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:

[srid]

@JonathanReeve Looks like you are using Ema's master branch example code on a project that uses an older version of Ema. See history of Ex02_Basic.hs here, https://github.com/srid/ema/commits/master/src/Ema/Example/Ex02_Basic.hs

You have two options:

• Use an older version of Ex03_Basic.hs, or
• Upgrade Ema by running nix flake lock --update-input ema (as documented here)

The second option is ideal, and you can use the history link above to know the series of changes required.

[srid]

On a more general note, you will want to add type declaration to the routeHref function, which will narrow the issue down (which comes to Ema.routeUrl now taking an extra argument, that model).