EOF后如何再次从标准输入中读取？

Question

我有以下C程序：

#include <stdio.h>
#include <unistd.h>

void readAndEchoAll(void) {
    for(;;) {
        char buf[100];
        ssize_t size = read(STDIN_FILENO, buf, sizeof(buf));
        if(size <= 0) {
            return;
        }
        fwrite(buf, 1, size, stdout);
    }
}

int main(void) {
    puts("Reading and echoing STDIN until first EOF...");
    readAndEchoAll();
    puts("Got first EOF. Now reading and echoing STDIN until second EOF...");
    readAndEchoAll();
    puts("Got second EOF.");
    return 0;
}

当我运行它时，它会按照我想要的方式工作。这是它的作用：

Reading and echoing STDIN until first EOF...
asdf
^Dasdf
Got first EOF. Now reading and echoing STDIN until second EOF...
fdsa
^Dfdsa
Got second EOF.

我正在尝试创建一个等效的Haskell程序。这是我的尝试：

readAndEchoAll :: IO ()
readAndEchoAll = do
    buf <- getContents
    putStr buf

main :: IO ()
main = do
    putStrLn "Reading and echoing STDIN until first EOF..."
    readAndEchoAll
    putStrLn "Got first EOF. Now reading and echoing STDIN until second EOF..."
    -- ???
    readAndEchoAll
    putStrLn "Got second EOF."

这行不通。这是它的作用：

Reading and echoing STDIN until first EOF...
asdf
^Dasdf
Got first EOF. Now reading and echoing STDIN until second EOF...
readtwice.hs: <stdin>: hGetContents: illegal operation (handle is closed)

如何使这项工作像C程序一样？我假设我需要把clearerr(stdin);我所拥有的位置等同起来-- ???，但是我不确定那是什么。

Update: Turns out clearerr is a bit of a red herring, as it's exclusive to the standard C API. When using the POSIX API, you can just read again without needing to do anything equivalent to it. So rather than make Haskell do anything extra, I need to make it not do something: not prevent further reads once it sees EOF.

Answer 1

您不能使用getContents，因为hGetContents（半）关闭了它传递的句柄并getContents调用hGetContents。但是，在EOF之后使用标准库中的大多数其他功能再次从句柄读取没有问题。这是不使用读取所有字符的简单但效率低下的示例getContents：

import Control.Exception
import System.IO.Error

readAll = go [] where
    handler cs err = if isEOFError err
        then return (reverse cs)
        else throwIO err
    go cs = catch (do
        c <- getChar
        go (c:cs))
        (handler cs)

main = do
    all <- readAll
    putStrLn $ "got: " ++ all
    putStrLn "go again, mate"
    all <- readAll
    putStrLn $ "got: " ++ all

如果要提高效率，可以使用各种功能来一次读取行或在标准库中读取其他大块数据，而不是一次读取一个字符。