I am also a beginner in Haskell. Following the same documentation, I was trying to move the schema definition to another module and import it in main. It simply didn't work with the same error message.
After 2-3 hours of trying out copy/pasting code here and there, and testing it with postgres, I realized something during the process. In the documentation the comments containing Language features appeared on top of file before anything else.
Simply move the comments to the top before everything else
{-# LANGUAGE EmptyDataDecls #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE UndecidableInstances #-}
module Sql.Schema where
import Database.Persist
import Database.Persist.TH
share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase|
Person
firstName String
lastName String
age Int Maybe
PersonName firstName lastName
deriving Show
BlogPost
title String
authorId PersonId
deriving Show
|]
You lovely beautiful person! I had actually given up on this as I could not find anything which helped! Such a simple solution to a silly problem - thank you!