Haskell 实现Unix“touch”命令

Question

I'm trying to implement the touch command from the unix command line, but it seems that my last line throws an exception: ** Exception: ~/.todo: openFile: does not exist (No such file or directory)

main = touch "~/.todo"

touch :: FilePath -> IO ()
touch name = do
  exists <- doesFileExist name
  unless exists $ appendFile name ""

If there is any OS specific behavior, I'm testing from macOS Sierra.

I feel as if this error is strange in that the documentation for openFile states that

If the file does not exist and it is opened for output, it should be created as a new file.

Any suggestions as to how to fix this?

Edit: According to @chi, the touch command should always open the file, even if it already exists, because it will then update the file's last modified date.

touch :: FilePath -> IO ()
touch name = appendFile name ""

Answer 1

Use touchFile from the unix package ( System.Posix.Files.ByteString ).

---

appendFile name "" does not work like touch ; appendFile is a no-op when the string to append is empty.

You can confirm this by running stat on the file before and after and comparing the modification times.

Answer 2

In the future please paste all the code you are using that creates the error. This includes both the imports and the invocation. In your case it seems you are running something with a shell expansion character:

*Main> touch "~/foobar"
*** Exception: ~/foobar: openFile: does not exist (No such file or directory)

The ~ is typically expanded by a shell (there also exists a C library that can do that rewriting for you). Most languages actually interpret that as a literal part of the path... but the ~ directory probably doesn't exist or that symbol might not even be valid depending on your platform.

Instead try a valid file path:

*Main> touch "/tmp/thisfile"
*Main>
Leaving GHCi.
% ls -l /tmp/thisfile
-rw-rw-r--. 1 theuser theuser 0 Feb  3 12:51 /tmp/thisfile