[Haskell-beginners] Convert bits to bytes

[Daniel Fischer]

On Thursday 02 September 2010 19:29:23, Alec Benzer wrote:
> I came up with this (dealing with non-multiple of 8 numbers of bits)
>
> bitsToBytes :: [Bool] -> [Word8]
> bitsToBytes [] = []
> bitsToBytes bits = map bitsToByte (getChunks bits)
>   where bitsToByte = foldl' (\byte bit -> byte*2 + if bit then 1 else 0)
> 0
>
> getChunks :: [Bool] -> [[Bool]]
> getChunks [] = []
> getChunks xs
>   | length xs < 8 = getChunks $ take 8 (xs ++ repeat False)

Pet peeve.
Don't use `length xs < k' (or <=, ==, >=, >).
That fails hard on infinite lists, also it is slow and prone to cause a 
space leak on long lists.
Remember, length must walk the entire list.
Instead of length xs < k use null (drop (k-1) xs), other combinations of 
not, null and drop for the other tests.
Or you can use lazy Peano numbers and check

genericLength xs < (8 :: Peano).

>   | otherwise =
>     let (these,rest) = splitAt 8 xs
>     in these:getChunks rest