Monad escaping inside a StateT context

Question

I am trying to get back a value that is in a json feed (via Aeson) directly inside a StateT stacked on IO:

{-# LANGUAGE DeriveGeneric #-}
module MyFeed where

import Data.Aeson
import Network.URI (parseURI, URI(..))
import Data.Maybe (fromJust)
import Data.Text (Text, unpack)
import Control.Monad.State
import Network.HTTP
import GHC.Generics
import Control.Applicative
import Network.HTTP.Conduit (simpleHttp)
import qualified Data.ByteString.Lazy as B

type Feed a = StateT MyIndex IO a

data MyIndex = MyIndex {
    index :: Int
}

data FooBar = Foo | Bar

data MyFeed = MyFeed {
    idx :: !Text,
    key :: !Text
    } deriving (Show,Generic)

instance FromJSON MyFeed
instance ToJSON MyFeed

getJSON :: String -> IO B.ByteString
getJSON url = simpleHttp url

getFeed :: String -> IO (Maybe MyFeed)
getFeed url = (decode <$> getJSON url) :: IO (Maybe MyFeed)

getIndex :: FooBar -> Feed MyIndex
getIndex fb = do
  cursor <- get
  let newCursor = case fb of
                            Foo -> do  myFeed <- liftIO $ getFeed "http://echo.jsontest.com/key/value/idx/1"
                                       let i = read $ unpack $ idx $ fromJust myFeed
                                       return $ cursor { index = i }

                            Bar -> return cursor
  put newCursor
  return newCursor

In the Foo case I fetch the feed as expected but when the required value is returned I get:

src/MyFeed.hs:47:10:
    Couldn't match expected type ‘MyIndex’
                with actual type ‘m0 MyIndex’
    Relevant bindings include
      newCursor :: m0 MyIndex (bound at src/MyFeed.hs:40:7)
    In the first argument of ‘return’, namely ‘newCursor’
    In a stmt of a 'do' block: return newCursor

The Actual Type looks still in a Monad context (do {...}). Is there a way to take it out or I am using a wrong approach?

Can you add all the definitions that you are using. It is difficult to diagnose the problem without being able to run it through GHC myself. — Matt, Jun 27 '15 at 17:49
Sorry you are perfectly right. I mixed up wrong parts. Anyway I have re-edited the question and put the solution below. — Randomize, Jun 27 '15 at 19:36
@Randomize It is considered good practice to put the solution in an answer instead of in the question. It is fine etiquette to answer your own question. — Daniel Wagner, Jun 27 '15 at 19:45

score 0 · Accepted Answer · answered Jun 27 '15 at 19:46

0

The error was due to the fact I used:

let newCursor = case fb of

instead of

newCursor <- case fb of

For this reason the final value never get "unwrapped" from its monad context.

answered Jun 27 '15 at 19:46

Randomize

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Monad escaping inside a StateT context

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