Puzzling Data.Encoding Error

Question

I am trying to use a decoder from Data.Encoding on some text that I'm not sure is encoded correctly. When I attempt to load the following code into haskell:

import Data.Encoding (DynEncoding,decodeLazyByteStringExplicit,encodingFromStringExplicit)
{- ...more imports ... -}
import qualified Data.ByteString.Lazy as B (ByteString,takeWhile)

{- ... more code ... -}
decodeMyString :: Encoding enc => B.ByteString -> enc -> Either String String
decodeMyString str decoder = case (decodeLazyByteStringExplicit decoder str) of
    Left s -> Left $ show s
    Right decoded -> Right decoder

I get the following error:

Couldn't match expected type `bytestring-0.10.0.2:Data.ByteString.Lazy.Internal.ByteString'
            with actual type `B.ByteString'
In the second argument of `decodeLazyByteStringExplicit', namely
  `str'
In the expression: (decodeLazyByteStringExplicit decoder str)
In the expression:
  case (decodeLazyByteStringExplicit decoder str) of {
    Left s -> Left $ show s
    Right decoded -> Right decoded }

Can anyone explain why? I have the latest versions of the libraries bytestring (10.4) and encoding (0.7). But it should not matter, because encoding's hackage page does not specify a required version of bytestring.

At this point, I'm almost tempted to write my own set of decoders for non-unicode encodings of bytestrings to convert them into utf-whatever bytestrings and use Data.Text.Encoding but I shouldn't have to given the existence of Data.Encoding .

Answer 1

That's a common problem when two different packages were assembled with different versions of the same library (ByteString in this case). A solution is here .