[Haskell-beginners] Ambiguous MonadIO and Monad

6 Mar 2012

      Hi,

I'm trying to understand how Network.OAuth.Consumer works, and
run sample code found here with Twitter API. (the full code is
attached at the bottom)
http://hackage.haskell.org/packages/archive/hoauth/0.3.3/doc/html/Network-OA...

When I load the test code with GHCi 7.0.2, it raises error:

--- Error message begin
TestOAuth.hs:21:103:
    Ambiguous type variable `m0' in the constraints:
      (MonadIO m0) arising from a use of `serviceRequest'
                   at TestOAuth.hs:21:103-116
      (Monad m0) arising from a use of `>>=' at TestOAuth.hs:21:99-101
    Possible cause: the monomorphism restriction applied to the following:
      response :: m0 Response (bound at TestOAuth.hs:17:1)
    Probable fix: give these definition(s) an explicit type signature
                  or use -XNoMonomorphismRestriction
    In the second argument of `(>>=)', namely
      `serviceRequest CurlClient'
    In the expression:
        signRq2 HMACSHA1 (Just $ Realm "realm") srvUrl
      >>=
        serviceRequest CurlClient
    In the second argument of `($)', namely
      `do { signRq2 PLAINTEXT Nothing reqUrl >>= oauthRequest CurlClient;
          cliAskAuthorization authUrl;
            signRq2 PLAINTEXT Nothing accUrl >>= oauthRequest CurlClient;
            signRq2 HMACSHA1 (Just $ Realm "realm") srvUrl
          >>=
            serviceRequest CurlClient }'
--- Error message end

I seem using NoMonomorphismRestriction is workaround, so I want to add
a proper type signature, but I cannot figure out what it is.

What confuses me is why GHC didn't just take MonadIO as MonadIO is a
Monad according to the definition.
http://hackage.haskell.org/packages/archive/transformers/0.2.2.0/doc/html/Co...

I tried to add a type signature by replacing the second line from the
bottom with this, in vain.
  ((signRq2 HMACSHA1 (Just $ Realm "realm") srvUrl) :: (MonadIO m) =>
OAuthMonadT m OAuthRequest) >>= serviceRequest CurlClient

Can you help me find what's wrong with this?

Thanks in advance,
Ken

--- Test code (TestOAuth.hs)
import Control.Monad.IO.Class (MonadIO)
import Data.Maybe (fromJust)

import Network.OAuth.Consumer
import Network.OAuth.Http.CurlHttpClient
import Network.OAuth.Http.Request
import Network.OAuth.Http.Response

consumerKey = "MY CONSUMER KEY"
consumerSec = "MY CONSUMER SEC"

reqUrl    = fromJust . parseURL $ "https://api.twitter.com/oauth/request_token"
accUrl    = fromJust . parseURL $ "https://api.twitter.com/oauth/access_token"
srvUrl    = fromJust . parseURL $ "http://service/path/to/resource/"
authUrl   = ("https://api.twitter.com/oauth/authorize?oauth_token="++)
. findWithDefault ("oauth_token","ERROR") . oauthParams
app       = Application consumerKey consumerSec OOB
response  = runOAuthM (fromApplication app) $ do { signRq2 PLAINTEXT
Nothing reqUrl >>= oauthRequest CurlClient
                                                 ; cliAskAuthorization authUrl
                                                 ; signRq2 PLAINTEXT
Nothing accUrl >>= oauthRequest CurlClient
                                                 ; signRq2 HMACSHA1
(Just $ Realm "realm") srvUrl >>= serviceRequest CurlClient
                                                 }

[Haskell-beginners] Ambiguous MonadIO and Monad

Ken KAWAMOTO