String -> ByteString and reverse

Question

In my Haskell Program I need to work with Strings and ByteStrings:

import Data.ByteString.Lazy as BS  (ByteString)
import Data.ByteString.Char8 as C8 (pack)
import Data.Char                   (chr)

stringToBS :: String -> ByteString
stringToBS str = C8.pack str

bsToString :: BS.ByteString -> String
bsToString bs = map (chr . fromEnum) . BS.unpack $ bs

bsToString works fine, but stringToBS results with following error at compiling:

Couldn't match expected type ‘ByteString’
                with actual type ‘Data.ByteString.Internal.ByteString’
    NB: ‘ByteString’ is defined in ‘Data.ByteString.Lazy.Internal’
        ‘Data.ByteString.Internal.ByteString’
          is defined in ‘Data.ByteString.Internal’
    In the expression: pack str
    In an equation for ‘stringToBS’: stringToBS str = pack str

But I need to let it be ByteString from Data.ByteString.Lazy as BS (ByteString) for further working functions in my code.

Any idea how to solve my problem?

BTW, I hope you know what you are doing. A ByteString is a sequence of bytes, and not a string: conversion between them fundamentally depends on a given encoding. In particular, Char8 assumes that everything is encoded using latin-1, and silently truncates everything else. — chi
Is there a particular reason why you are using String and not Text the conversion with encoding/decoding is a bit more straightforward. — epsilonhalbe

ErikR ErikR · Accepted Answer · 2016-05-09T17:09:51

You are working with both strict ByteStrings and lazy ByteStrings which are two different types.

This import:

import Data.ByteString.Lazy as BS  (ByteString)

makes ByteString refer the lazy ByteStrings, so the type signature of your stringToBS doesn't match it's definition:

stringToBS :: String -> ByteString  -- refers to lazy ByteStrings
stringToBS str = C8.pack str        -- refers to strict ByteStrings

I think it would be a better idea to use import qualified like this:

import qualified Data.ByteString.Lazy as LBS
import qualified Data.ByteString.Char8 as BS

and use BS.ByteString and LBS.ByteString to refer to strict / lazy ByteStrings.

String -> ByteString and reverse

2 Answers