Updating a value in Data.ByteString

Question

The C language provides a very handy way of updating the nth element of an array: array[n] = new_value. My understanding of the Data.ByteString type is that it provides a very similar functionality to a C array of uint8_t - access via index :: ByteString -> Int -> Word8. It appears that the opposite operation - updating a value - is not that easy.

My initial approach was to use the take, drop and singleton functions, concatetaned in the following way:

updateValue :: ByteString -> Int -> Word8 -> ByteString
updateValue bs n value = concat [take (n-1) bs, singleton value, drop (n+1) bs] 

(this is a very naive implementation as it does not handle edge cases)

Coming with a C background, it feels a bit too heavyweight to call 4 functions to update one value. Theoretically, the operation complexity is not that bad:

• take is O(1)
• drop is O(1)
• singleton is O(1)
• concat is O(n), but here I am not sure if the n is the length of the concatenated list altogether or if its just, in our case, 3.

My second approach was to ask Hoogle for a function with a similar type signature: ByteString -> Int -> a -> ByteString, but nothing appropriate appeared.

Am I missing something very obvious, or is really that complex to update the value?

I would like to note that I understand the fact that the ByteString is immutable and that changing any of its elements will result into a new ByteString instance.

EDIT: A possible solution that I found while reading about the Control.Lens library uses the set lens. The following is an outtake from GHCi with omitted module names:

> import Data.ByteString
> import Control.Lens
> let clock = pack [116, 105, 99, 107]
> clock
"tick"
> let clock2 = clock & ix 1 .~ 111
> clock2
"tock"

Answer 1

One solution is to convert the ByteString to a Storable Vector, then modify that:

import Data.ByteString (ByteString)
import Data.Vector.Storable (modify)
import Data.Vector.Storable.ByteString  -- provided by the "spool" package
import Data.Vector.Storable.Mutable (write)
import Data.Word (Word8)

updateAt :: Int -> Word8 -> ByteString -> ByteString
updateAt n x s = vectorToByteString . modify inner . byteStringToVector
  where
    inner v = write v n x

See the documentation for spool and vector.