Plugins are useful to apply certain action to all routes in the server. For example we can add generic logger or authorization bazed on common query parameter or field of the body request that contains token of the session.

The downside is that we work on low level of Requesnce/Response as we have rendered all routes to ServerFun. But thw good part of it is that we can add generic action to every route.

Let's consider a simple example of adding logger to lall routes:

logRoutes :: Server IO -> Server IO
logRoutes = applyPlugin $ \(PathInfo path) -> prependServerAction $
   when (path /= ["favicon.ico"] && headMay path /= Just "static") $ do
     logRoute site (Text.intercalate "/" path)

-- | Logs the route info
logRoute :: Site -> Text -> IO ()
logRoute site route = do
  time <- getCurrentTime
  site.logInfo $ route <> " page visited at: " <> Text.pack (show time)

Here we use instance of ToPlugin for PathInfo to read full path for any route and we use this information in the logger.

We have various instances for everything that we can query from the request and we can use this information to transform the server functions inside the routes.

The instances work in the same manner as route handlers we can use as many arguments as we wish and we use typed wrappers to query specific part of the request. Thus we gain type-safety and get convenient interface to request the various parts of request.