Couldn't match expected type `BitcoinData'
                with actual type `Maybe a0'
    Expected type: Maybe BitcoinData
      Actual type: Maybe (Maybe a0)
    In a stmt of a 'do' block:
        return $ A.decode json :: Maybe BitcoinData
    In the expression:
      do { json <- simpleHttp
                     "http://api.bitcoincharts.com/v1/markets.json";
             return $ A.decode json :: Maybe BitcoinData }