文件操作

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2023-12-01

这是一篇讲解只用OCaml标准库中提供的操作来实现基本文件操作的指南。

请看官方模块文档:Pervasives, Printf.

标准库不提供可以直接读取文件成字符串或保存字符串到文件中的函数。这样的函数可以在第三方类如Extlib找到. 请见 Std.input_file与 Std.output_file.

缓冲通道

The normal way of opening a file in OCaml returns a channel. There are two kinds of channels:

channels that write to a file: type out_channel
channels that read from a file: type in_channel

写

For writing into a file, you would do this:

Open the file to obtain an out_channel
Write stuff to the channel
If you want to force writing to the physical device, you must flush the channel, otherwise writing will not take place immediately.
When you are done, you can close the channel. This flushes the channel automatically.

Commonly used functions: open_out, open_out_bin, flush, close_out, close_out_noerr

Standard out_channels: stdout, stderr

读

For reading data from a file you would do this:

Open the file to obtain an in_channel
Read characters from the channel. Reading consumes the channel, so if you read a character, the channel will point to the next character in the file.
When there are no more characters to read, the End_of_file exception is raised. Often, this is where you want to close the channel.

Commonly used functions: open_in, open_in_bin, close_in, close_in_noerr

Standard in_channel: stdin

查找

Whenever you write or read something to or from a channel, the current position changes to the next character after what you just wrote or read. Occasionally, you may want to skip to a particular position in the file, or restart reading from the beginning. This is possible for channels that point to regular files, use seek_in or seek_out.

注意事项

Don't forget to flush your out_channels if you want to actually write something. This is particularly important if you are writing to non-files such as the standard output (stdout) or a socket.
Don't forget to close any unused channel, because operating systems have a limit on the number of files that can be opened simultaneously. You must catch any exception that would occur during the file manipulation, close the corresponding channel, and re-raise the exception.
The Unix module provides access to non-buffered file descriptors among other things. It provides standard file descriptors that have the same name as the corresponding standard channels: stdin, stdout and stderr. Therefore if you do open Unix, you may get type errors. If you want to be sure that you are using the stdout channel and not the stdout file descriptor, you can prepend it with the module name where it comes from: Pervasives.stdout. Note that most things that don't seem to belong to any module actually belong to the Pervasives module, which is automatically opened.
open_out and open_out_bin truncate the given file if it already exists! Use open_out_gen if you want an alternate behavior.

示例

open Printf
  
let file = "example.dat"
let message = "Hello!"
  
let () =
  (* 向文件写信息 *)
  let oc = open_out file in    (* 新建或修改文件,返回通道 *)
  fprintf oc "%s\n" message;   (* 写一些东西 *)
  close_out oc;                (* 写入并关闭通道 *)
  
  (* 读取文件并显示第一行 *)
  let ic = open_in file in
  try 
    let line = input_line ic in  (* 从输入通道读入一行并丢弃'\n'字符 *)
    print_endline line;          (* 输出结果到标准输出端 *)
    flush stdout;                (* 现在写入默认设备 *)
    close_in ic                  (* 关闭输入通道 *) 
  
  with e ->                      (* 一些不可预见的异常发生 *)
    close_in_noerr ic;           (* 紧急关闭 *)
    raise e                      (* 以出错的形式退出: 文件已关闭,但通道没有写入东西 *)
  
  (* 正常退出: 所有的通道都已写入且关闭 *)