Replace Scotty with Twain

This works when running `cabal run`, but I can't build it with nix,
probably because of versions.

I don't know if this version if cleaner than the one with Scotty, but
it's a bit cleaner than the previous version of this PR.

app/Main.hs

@@ -29,6 +29,7 @@ import           Data.Time.Format                         ( defaultTimeLocale
 import           Lens.Micro.Platform                      ( set
                                                           , view
                                                           )
+import           Network.Wai.Handler.Warp                 ( run )
 import           Network.Wai.Middleware.RequestLogger     ( logStdout )
 import           Network.Wai.Middleware.StaticEmbedded    ( static )
 import           System.Console.GetOpt                    ( ArgDescr(..)
@@ -40,11 +41,13 @@ import           System.Console.GetOpt                    ( ArgDescr(..)
 import           System.Directory                         ( createDirectoryIfMissing )
 import           System.Environment                       ( getArgs )
 import           System.IO                                ( stdout )
-import           Web.Scotty                               ( get
+import           Data.Text.Lazy.Encoding                  ( encodeUtf8 )
+import           Web.Twain                                ( ResponderM
+                                                          , get
                                                           , html
-                                                          , middleware
-                                                          , redirect
-                                                          , scotty
+                                                          , notFound
+                                                          , redirect302
+                                                          , send
                                                           )
 
 import           Config
@@ -106,8 +109,23 @@ webserver
   -> IORef View -- ^ View model
   -> MVar () -- ^ Update signal
   -> IO ()
-webserver conf viewRef upd = scotty (view cPort conf) $ do
-  middleware logStdout
-  middleware (static $(embedDir "static"))
-  get "/"  ((html . render) =<< liftIO (readIORef viewRef))
-  get "/r" (liftIO (tryPutMVar upd ()) >> redirect "/") -- force update
+webserver Config{_cPort=webserverPort} viewRef upd =
+  run webserverPort $ foldr ($) (notFound missing)
+    [ middleware . get "/"  index
+    , middleware . get "/r" forceUpdate
+    ]
+ where
+   index :: ResponderM a
+   index = do
+     theCurrentView <- liftIO (readIORef viewRef)
+     (send . html . encodeUtf8 . render) theCurrentView
+
+   forceUpdate :: ResponderM a
+   forceUpdate = do
+     _ <- liftIO $ tryPutMVar upd ()
+     send $ redirect302 "/"
+
+   missing :: ResponderM a
+   missing = send $ html "Not found..."
+
+   middleware = logStdout . static $(embedDir "static")