Parsing JSON data from a URL in Haskell using Aeson - json

Question

I'm trying to create a web app in Haskell that takes some JSON input from a URL.
{-# LANGUAGE OverloadedStrings, DeriveGeneric #-}
import Data.Aeson as Q
import Data.Text
import Control.Applicative
import Control.Monad
import qualified Data.ByteString.Lazy as B
import Network.HTTP.Conduit (simpleHttp)
import GHC.Generics
--import Data.DateTime
data Temperatures =
Temperatures { date :: String
, temperature :: Int
} deriving (Show, Generic)
instance FromJSON Temperatures
instance ToJSON Temperatures
jsonURL :: String
jsonURL = "http://www.phoric.eu/temperature"
getJSON :: IO B.ByteString
getJSON = simpleHttp jsonURL
main :: IO ()
main = do
d <- (eitherDecode <$> getJSON) :: IO (Either String Temperatures)
case d of
Left e -> putStrLn e
Right stuff -> print stuff
However, I'm getting the error at runtime:
Bradley$ runhaskell test.hs
The key "date" was not found
The JSON is below # the URL which is in the code.
{"temperatures":[
{"date":"2015-02-28T20:16:12+00:00", "temperature":0},
{"date":"2015-01-01T21:46:55+00:00", "temperature":2},
{"date":"2015-04-08T21:46:53+00:00", "temperature":3},
{"date":"2015-04-09T21:46:01+00:00", "temperature":4},
{"date":"2015-04-10T21:46:40+00:00", "temperature":5},
{"date":"2015-04-11T21:46:36+00:00", "temperature":6},
{"date":"2015-04-12T20:36:25+00:00", "temperature":7}
]}
I have no idea why it is not recognising the keys when they're clearly present in the JSON object, any ideas?

Answer 1

In its current form, your code can only parse a single line of the form {"date":"2015-04-12T20:36:25+00:00", "temperature":7} : it tries to parse the input into a Temperatures, but can't find a date key into the root object because it only has a temperatures key.
The problem is that your Temperatures datatype does not really match your input. Here is something that should match:
data DataPoint = DataPoint { date :: String
, temperature :: Int
} deriving (Show, Generic)
data Temperatures = Temperatures { temperatures :: [DataPoint]
} deriving (Show, Generic)