Haskell 记录语法并从文件中读取。记录语法的字符串。 *** 异常：Prelude.read：没有解析

Question

我有以下记录语法。

type Title = String
type Author = String
type Year = Int
type Fan = String


data Book = Book { bookTitle :: Title
                 , bookAuthor:: Author
                 , bookYear  :: Year
                 , bookFans  :: [Fan]
                 }
                 deriving (Show, Read)


type Database = [Book]

bookDatabase :: Database 
bookDatabase = [Book "Harry Potter" "JK Rowling" 1997 ["Sarah","Dave"]]

我想创建一个读取 books.txt 文件并将其转换为数据库类型的 IO 函数。我认为它需要类似于我下面的尝试。

main :: IO()
main = do fileContent <- readFile "books.txt";
          let database = (read fileContent :: Database)

books.txt 文件的正确格式是什么？

以下是我当前的 books.txt 内容（与 bookDatabase 相同）。

[Book "Harry Potter" "JK Rowling" 1997 ["Sarah","Dave"]]

*** Exception: Prelude.read: no parse

Answer 1

记录的派生 Read 实例只能读取记录语法。它无法读取按顺序应用于参数的构造函数格式的记录。尝试将以下内容（show bookDatabase 的结果）放入 books.txt.

[Book {bookTitle = "Harry Potter", bookAuthor = "JK Rowling", bookYear = 1997, bookFans = ["Sarah","Dave"]}]

Answer 2

您的示例中有几处需要修复。

首先，在您的数据类型声明中，您需要派生一个 Read 实例。

data Book = Book {
      bookTitle :: String
    , bookAuthor :: String
    , bookYear   :: Int
    , bookFans   :: [String]
    } deriving (Show,Read)

接下来，如果你看read的类型：

> :t read
read :: Read a => String -> a

可以看到它的return类型是多态的。因此，要正确解析它，您需要让 GHC 知道您希望解析哪种类型。通常，您可以像往常一样使用解析后的值，GHC 将能够推断出类型：

getFans = do
    book <- readFile "book.txt"
    return $ bookFans book

但在 GHCi 中你必须给它一个提示：

> let book = Book "the sound and the fury" "faulkner" 1929 ["me"] 
> show book
"Book {bookTitle = \"the sound and the fury\", bookAuthor = \"faulkner\", bookYear = 1929, bookFans = [\"me\"]}"
> read $ show book :: Book
Book {bookTitle = "the sound and the fury", bookAuthor = "faulkner", bookYear = 1929, bookFans = ["me"]}

Haskell 记录语法并从文件中读取。记录语法的字符串。 *** 异常：Prelude.read：没有解析

Haskell record syntax and read from file. String to record syntax. *** Exception: Prelude.read: no parse

syntax

haskell

exception

record