IsRoute

The IsRoute typeclass is used to mainly define a Route Prism-based encoding for your route types. In other words, deriving IsRoute for the example route,

-- An example of nested routes
-- Route's expected encoding is given as a comment.

data Route
  = Route_Index          -- index.html
  | Route_About          -- about.html
  | Route_Contact        -- contact.html
  | Route_Blog BlogRoute -- blog/<BlogRoute>

data BlogRoute
  = BlogRoute_Index      -- index.html
  | BlogRoute_Post Slug  -- post/<Slug>

newtype Slug = Slug { unSlug :: String }

enables us to convert between Route_Blog BlogRoute_Index and /blog/index.html and vice versa (as shown in the tutorial).

Explanation

IsRoute is defined as:

class IsRoute r where
  -- (1)
  type RouteModel r :: Type
  -- (2)
  routePrism :: RouteModel r -> Prism_ FilePath r
  -- (3)
  routeUniverse :: RouteModel r -> [r]

1. RouteModel is (optionally) used to specify the value that is used for encoding routes (as defined by routePrism).
   1. For the blog route example above, this maybe be a type that contains your blog posts. It can also be () if you do not care about it.
2. routePrism gives us an optics prism (see Route Prism) that can be used to encode and decode between the route type and the .html filepath.
3. routeUniverse simply returns a list of routes to statically generate.

Generic deriving

IsRoute may be derived genericallly; see Generic deriving.