry.

# Reporting some errors differently

You can use method overloading to report some errors differently than others. For example, let’s say we have a SuperScaryError that we want to report by sending a text to the CEO. We can add a report method that handles that error:

def report(error : SuperScaryError)!

Now SuperScaryError will be handled by report(error : SuperScaryError), and all other errors will be handled by the regular report(error : Exception).

# Skipping reporting

Some errors don’t need to be reported. Errors::Show has a dont_report macro that accepts an array of classes that should not be reported. By default Lucky does not report Lucky::RouteNotFoundError, but you can add any errors there that you don’t want reported.

dont_report [
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