Couldn't match expected type `Data.ByteString.Internal.ByteString' with actual type `ByteString'

Question

Running the following code:

import Crypto.BCrypt
import Data.ByteString.Lazy.Char8

main = do
  maybe_pwhash <- hashPasswordUsingPolicy slowerBcryptHashingPolicy (pack "hunter2")
  print $ maybe_pwhash

I get the following compilation error:

test.hs:5:70:
    Couldn't match expected type `Data.ByteString.Internal.ByteString'
                with actual type `ByteString'
    In the return type of a call of `pack'
    In the second argument of `hashPasswordUsingPolicy', namely
      `(C.pack "hunter2")'
    In a stmt of a 'do' block:
      maybe_pwhash <- hashPasswordUsingPolicy
                        slowerBcryptHashingPolicy (pack "hunter2")

I am confused because I don't see why there is a difference between a Data.ByteString.Internal.ByteString and a ByteString.

Cordite

Answering my own question because I had already written most of it by the time I managed to figure it out... Anyway, I still don't really understand the difference between all the different bytestring types and modules (´Lazy` vs Strict, ByteString.Char8 vs ByteString, etc) and the haskellwiki article on that seems to be just a stub. Any explanations on that would be much appreciated. – Cordite

There are two different types of ByteStrings: lazy and strict. If the module name is not qualified with Lazy, then it uses strict ByteString. A lazy ByteString is essentially a linked list of strict ByteString. – Ralph

Answer 1

According to the bcrypt docs, you should be using strict bytestrings

import Data.ByteString.Char8

instead of the lazy ones:

import Data.ByteString.Lazy.Char8

Cordite

To clarify: There are two kinds of ByteStrings: Lazy and strict. Data.ByteString.(Internal.)ByteString is strict. Data.ByteString.Lazy.(Internal.)ByteString is lazy. If you see a plain old unqualified ByteString type not matching a qualified ByteString type, then it's the other one. – Hunker

.Char8 is evil/broken/experts-only. You shouldn't use it unless you know exactly what it does and what's wrong with it. ByteStrings are byte sequences, not character sequences. With that said, Data.ByteString.Char8.ByteString is the same as strict ByteString and Data.ByteString.Lazy.Char8.ByteString is the same as lazy ByteString. There are only two ByteStrings. – Hunker