Haskell記錄語法並從文件中讀取。字符串來記錄語法。 ***例外：Prelude.read：不解析

Question

我有下面的記錄語法。

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"]] 

我想創建一個讀取books.txt文件並將其轉換爲數據庫類型的IO函數。我認爲它需要與我下面的嘗試類似。

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

什麼是books.txt文件的正確格式？

以下是我目前的books.txt內容（與bookDatabase相同）。

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

***例外：Prelude.read：沒有解析

Answer 1

的記錄只能讀取記錄語法派生Read實例。它無法讀取按順序應用於參數的構造函數格式的記錄。嘗試將以下（這是show bookDatabase的結果）放入books.txt。

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

Answer 2

在您的示例中有幾件事需要解決。

首先，在您的數據類型聲明中，您需要派生一個Read實例。

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

接下來，如果你看看read類型：

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

你可以看到它的返回類型是多態的。因此，爲了讓它正確解析，您需要讓GHC知道您希望解析的是什麼類型的類型。通常情況下，你可以使用分析得到的值，你通常會和GHC就能推斷出類型：

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

但GHCI你必須給它一個暗示：

> 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"]}