show me an example how to design a C++ monadic I/O on reading and writing a regular file.

设计C++的monadic I/O（函数式编程风格的I/O操作）通常会利用`std::experimental::io_context`和相关的流适配器，比如`std::io_context::work`, `std::async`, 和`std::piped_stream`等。以下是一个简单的例子，展示如何创建一个读写文件的基本monadic API：

#include <iostream>
#include <fstream>
#include <future>
#include <asio.hpp>

// Monadic IO function for reading from a file
template <typename Stream, typename Result>
auto read_file(Stream& stream, Result&& result)
{
    return [stream = std::move(stream), result = std::forward<Result>(result)] {
        auto future_read = std::async(std::launch::async, [] (Stream& s) -> std::string {
            std::stringstream ss;
            ss << s.rdbuf();
            return ss.str();
        }, std::ref(stream));

        // Wait for the async operation to complete
        auto content = future_read.get();

        // Use the result in your monadic chain
        result(content);
    };
}

// Monadic IO function for writing to a file
template <typename Stream, typename Input>
auto write_file(Stream& stream, Input input)
{
    return [stream = std::move(stream)](Input input) mutable {
        stream << input;
        return stream.commit(); // Commit the write operation
    };
}

int main()
{
    try
    {
        std::ifstream reader("input.txt");
        std::ofstream writer("output.txt");

        // Monadic chaining of read and write operations
        io_context ioc;
        auto read_task = read_file(reader, [&](const std::string& content) {
            auto write_task = write_file(writer, content);
            ioc.run(write_task);
        });

        ioc.run(read_task);
    }
    catch (std::exception& e)
    {
        std::cerr << "Error: " << e.what() << '\n';
    }

    return 0;
}

在这个示例中，`read_file`和`write_file`都是接受输入流并返回一个新的函数，这个新函数用于继续链式操作。通过这种方式，你可以将一系列复杂的I/O操作组合在一起，形成一个简洁、易理解的序列。