Developing Web Apps with Haskell and Yesod, 2nd Edition

Chapter 29. Route Attributes

Route attributes allow you to set some metadata on each of your routes, in the route description itself. The syntax is trivial: just an exclamation point followed by a value. Using it is also trivial: just use the routeAttrs function.

It’s easiest to understand how all this fits together, and when you might want to use it, with a motivating example. The case I personally most use this for is annotating administrative routes. Imagine having a website with about 12 different admin actions. You could manually add a call to requireAdmin or some such at the beginning of each action, but:

• It’s tedious.

• It’s error prone: you could easily forget one.

• Worse yet, it’s not easy to notice that you’ve missed one.

Modifying your isAuthorized method with an explicit list of administrative routes is a bit better, but it’s still difficult to see at a glance when you’ve missed one.

This is why I like to use route attributes for this: you add a single word to each relevant part of the route definition, and then you just check for that attribute in isAuthorized. Let’s see the code!

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings     #-}
{-# LANGUAGE QuasiQuotes           #-}
{-# LANGUAGE TemplateHaskell       #-}
{-# LANGUAGE TypeFamilies          #-}
import           Data.Set         (member)
import           Data.Text        (Text)
import           Yesod
import           Yesod.Auth
import           Yesod.Auth.Dummy

data App = App

mkYesod "App" [parseRoutes|
/ HomeR GET
/unprotected UnprotectedR GET
/admin1 Admin1R GET !admin
/