How to clear EOF flag in Haskell?

I have small and primitive code reading a file – the file’s content is appending from another application though – it’s something as Linux utility less. It works fine, but if the reader reaches EOF, the "EOF" flag of the file is set. Then any other read operations on the file lead to empty read/unchanged size of file (hFileSize, hTell). The solution would be to call clearerr or rewind – they clear the EOF flag, I suppose. How to do it in Haskell? And if there is some way, is it possible to do it in cross-platform way (if it is not cross-platform)?

PS. Currently I reopen the file, but I think that permanently reopening during poll, with period of 1-2 seconds maybe is not so good idea if there are better solutions.

>Solution :

In Haskell, you can use the System.IO module to work with files and manage file handles. To clear the EOF flag and reset the file pointer, you can use the hSeek function. This function allows you to set the position of the file handle to a given offset.

Here’s how you can clear the EOF flag and reset the file pointer in Haskell:

import System.IO

resetFileHandle :: Handle -> IO ()
resetFileHandle h = hSeek h AbsoluteSeek 0

main :: IO ()
main = do
    h <- openFile "example.txt" ReadMode
    content <- hGetContents h
    putStrLn content
    -- At this point, the file pointer is at the end of the file and the EOF flag is set.
    
    -- Clear the EOF flag and reset the file pointer
    resetFileHandle h

    -- Now you can read the file again, and the content will not be empty.
    content2 <- hGetContents h
    putStrLn content2

    hClose h

In the above code, we define the resetFileHandle function, which takes a file handle (Handle) as an argument. The hSeek function is used to set the position of the file handle to the beginning of the file (AbsoluteSeek 0), effectively clearing the EOF flag.

However, please note that this solution assumes that the file is not being modified by another process while you are reading it. If the file is being continuously appended to by another application, and you want to read the new content as it arrives, reopening the file with each poll might be a better approach.

If you want a cross-platform solution, using hSeek is generally portable across different platforms. However, be aware that file handling and behavior may vary across operating systems, so it’s always a good idea to thoroughly test your code on the target platforms.