Bit-packing Word32 and Word8 into Word64 in Haskell.

BitPack.hs

import Data.Bits (Bits (shiftL, shiftR, (.&.), (.|.)))
import Data.Word (Word32, Word64, Word8)

word64 :: (Word32, Word8) -> Word64
word64 (w32, w8) = ((fromIntegral w32 :: Word64) `shiftL` 8) .|. (fromIntegral w8 :: Word64)
{-# INLINE [0] word64 #-}

unword64Fst :: Word64 -> Word32
unword64Fst w = fromIntegral (w `shiftR` 8)
{-# INLINE [0] unword64Fst #-}

unword64Snd :: Word64 -> Word8
unword64Snd w = fromIntegral (w .&. 255)
{-# INLINE [0] unword64Snd #-}

unword64 :: Word64 -> (Word32, Word8)
unword64 w = (unword64Fst w, unword64Snd w)
{-# INLINE [0] unword64 #-}

{-# RULES
"bit_pack_cancel" forall a b. unword64 (word64 (a, b)) = (a, b)
"bit_unpack_fst" forall a. fst (unword64 a) = unword64Fst a
"bit_unpack_snd" forall a. snd (unword64 a) = unword64Snd a
  #-}