[Haskell-beginners] help understanding JSON

10 May 2011

      I have some JSON which looks like this:

{"count":2, "result":[{"LastTrade":"31.24", "Symbol":"FOO"},
                                  {"LastTrade":"345.12", "Symbol":"BAR"}]}
(named testQuoteResult below)

When I try to parse this into types defined below it failed because the Double is encoded as a String. If I change the type of LastTrade in my code to String everything is ok but I am left read'ing the String.

I have defined the following using Language.JsonGrammar:

import Data.Iso
import Language.JsonGrammar
import Prelude hiding (id, (.), head, either)
import Control.Category ((.))

import qualified Data.Aeson as JS
import qualified Data.ByteString.Char8 as BS
import qualified Data.Attoparsec as P

data MyQuote = MyQuote { price  :: Double,
                                                 symbol :: String }
  deriving(Show)

myQuote = $(deriveIsos ''MyQuote)

instance Json MyQuote where
  grammar = myQuote . object
    (
      prop "LastTrade"
    . prop "Symbol"
    )

data MyQuoteResult = MyQuoteResult { count :: Int,
                                                                         result :: [MyQuote] }
  deriving(Show)

myQuoteResult = $(deriveIsos ''MyQuoteResult)

instance Json MyQuoteResult where
  grammar = myQuoteResult . object
    (  prop "count"
    .  prop "result"
    )

tryJSON input = 
  case P.parse JS.json (BS.pack input) of
    P.Done s v -> Right v
    other      -> Left ("failed parse " ++ show other)

Using it like this:
...
let Right qr = tryJSON testQuoteResult
fromJson qr :: Maybe MyQuoteResult
Nothing
Is there some way to convince the JSON text parser to turn "12.34" into (Double 12.34)??

An example using pure aeson would be fine if required. I just decided to play with JsonGrammar since it was announced today and it sounded interesting.

[Haskell-beginners] help understanding JSON

Sean Perry