In Haskell, I want to read a file and then write to it. Do I need strictness annotation?

Question

Still quite new to Haskell..

I want to read the contents of a file, do something with it possibly involving IO (using putStrLn for now) and then write new contents to th

Answer 1

You can duplicate the file Handle, do lazy write with original one (to the end of file) and lazy read with another. So no strictness annotation involved in case of appending to file.

import System.IO
import GHC.IO.Handle

main :: IO ()
main = do
    h <- openFile "filename" ReadWriteMode
    h2 <- hDuplicate h

    hSeek h2 AbsoluteSeek 0
    originalFileContents <- hGetContents h2
    putStrLn originalFileContents

    hSeek h SeekFromEnd 0
    hPutStrLn h $ concatMap ("{new_contents}" ++) (lines originalFileContents)

    hClose h2
    hClose h

The hDuplicate function is provided by GHC.IO.Handle module.

Returns a duplicate of the original handle, with its own buffer. The two Handles will share a file pointer, however. The original handle's buffer is flushed, including discarding any input data, before the handle is duplicated.

With hSeek you can set position of the handle before reading or writing.

But I'm not sure how reliable would be using "AbsoluteSeek 0" instead of "SeekFromEnd 0" for writing, i.e. overwriting contents. Generally I would suggest to write to a temporary file first, for example using openTempFile (from System.IO), and then replace original.