servant-router: Servant router for non-server applications.

servant-router

servant-router routes a URI given a Servant API and an appropriate handler. In web applications, this is used to make single page applications (SPAs) with front-end routing, letting you share portions of your Servant APIs between the client and server.

servant-router does not depend on reflex or any GHCJS packages. It's intended to be a general purpose URI router on any platform. Combined with reflex-dom, servant-reflex, and reflex-dom-contrib, this makes for a very satisfactory front-end Haskell experience.

{-# LANGUAGE DataKinds     #-}
{-# LANGUAGE TypeOperators #-}

module Main where

import           Control.Monad.Except
import           Data.Proxy
import           Reflex.Dom
import           Reflex.Dom.Contrib.Router
import           Servant.API
import           Servant.Router

type MyApi = "books" :> Capture "id" Int :> View
        :<|> "search" :> QueryParam "keywords" String :> View
myApi :: Proxy MyApi
myApi = Proxy

main :: IO ()
-- reflex-dom-contrib provides a 'routeSite' function
-- for routing single page applications with reflex.
-- servant-router uses the provided uri String to perform the routing.
main = routeSite $ \uri -> do
  let handler = books :<|> search
      books i = do
        -- Here, you would get and display a book.
        -- Return a Reflex event for changing the browser location.
        return never
      search Nothing = do
        -- Here, you would display a search bar.
        return never
      search (Just keywords) = do
        -- Here you would display the search bar plus results.
        return never
  -- With the handler constructed, run the router with the uri.
  result <- runRoute uri myApi handler
  case result of
    -- If 'Left', there was no correct route for the uri.
    Left _ -> do
      el "div" $ text "No such page"
      return never
    -- If 'Right', the result of the route is returned.
    Right e -> return e

Serving

When using servant-router on the front-end in Single Page Applications (SPAs), you still need to serve the application from the back-end. To share the routing layout between the front-end and back-end, the View endpoints need to be converted to a verb like Get. Plus, with an SPA, all the end-points should serve the same HTML on the back-end. To do this, use the ViewTransform type family, and constHandler function.

type Views = "books" :> View
        :<|> "search" :> QueryParam "query" String :> View

-- Equivalent to:
--
--   type ViewsServer = "books" :> Get '[HTML] Blaze.Html
--          :<|> "search" :> QueryParam "query" String :> Get '[HTML] Blaze.Html
type ViewsServer = ViewTransform Views (Get '[HTML] Blaze.Html)

viewsServer :: Server ViewsServer
viewsServer = constHandler
	    (Proxy :: Proxy Views)
	    (Proxy :: Proxy Handler) $
	    docTypeHtml $ do
	      H.head $ return ()
	      body $
	        script ! src "app.js" $ return ()