Haskell Type Error while using HTTP-Conduit

Question

I am working on OAuth2 authentication for a Yesod application and I am having a type error that I really really don't understand. The code is broken at the moment, and I have a few :: IO () 's and undefined s thrown around to help me isolate the type error, but the relevant code is:

getAccessToken :: Manager -> OAuth2 -> ExchangeToken -> IO (OAuth2Result Errors OAuth2Token)
getAccessToken manager oa code = do

  let (uri, defaultBody) = accessTokenUrl oa code
  let body = defaultBody <> [ ("client_id", TE.encodeUtf8 . oauthClientId $ oa )
                            , ("client_secret", TE.encodeUtf8 . oauthClientSecret $ oa)
                            , ("resource", TE.encodeUtf8 . oauthClientId $ oa)
                            ]

  response <- performOAuth2PostRequest manager oa uri body

  return undefined

performOAuth2PostRequest :: Manager -> OAuth2 -> URI -> PostBody -> IO (Response ByteString)
performOAuth2PostRequest manager oa uri body  = do
  defaultReq <- uriToRequest uri

  let addBasicAuth = applyBasicAuth (TE.encodeUtf8 . oauthClientId $ oa)
                                    (TE.encodeUtf8 . oauthClientSecret $ oa)

  let req = (addBasicAuth . updateRequestHeaders Nothing) defaultReq

  (httpLbs (urlEncodedBody body req) manager) :: IO (Response ByteString)

Notice that I am specifically setting the type of the httpLbs (urlEnc...) manager action as an IO (Response ByteString) using the ScopedTypeVariables extension. Also, that line of code should be an IO action because it's being performed at the top level of an IO action.

In fact, I ran a GHCi session and did:

Network.OAuth.OAuth2.HttpClient Network.OAuth.OAuth2.Internal 
Network.HTTP.Conduit Data.Functor Prelude> :t httpLbs
httpLbs
  :: Control.Monad.IO.Class.MonadIO m =>
     Request
     -> Manager -> m (Response Data.ByteString.Lazy.Internal.ByteString)

Which confirms my understanding that httpLbs should yield a MonadIO m => m (Response ByteString) .

But here is the error I get:

• Couldn't match type ‘Response
                         Data.ByteString.Lazy.Internal.ByteString’
                 with ‘IO (Response ByteString)’
  Expected type: Manager -> IO (Response ByteString)
    Actual type: Manager
                 -> Response Data.ByteString.Lazy.Internal.ByteString
• The function ‘httpLbs’ is applied to two arguments,
  its type is ‘Request
               -> m1 (Response Data.ByteString.Lazy.Internal.ByteString)’,
  it is specialized to ‘Request
                        -> Manager -> Response Data.ByteString.Lazy.Internal.ByteString’

Why is GHC specializing m to Response instead of IO ? How do I fix it?

Answer 1

You haven't included your import statements, making it difficult to debug this. My best guess though is that you've imported Network.HTTP.Simple , which provides functions that do not require an explicit Manager argument. I'm guessing this from the error message providing the expected type:

Request -> m1 (Response Data.ByteString.Lazy.Internal.ByteString)

Solution: either change the import, or drop the Manager argument.