Memory Mapping Large File Haskell

I am experimenting with the Haskell mmap package and I am quite new to Haskell, so I am trying to get started by writing a little program to write a small amount of data to a memory mapped file.

This code correctly creates and file size but doesn't seem to flush the data from the vector to the memory mapped file; I verified this using hexdump - it's just all 0s.

What is going wrong?

import Control.Monad
import Data.Vector.Storable
import Foreign.Marshal.Array
import System.Directory
import System.IO
import System.IO.MMap

createFile :: FilePath -> Integer -> IO ()
createFile path size = do
    h <- openBinaryFile path WriteMode
    hSetFileSize h size

n = 10
size = 10 * 8
path = "test.dat" :: FilePath

main :: IO ()
main = do
    createFile "signal.ml" size
    let v = generate n (\i -> i) :: Vector Int
    putStrLn $ show v
    (ptr, s, _, _) <- mmapFilePtr path ReadWrite Nothing
    unsafeWith v (\srcPtr -> copyArray ptr srcPtr n)
    munmapFilePtr ptr s

Many thanks.