问题描述
过去几个月来我一直在学习Haskell,并且正在尝试遵循有关Haskell + Persistent的本教程:
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(他们在教程中谈论的是“ QuasiQuotes”,所以我猜这就是这个意思),但是我认为,因为我直接从教程中获取了代码,因此应该进行编译。但是,不会,并且出现以下错误:
/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
这是我的阴谋文件中的配置:
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
如果有人可以帮助我,那就太好了。
解决方法
我还是Haskell的初学者。遵循相同的文档,我试图将模式定义移至另一个模块并将其导入main。根本无法使用相同的错误消息。
经过2-3小时的到处尝试复制/粘贴代码,并使用postgres进行测试之后,我在此过程中意识到了一些问题。在文档中,包含语言功能的注释首先出现在文件顶部。
只需将评论移至其他所有内容的顶部
{-# 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
|]