chunksOf analog for ByteString?

Question

I need to split bytestring into list of bytestrings diving first one by 100 characters. For lists, I can use chunksOf but not for ByteString.

Is there some proper way to do this?

Answer 1

Both ByteString and Lazy.Bytestring have splitAt functions which you can use to unfoldr a list.

import Data.List (unfoldr)
import Data.Int (Int64)

import qualified Data.ByteString as BS
import qualified Data.ByteString.Lazy as Lazy

justWhen :: (a -> Bool) -> (a -> b) -> (a -> Maybe b)
justWhen f g a = if f a then Just (g a) else Nothing

nothingWhen :: (a -> Bool) -> (a -> b) -> (a -> Maybe b)
nothingWhen f = justWhen (not . f)

chunksOf :: Int -> BS.ByteString -> [BS.ByteString]
chunksOf x = unfoldr (nothingWhen BS.null (BS.splitAt x))

chunksOf' :: Int64 -> Lazy.ByteString -> [Lazy.ByteString]
chunksOf' x = unfoldr (nothingWhen Lazy.null (Lazy.splitAt x))

Building bytestrings for an example is easier with OverloadedStrings

{-# LANGUAGE OverloadedStrings #-}

main = do
    print $ chunksOf 3 ""
    print $ chunksOf 3 "Hello World!"
    print $ chunksOf' 3 ""
    print $ chunksOf' 3 "Hello, World"