Servant router for non-server applications.

Latest on Hackage:0.9.0

This package is not currently in any snapshots. If you're interested in using it, we recommend adding it to Stackage Nightly. Doing so will make builds more reliable, and allow to host generated Haddocks.

BSD3 licensed by Will Fancher


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


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 ()
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