Saving and restoring text positions

tell() returns a u64 position cookie. Pass it back to seek() on the same file to resume reading there, or use seek(0u64) to rewind. Do not calculate with these values: they include newline state and are not byte or character counts. Saved positions are only meaningful while the file contents remain unchanged.

def demo() -> Result[(), IoError]:
    write_text("positions.txt", "one\r\ntwo")?
    with open("positions.txt")? as file:
        file.readline()?
        saved = file.tell()?
        print(file.read()?)?
        file.seek(saved)?
        print(file.read()?)?
    Ok(())
def main() -> Result[(), IoError]:
    demo()?
    Ok(())

two
two

truncate(size) resizes a writable file in bytes and returns its new length. With no size, it truncates at the current physical position. Neither form moves the cursor. A text-position cookie is not a byte size. Cutting through a multibyte character makes the file invalid UTF-8; a later read reports that error.

def demo() -> Result[(), IoError]:
    write_text("short.txt", "keep rest")?
    with open("short.txt", "r+")? as file:
        file.read(4)?
        print(file.truncate()?)?
    print(read_text("short.txt")?)?
    Ok(())
def main() -> Result[(), IoError]:
    demo()?
    Ok(())

4
keep