How to append text to a file🔗

To append text to a file instead of overwriting it, you can use the IO.FS.Mode.append mode when creating the file handle. This will allow you to add new content to the end of the file without deleting the existing content. Note that it will not add a newline character automatically, you would have to include it.

Important: flush is necessary to ensure that the file handler writes the content to the file immediately. Otherwise, the content may be buffered and not written until later.

def appendToFile (path s : String) : IO Unit := do
  let file := ← IO.FS.Handle.mk path IO.FS.Mode.append
  file.putStr s
  file.flush

-- Another way
def appendToFile' (path : System.FilePath) (s : String)
    : IO Unit := do
  IO.FS.withFile path IO.FS.Mode.append fun handle =>
    handle.putStr s

Note, IO.FS.withFile is recommended because it ensures the handle is closed and the buffer is flushed, even if an exception is thrown.

Now if you wanted to write the string in the beginning of the file and keep the existing content, you can read the existing content first, then write the new string followed by the old content.

def prependToFile (path s : String) : IO Unit := do
  let file := ← IO.FS.Handle.mk path IO.FS.Mode.read
  let oldContent ← file.readToEnd
  let file := ← IO.FS.Handle.mk path IO.FS.Mode.write
  file.putStr (s ++ oldContent)
  file.flush

-- Another way
def prependToFile' (path : System.FilePath) (s : String)
    : IO Unit := do
  let oldContent ← IO.FS.readFile path
  IO.FS.writeFile path (s ++ oldContent)