I am trying to evaluate the polymorphism technique used in Hackage library.  I like to know if the approach taken is "right" or not.

The code in question is in the Hoaut package   http://hackage.haskell.org/package/hoauth/

As far as I can understand the code wants to have polymorphism on HTTP client such that it can use different underlying HTTP.  The package comes with Curl and Debug implementation of its HTTPClient class.

My question is with how the polymorphism is used.  In function such as "serviceRequest" in the Network.OAuth.Consumer module you have:
   

-- | Performs a signed request with the available token.
serviceRequest :: (HttpClient c,MonadIO m) => c -> OAuthRequest -> OAuthMonadT m Response

serviceRequest c req = do { result <- lift $ runClient c (unpackRq req)

                          ; case (result)

                            of Right rsp -> return rsp

                               Left err  -> fail $ "Failure performing the request. [reason=" ++ err ++"]"

                          }



The function expects the HTTPClient implementation to be the first argument to the function.  Is that the right thing to do?   Or should
the instance of the client be a type parameter in the computation (in this case OAuthMonadT)? 

thanks,

Daryoush