ByteString expects different ByteString

Question

This code doesn't typecheck:

import Network.HTTP.Conduit
import qualified Data.ByteString.Char8 as BS

main :: IO ()
main = do
  resp <- simpleHttp "http://www.google.com"
  putStrLn $ BS.unpack resp

Throws the following error:

Couldn't match expected type `BS.ByteString'
            with actual type `Data.ByteString.Lazy.Internal.ByteString'
In the first argument of `BS.unpack', namely `resp'
In the second argument of `($)', namely `BS.unpack resp'
In a stmt of a 'do' block: putStrLn $ BS.unpack resp
Failed, modules loaded: none.

How to fix this ? Changing to other ByteString variant doesn't work.

The type of simpleHttp function is like this: simpleHttp :: Control.Monad.IO.Class.MonadIO m => String -> m Data.ByteString.Lazy.Internal.ByteString. So I try to get the ByteString within the IO monad and try to unpack it, but this results in an error.

Answer 1

There are two separate ByteString modules, one for lazy ByteStrings and one for strict ByteStrings. simpleHTTP returns a lazy bytestring but you imported the strict bytestring module so unpack is expecting a strict bytestring.

Try changing

import qualified Data.ByteString.Char8 as BS

to

import qualified Data.ByteString.Lazy.Char8 as BS

---

That said, you need to be careful if you use the Char8 version of the bytestring modules, since the String <-> ByteString conversions only work if you use ASCII encoding. I would recommend converting your bytestrings to Text with an appropriate encoding function and then printing that.