Converting a bytestring to a list of 7 bits bytes - haskell

Question

I have to convert a ByteString into a list of 7 bits bytes. For example, bytes with a, b, c, d etc. bits:
abcdefgh ijklmnop qrstuvwx yz...
should be converted to:
abcdefg hijklmn opqrstu vwxyz...
I use the Binary-Bits package in order to do it. My convert8to7 function is recursive but the Binary-Bits does not provide any mean to check for the lack of bits whereas the Get monad does have isEmpty or remaining functions.
Here’s my code:
import Data.Word
import Data.Binary.Bits.Get
import Data.Binary.Get (runGet)
import Data.ByteString.Lazy.Char8
convert8to7 :: BitGet [Word8]
convert8to7 = do
bits <- getWord8 7
rest <- convert8to7
return (bits : rest)
main :: IO ()
main = do
let datas = pack "Hello world!"
print $ runGet (runBitGet convert8to7) datas
When I run this code, it logically says:
Data.Binary.Get.runGet at position 12: demandInput: not enough bytes
Can I do this conversion with Binary-Bits or should I look for an other package ?
Update
Here’s my code based on user5402 answer:
import Data.Word
import Data.Bits
import Data.Binary.Bits.Get
import Data.Binary.Get (runGet)
import qualified Data.ByteString.Lazy.Char8 as BS
convert87 :: Int -> BitGet [Word8]
convert87 n
| n == 0 = return []
| n < 7 = do bits <- getWord8 n
return [shiftL bits (7 - n)]
| otherwise = do bits <- getWord8 7
rest <- convert87 (n-7)
return $ bits : rest
to87 :: BS.ByteString -> [Word8]
to87 datas = runGet (runBitGet (convert87 len)) datas
where len = fromIntegral $ BS.length datas * 8
main :: IO ()
main = do
let datas = BS.pack "Hello world!"
print $ to87 datas

Answer 1

The problem is that you need to keep track of the number of bits to decode - the BitGet monad doesn't know when the end of input has been reached.
Try this:
import Data.Word
import Data.Binary.Bits.Get
import Data.Binary.Get (runGet)
import Data.ByteString.Lazy.Char8
import qualified Data.ByteString.Lazy.Char8 as BS
convert87 :: Int -> BitGet [Word8]
convert87 n
| n < 7 = do bits <- getWord8 n
return [bits]
| otherwise = do bits <- getWord8 7
rest <- convert87 (n-7)
return $ bits : rest
main :: IO ()
main = do
let datas = pack "Hello world!"
len = fromIntegral $ BS.length datas * 8
print $ runGet (runBitGet (convert87 len)) datas
Update: Here is the way to detect end of input in the Get monad (on top of which the BitGet monad is implemented). It relies on the Alternative class for Get. The function chunks7 breaks up a byte string into chunks of 7 with any remainder going into the last chunk.
As far as I can tell, BitGet does not implement the Alternative class - although I'm sure it could.
import Data.Word (Word8)
import Data.Binary.Get
import Data.ByteString.Lazy.Char8
import qualified Data.ByteString as BSW
import qualified Data.ByteString.Lazy as BSL
import Control.Applicative -- used for (<|>)
chunks7 :: Get [[Word8]]
chunks7 = do
b <- isEmpty
if b
then return []
else do chunk <- fmap BSW.unpack (getByteString 7)
<|> fmap BSL.unpack getRemainingLazyByteString
rest <- chunks7
return $ chunk : rest
main :: IO ()
main = do
let datas = pack "Hello world! This is a test"
print $ runGet chunks7 datas