Yes you really have to do this in two files these days :(

App.hs

{-# LANGUAGE OverloadedStrings, QuasiQuotes, TemplateHaskell, TypeFamilies #-}
module App where

import ClassyPrelude hiding (Handler)

import Control.Exception.Safe (throw)
import Data.Aeson
import Deriving.Aeson
import GHC.Generics
import Prelude ()
import Yesod

data Book =
  Book { isbn :: Text
       , author :: Text
       , bTitle :: Text
       } deriving (Eq, Generic, Ord, Show)

instance FromJSON Book
instance ToJSON Book

data Movie =
  Movie { mTitle :: Text
        , director :: Text
        , cast :: [Text]
        } deriving (Eq, Generic, Ord, Show)

instance FromJSON Movie
instance ToJSON Movie

data CatalogItem = CatalogBook Book
                 | CatalogMovie Movie
                 deriving (Eq, Generic, Ord, Show)

instance FromJSON CatalogItem
instance ToJSON CatalogItem

data App = App
  { libraryCatalog :: TVar [CatalogItem]
  }

mkYesodData "App" [parseRoutes|
/books#Text BooksR GET
/movies#Text MoviesR GET
/purchaseBook PurchaseBookR PUT
/purchaseMovie PurchaseMovieR PUT
/health HealthR GET
|]

Main.hs

#!/usr/bin/env stack --verbose script --resolver=lts20.4 --verbose

{-# LANGUAGE OverloadedStrings, QuasiQuotes, TemplateHaskell, TypeFamilies #-}

import ClassyPrelude hiding (Handler)

import Control.Exception.Safe (throw)
import Data.Aeson
import Deriving.Aeson
import GHC.Generics
import Prelude ()
import Yesod
import App

instance Yesod App

getBooksR :: Handler Html
getBooksR = undefined

getMoviesR :: Handler Html
getMoviesR = undefined

putPurchaseBookR :: Handler Html
putPurchaseBookR = undefined

putPurchaseMovieR :: Handler Html
putPurchaseMovieR = undefined

getHealthR :: Handler Html
getHealthR = do
  putStrLn "Health!"
  defaultLayout [whamlet|"Health!"|]

mkYesodDispatch "App" resourcesApp

main :: IO ()
main = do
  putStrLn "in main"
  let
    initialCatalog = [ CatalogBook Book{isbn="1", bTitle="Dune", author="Frank Herbert"}
                       , CatalogMovie Movie{mTitle="Dune", director="Denis Villeneuve", cast=[]}
                       , CatalogBook Book{isbn="2", bTitle="How To Win Twilight Imperium", author="A Space Lion"}
                       ]
  icTVar <- atomically $ newTVar initialCatalog
  warp 3000 App { libraryCatalog = icTVar }