Haskell在输入'let'上解析错误

Question

我正在尝试运行此代码，但在输入“let”时出现解析错误：

 let database = tempFilm:database

我找不到问题出在哪里，你能看看吗？

options 7 (username, database) = do   putStrLn "******************"
                                      putStrLn "   Rate a film    "
                                      putStrLn "******************"
                                      putStrLn ""
                                      putStr "Enter the title of the film or nothing to return to the main menu: "
                                      title <- getLine
                                      if title == ""
                                        then return(username, database)
                                      else do let filmCheck = findFilm title database
                                              if filmCheck == []
                                                then do putStrLn "That film does not exists."
                                                        return (username, database) 
                                                else do putStr "Enter your rate: "
                                                        tempRate <- getLine
                                                        case reads tempRate :: [(Integer, String)] of
                                                             [(n, "")] -> do let rate = read tempRate :: Int
                                                                             let tempFilm = rateFilm username (username, rate) filmCheck
                                                                             if checkIfRated username tempFilm == True 
                                                                              then do putStrLn "You already voted for this film\n"
                                                                                      putStrLn "Do you want to modify your voted?\n"
                                                                                      putStrLn "Press y to modify or nothing to return to the main menu:"
                                                                                      input <- getLine
                                                                                      if input /= "y"
                                                                                        then return (username, database)
                                                                                      else putStrLn "Your vote will be modified."
                                                                             let database = tempFilm:database
                                                                             putStrLn "You rating has been  sumbited successfully!"
                                                                             putStrLn (displayFilm tempFilm)
                                                                             return (username, database) 

编辑： 何时使用

options 7 (username, database) = do   putStrLn "******************"
                                      putStrLn "   Rate a film    "
                                      putStrLn "******************"
                                      putStrLn ""
                                      putStr "Enter the title of the film or nothing to return to the main menu: "
                                      title <- getLine
                                      if title == ""
                                        then return(username, database)
                                      else do let filmCheck = findFilm title database
                                              if filmCheck == []
                                                then do putStrLn "That film does not exists."
                                                        return (username, database) 
                                                else do putStr "Enter your rate: "
                                                        tempRate <- getLine
                                                        case reads tempRate :: [(Integer, String)] of
                                                             [(n, "")] -> do let rate = read tempRate :: Int
                                                                             let tempFilm = rateFilm username (username, rate) filmCheck
                                                                             when (checkIfRated username tempFilm == True) $
                                                                                 do putStrLn "You already voted for this film\n"
                                                                                    putStrLn "Do you want to modify your vote?\n"
                                                                                    putStrLn "Press y to modify or nothing to return to the main menu:"
                                                                                    input <- getLine
                                                                                    if input /= "y"
                                                                                      then return (username, database)
                                                                                    else putStrLn "Your vote will be modified."
                                                                             let database = tempFilm:database
                                                                             putStrLn "You rating has been  sumbited successfully!"
                                                                             putStrLn (displayFilm tempFilm)
                                                                             return (username, database) 

我以前从未使用过的语法可能不正确

Answer 1

if checkIfRated ... then do putStrLn "you already voted"... 没有 else

Answer 2

if 关键字表示表达式的开头，该表达式具有三个组成部分：谓词（计算结果为 Bool 类型）、then 分支（计算结果为 say A）和 else 分支（也必须评估为类型 A）。 if if 表达式的类型是类型 A，任何有效的 Haskell 类型。

我在此引用 Real World Haskell 一书的第 2 章，类型和函数，在 条件评估 下：

回想一下，Haskell 是一种面向表达式的语言。在命令式语言中，从 if 中省略 else 分支是有意义的，因为我们使用的是语句，而不是表达式。但是，当我们使用表达式时，如果谓词评估为 False，则缺少 else 的 if 将不会有结果或类型，因此这是荒谬的。

在命令式语言中，if 语句告诉计算机执行某些操作，但它不必返回类型化对象。在 Haskell 中，它是一种尊重类型系统以避免意外副作用的表达式。