How to get nth byte from ByteString?

Question

How can I get nth byte of ByteString in Haskell?

I tried to find function like !! for ByteStrings, but found nothing.

[ByteString.index](https://hackage.haskell.org/package/bytestring-0.10.8.2/docs/Data-ByteString.html#v:index) is probably what you're looking for. — David Maze, Dec 22 '18 at 12:44

score 6 · Accepted Answer · answered Dec 22 '18 at 13:15

ByteString.index is the function you're looking for.

Most of the "containerish" types emulate the extended list interface; you also want to be careful because that index function will crash the program if you feed it a string that's too short (as will !! on ordinary lists). A better implementation might be

import Data.ByteString as B
nthByte :: Int -> B.ByteString -> Maybe Word8
nthByte n bs = fst <$> B.uncons (B.drop n bs)

which, reading inside out, drops the first n bytes (maybe producing an empty byte string), then attempts to split the first character from the remainder, and if successful, ignores the rest of the string.

How to get nth byte from ByteString?

1 Answers1