How do I read (and parse) a file and then append to the same file without getting an exception?

Question

I am trying to read from a file correctly in Haskell but I seem to get this error.

*** Exception: neo.txt: openFile: resource busy (file is locked) This is my code.<

Answer 1

Whoopsy daisy, being lazy
tends to make file changes crazy.
File's not closed, as supposed
thus the error gets imposed.
This small guile, by loadFile
is what you must reconcile.
But don't fret, least not yet,
I will show you, let's get set.

---

As many other functions that work with IO in System.IO, readFile doesn't actually consume any input. It's lazy. Therefore, the file doesn't get closed, unless all its content has been consumed (it's then half-closed):

The file is read lazily, on demand, as with getContents.

We can demonstrate this on a shorter example:

main = do
  let filename = "/tmp/example"
  writeFile filename "Hello "
  contents <- readFile filename
  appendFile filename "world!"   -- error here

This will fail, since we never actually checked contents (entirely). If you get all the content (for example with printing, length or similar), it won't fail anymore:

main = do
  let filename = "/tmp/example2"
  writeFile filename "Hello "
  content <- readFile filename
  putStrLn content
  appendFile filename "world!"   -- no error

Therefore, we need either something that really closes the file, or we need to make sure that we've read all the contents before we try to append to the file.

For example, you can use withFile together with some "magic" function force that makes sure that the content really gets evaluated:

readFile' filename = withFile filename ReadMode $ \handle -> do
  theContent <- hGetContents handle
  force theContent

However, force is tricky to achieve. You could use bang patterns, but this will evaluate the list only to WHNF (basically just the first character). You could use the functions by deepseq, but that adds another dependency and is probably not allowed in your assignment/exercise.

Or you could use any function that will somehow make sure that all elements are evaluated or sequenced. In this case, we can use a small trick and mapM return:

readFile' filename = withFile filename ReadMode $ \handle -> do
  theContent <- hGetContents handle
  mapM return theContent

It's good enough, but you would use something like pipes or conduit instead in production.

The other method is to make sure that we've really used all the contents. This can be done by using another parsec parser method instead, namely runParserT. We can combine this with our withFile approach from above:

parseFile :: ParsecT String () IO a -> FilePath -> IO (Either ParseError a) 
parseFile p filename = withFile filename ReadMode $ \handle ->
  hGetContents handle >>= runParserT p () filename

Again, withFile makes sure that we close the file. We can use this now in your loadFilm:

loadFile :: FilePath -> IO (Either ParseError [Film])
loadFile filename = parseFile films filename

This version of loadFile won't keep the file locked anymore.