haskell - 即使 DeriveGeneric 打开，派生 Generic 也不起作用

Question

我正在尝试遵循 Beam Haskell 库的教程:https://tathougies.github.io/beam/tutorials/tutorial1/

module Lib
    ( someFunc
    ) where

{-# LANGUAGE
    DeriveGeneric
  , GADTs
  , OverloadedStrings
  , FlexibleContexts
  , FlexibleInstances
  , TypeFamilies
  , TypeApplications
 #-}

import Database.Beam
import Database.Beam.Postgres
import GHC.Generics

import Data.Text (Text)

data UserT f
    = User
    { _userEmail     :: Columnar f Text
    , _userFirstName :: Columnar f Text
    , _userLastName  :: Columnar f Text
    , _userPassword  :: Columnar f Text }
    deriving Generic

someFunc :: IO ()
someFunc = putStrLn "someFunc"

这会导致以下错误:

    • Can't make a derived instance of ‘Generic (UserT f)’:
        You need DeriveGeneric to derive an instance for this class
    • In the data declaration for ‘UserT’
   |
27 |     deriving Generic
   |              ^^^^^^^

请注意，存在 DeriveGeneric 语言编译指示。

我在这里缺少什么？

构建环境:

• 堆栈 lts-11.9
• Linux

Answer 1

{-# LANGUAGE #-} 声明需要位于文件的最顶部，位于 module 声明之前。