A readVarInt implementation using recursion.

readVarInt.hs

import Data.Bits
import Data.Binary.Get
import Data.ByteString.Lazy (pack)

type VarInt = Integer

readVarInt :: Get VarInt
readVarInt = do
    bs <- loop 0
    return bs
    where
        loop acc = do
            b <- getWord8
            let currentByte = toInteger $ b .&. 0x7f
                acc' = (acc `shiftL` 7) .|. currentByte
            if (b .&. 0x80 /= 0)
                then loop acc'
                else return acc'
 
main :: IO ()
main = print $ runGet (readVarInt) (pack [0x7f]) -- this must return 127