Tutorial example fails with "No instance for (H.ToValue ... arising from a use of 'routeElem'"

[JonathanReeve]

Just using the Ema tutorial code,

{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE AllowAmbiguousTypes #-}

module Main where

import qualified Ema
import Ema (Ema (..))
import Control.Concurrent (threadDelay)
import qualified Data.LVar as LVar
import qualified Ema.CLI
import Text.Blaze.Html5 ((!))
import qualified Text.Blaze.Html5 as H
import qualified Text.Blaze.Html5.Attributes as A
import qualified Text.Blaze.Html.Renderer.Utf8 as RU

data Route
  = Index  -- Corresponds to /
  | About  -- Corresponds to /about
  deriving (Bounded, Enum, Show)

data Model = Model { speaker :: Text }

instance Ema Model Route where
  encodeRoute _model = \case
    Index -> "index.html"  -- To /
    About -> "about.html"  -- To /about
  decodeRoute _model = \case
    "index.html" -> Just Index  -- From /
    "about.html" -> Just About  -- From /about
    _ -> Nothing                -- Everything else, are bad routes


render :: Ema.CLI.Action -> Model -> Route -> Ema.Asset LByteString
render _emaAction model r = Ema.AssetGenerated Ema.Html . RU.renderHtml $
  H.html $ do
    H.head $ do
      H.title "Basic site"
      H.base ! A.href "/" -- This is important.
    H.body $ do
      H.div ! A.class_ "container" $ do
        case r of
          Index -> do
            H.toHtml $
              "You are on the index page. The name is " <> speaker model
            routeElem About "Go to About"
          About -> do
            "You are on the about page. "
            routeElem Index "Go to Index"
  where
    routeElem targetRoute w =
      H.a
        ! A.style "text-decoration: underline"
        ! A.href (H.toValue $ Ema.routeUrl targetRoute) $ w

main :: IO ()
main = do
  Ema.runEma render $ \_act model -> do
    LVar.set model $ Model "Ema"
    liftIO $ threadDelay maxBound

I'm getting this error:

src/Main.hs:46:13-41: error:
    • No instance for (H.ToValue (r0 -> Text))
        arising from a use of ‘routeElem’
        (maybe you haven't applied a function to enough arguments?)
    • In a stmt of a 'do' block: routeElem About "Go to About"
      In the expression:
        do H.toHtml
             $ "You are on the index page. The name is " <> speaker model
           routeElem About "Go to About"
      In a case alternative:
          Index
            -> do H.toHtml
                    $ "You are on the index page. The name is " <> speaker model
                  routeElem About "Go to About"
   |
46 |             routeElem About "Go to About"
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

src/Main.hs:49:13-41: error:
    • No instance for (H.ToValue (r1 -> Text))
        arising from a use of ‘routeElem’
        (maybe you haven't applied a function to enough arguments?)
    • In a stmt of a 'do' block: routeElem Index "Go to Index"
      In the expression:
        do "You are on the about page. "
           routeElem Index "Go to Index"
      In a case alternative:
          About
            -> do "You are on the about page. "
                  routeElem Index "Go to Index"
   |
49 |             routeElem Index "Go to Index"
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Failed, no modules loaded.
�]0;2 errors - ema-template
src/Main.hs:46:13-41: error:
    • No instance for (H.ToValue (r0 -> Text))
        arising from a use of ‘routeElem’
        (maybe you haven't applied a function to enough arguments?)
    • In a stmt of a 'do' block: routeElem About "Go to About"
      In the expression:
        do H.toHtml
             $ "You are on the index page. The name is " <> speaker model
           routeElem About "Go to About"
      In a case alternative:
          Index
            -> do H.toHtml
                    $ "You are on the index page. The name is " <> speaker model
                  routeElem About "Go to About"
   |
46 |             routeElem About "Go to About"
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
src/Main.hs:49:13-41: error:
    • No instance for (H.ToValue (r1 -> Text))
        arising from a use of ‘routeElem’
        (maybe you haven't applied a function to enough arguments?)
    • In a stmt of a 'do' block: routeElem Index "Go to Index"
      In the expression:
        do "You are on the about page. "
           routeElem Index "Go to Index"
      In a case alternative: