Re: [Haskell-beginners] help understanding JSON

10 May 2011

      Hello,

 I do not have the right answer for your question, I am just starting to 
 learn Haskell.

 Still, you may want to take a look at the JSON library example 
 described in real world Haskell, this can bring you some more clues:

 http://book.realworldhaskell.org/read/writing-a-library-working-with-json-da...

 Regards,
 Adrien

 On Tue, 10 May 2011 00:51:30 -0700, Sean Perry wrote:
...
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.
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Re: [Haskell-beginners] help understanding JSON

Adrien Haxaire