在涉及 ReadMode 的函数应用程序中意外执行块此消息是什么意思？

Question

此函数读取一个字符串并连接到一个 txt 文件

adicionaEmListaDeCategorias :: [Char] -> IO() 
adicionaEmListaDeCategorias categoria = do
    file <- openFile ("listagemCategorias.txt") ReadMode
    categorias <- hGetContents file
    let categoria1 = (read categorias :: [Char]) ++ categoria
    hClose file
    file2 <- openFile ("listagemCategorias.txt") WriteMode
    hPutStr file2 (categoria)
    hFlush file2
    hClose file2

但我收到此错误消息：

main.hs:190:41: error:
    Unexpected do block in function application:
        do file <- openFile ("listagemCategorias.txt") ReadMode
    You could write it with parentheses
    Or perhaps you meant to enable BlockArguments?
    |
190 | adicionaEmListaDeCategorias categoria = do
    |                                         ^^...

Answer 1

正如我在评论中所说 - 问题是您打算在第一行使用 tab

这个有效：

adicionaEmListaDeCategorias :: [Char] -> IO()
adicionaEmListaDeCategorias categoria = do
    file <- openFile "listagemCategorias.txt" ReadMode
    categorias <- hGetContents file
    let categoria1 = (read categorias :: [Char]) ++ categoria
    hClose file
    file2 <- openFile "listagemCategorias.txt" WriteMode
    hPutStr file2 categoria
    hFlush file2
    hClose file2

但是 Haskell 会给你一个警告，因为你没有使用 categoria1 我想你会想要这个：

adicionaEmListaDeCategorias :: [Char] -> IO()
adicionaEmListaDeCategorias categoria = do
    file <- openFile "listagemCategorias.txt" ReadMode
    categorias <- hGetContents file
    let categoria1 = (read categorias :: [Char]) ++ categoria
    hClose file
    file2 <- openFile "listagemCategorias.txt" WriteMode
    hPutStr file2 categoria1
    hFlush file2
    hClose file2

（写出拼接版）

同样根据你的问题描述，我认为你的版本不会达到你的预期（read 会失败，你不需要它）：

adicionaEmListaDeCategorias :: [Char] -> IO()
adicionaEmListaDeCategorias categoria = do
    file <- openFile "listagemCategorias.txt" ReadMode
    fileContent <- hGetContents file
    hClose file
    let newFileContent = fileContent ++ categoria
    file2 <- openFile "listagemCategorias.txt" WriteMode
    hPutStr file2 newFileContent
    hFlush file2
    hClose file2

在涉及 ReadMode 的函数应用程序中意外执行块此消息是什么意思？

Unexpected do block in function application involving ReadMode what does mean this message?

haskell

functional-programming

block

message

readfile