fixed-length vectors?

BitVectorTest.hs

import qualified Data.Vector.Unboxed as V
import Data.Bits(setBit)
import Data.Word(Word8)

main :: IO ()
main = do
  assertEqual 0x8c $ byte (bitsLE "0011" V.++ bitsLE "0001")
  putStrLn "passed"

-- Create a bit-vector from a little-endian string of 1's and 0's
bitsLE :: String -> V.Vector Bool
bitsLE = V.map (/= '0') . V.fromList

-- Convert to Word8.  Bits past first 8 silently ignored.
byte :: V.Vector Bool -> Word8
byte = V.ifoldl' setWhen 0
  where
    setWhen acc _ False = acc
    setWhen acc i True  = setBit acc i

assertEqual :: (Eq a, Show a) => a -> a -> IO ()
assertEqual actual expected
  | actual == expected = return ()
  | otherwise = error $ "assertion failed\n"
                     ++ "  expected: " ++ show expected ++ "\n"
                     ++ "   but got: " ++ show actual