The documentation for pokeByteOff indicates that the following equality holds:

  pokeElemOff addr idx x =

    poke (addr `plusPtr` (idx * sizeOf x)) x

Notably, this ignores alignment. It thus seems to imply that sizeOf must always be a multiple of alignment; otherwise, non-zero indices could access non-aligned addresses.

Was this intentional? If so, I believe sizeOf and alignment should document the law. If not, then I believe the {poke,peek}ElemOff laws need to change to something like