Mode of mapping. Four cases are supported.
file is mapped read-only, file must exist
file is mapped read-write, file must exist
file is mapped read-write, but changes aren't propagated to disk, file must exist
file is mapped read-write, if file does not exist it will be created with default permissions, region parameter specifies size, if file size is lower it will be extended with zeros
|:: forall s a . Storable a|
Path of the file to map
|-> Maybe (Int64, Int)|
|-> IO (MVector (PrimState IO) a)|
Map a file into memory as a mutable vector.