Haskell + 持续存在：解析错误可能是缩进不正确或括号不匹配

Question

我在过去几个月里一直在学习 Haskell 并且正在努力按照本教程学习 Haskell + Persistent:

https://www.yesodweb.com/book/persistent#persistent_code_generation

module Main where

{-# LANGUAGE GADTs                      #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE OverloadedStrings          #-}
{-# LANGUAGE QuasiQuotes                #-}
{-# LANGUAGE TemplateHaskell            #-}
{-# LANGUAGE TypeFamilies               #-}
import Database.Persist
import Database.Persist.TH
import Database.Persist.Sqlite
import Control.Monad.IO.Class (liftIO)

mkPersist sqlSettings [persistLowerCase| 
Person
    name String
    age Int
    deriving Show
|]

我以前没有遇到过这种语法（他们在教程中谈论 'QuasiQuotes' 所以我猜这就是它），但我认为我直接从教程中获取了代码它应该编译。但是，它不会，我收到以下错误：

/home/will/programming/learn-haskell/postgres-example/app/Main.hs:15:1: error:
    parse error (possibly incorrect indentation or mismatched brackets)
   |
15 | Person
   | ^


--  While building package postgres-example-0.1.0.0 using:
      /home/will/.stack/setup-exe-cache/x86_64-linux-tinfo6/Cabal-simple_mPHDZzAJ_3.0.1.0_ghc-8.8.4 --builddir=.stack-work/dist/x86_64-linux-tinfo6/Cabal-3.0.1.0 build exe:simple --ghc-options " -fdiagnostics-color=always"
    Process exited with code: ExitFailure 1

这是我的 cabal 文件中的配置：

name:                                postgres-example
version:                             0.1.0.0
synopsis:                            An example of using Haskell and PostGres with persistent
description:                         Please see README.md
license:                             BSD3
license-file:                        LICENSE
author:                              Will Taylor
copyright:                           2020, Will Taylor
category:                            Web
build-type:                          Simple
cabal-version:                       >=1.10

executable simple
  hs-source-dirs:       app
  main-is:              Main.hs
  ghc-options:          -threaded -rtsopts -with-rtsopts=-N
  build-depends:        base
                        , persistent
                        , persistent-template
                        , persistent-postgresql
                        , monad-logger
  default-language:       Haskell2010

如果有人能帮助我，那就太好了。

Answer 1

我也是Haskell的初学者。按照相同的文档，我试图将架构定义移动到另一个模块并将其导入 main.它根本无法使用相同的错误消息。

在各处尝试 copy/pasting 代码并使用 postgres 测试 2-3 小时后，我在这个过程中意识到了一些事情。在文档中，包含语言功能的注释先于其他内容出现在文件顶部。

只需将评论移至顶部即可

{-# 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
|]

Haskell + 持续存在：解析错误可能是缩进不正确或括号不匹配

Haskell + Persistent: parse error possibly incorrect indentation or mismatched brackets

haskell

cabal

yesod

haskell-persistent