What is the best way to convert String to ByteString

Data.ByteString.UTF8.fromString is also useful. The Char8 version will lose the unicode-ness and UTF8 will make a UTF8-encoded ByteString. You have to choose one or the other.

A safe approach will involve encoding the unicode string:

import qualified Data.ByteString as B
import qualified Data.Text as T
import Data.Text.Encoding (encodeUtf8)

packStr'' :: String -> B.ByteString
packStr'' = encodeUtf8 . T.pack

Regarding the other answers: Data.ByteString.Char8.pack is effectively the same as the version in the question, and is unlikely to be what you want:

import qualified Data.ByteString as B
import qualified Data.ByteString.Char8 as C
import qualified Data.Text as T
import Data.Text.Encoding (encodeUtf8)
import Data.Char (ord)

packStr, packStr', packStr'' :: String -> B.ByteString
packStr   = B.pack . map (fromIntegral . ord)
packStr'  = C.pack
packStr'' = encodeUtf8 . T.pack

*Main> packStr "hellö♥"
"hell\246e"
*Main> packStr' "hellö♥"
"hell\246e"
*Main> packStr'' "hellö♥"
"hell\195\182\226\153\165"

Data.ByteString.UTF8.fromString is fine, but requires the utf8-string package, while Data.Text.Encoding comes with the Haskell Platform.

Here is my cheat sheet for Haskell String/Text/ByteString strict/lazy conversion assuming the desired encoding is UTF-8. The Data.Text.Encoding library has other encodings available.

Please make sure to not write (using OverloadedStrings):

lazyByteString :: BL.ByteString
lazyByteString = "lazyByteString ä ß" -- BAD!

This will get encoded in an unexpected way. Try

lazyByteString = BLU.fromString "lazyByteString ä ß" -- good

instead.

String literals of type 'Text' work fine with regard to encoding.

Cheat sheet:

import Data.ByteString.Lazy as BL
import Data.ByteString as BS
import Data.Text as TS
import Data.Text.Lazy as TL
import Data.ByteString.Lazy.UTF8 as BLU -- from utf8-string
import Data.ByteString.UTF8 as BSU      -- from utf8-string
import Data.Text.Encoding as TSE
import Data.Text.Lazy.Encoding as TLE

-- String <-> ByteString

BLU.toString   :: BL.ByteString -> String
BLU.fromString :: String -> BL.ByteString
BSU.toString   :: BS.ByteString -> String
BSU.fromString :: String -> BS.ByteString

-- String <-> Text

TL.unpack :: TL.Text -> String
TL.pack   :: String -> TL.Text
TS.unpack :: TS.Text -> String
TS.pack   :: String -> TS.Text

-- ByteString <-> Text

TLE.encodeUtf8 :: TL.Text -> BL.ByteString
TLE.decodeUtf8 :: BL.ByteString -> TL.Text
TSE.encodeUtf8 :: TS.Text -> BS.ByteString
TSE.decodeUtf8 :: BS.ByteString -> TS.Text

-- Lazy <-> Strict

BL.fromStrict :: BS.ByteString -> BL.ByteString
BL.toStrict   :: BL.ByteString -> BS.ByteString
TL.fromStrict :: TS.Text -> TL.Text
TL.toStrict   :: TL.Text -> TS.Text

Please +1 Peaker's answer, because he correctly deals with encoding.